What can fisheries historians learn from marine science?
– The concept of catch per unit effort (CPUE)
René Taudal Poulsen & Poul Holm
Fisheries historians have a lot to learn from marine science. If properly applied, methodologies
commonly used by fisheries biologists can enrich fisheries history and offer completely new
insights to maritime historians. This article will discuss how one powerful set of analytical tools
from fisheries biology can clarify central concepts within the field of fisheries history.
For many years, fisheries historians have examined structural changes in the fishing
industries, both in national and international contexts. The growth and decline of fisheries have
fascinated historians, and economic, political, social and technological aspects have been
analysed to explain the fluctuations of fisheries. Recently, the role of the marine environment has
also received attention from historians. After all, fishermen’s successes ultimately depend on fish
abundances. Both European, North American and Asian historians have argued that nineteenth
and twentieth centuries fishermen introduced more efficient gear and sailed to more distant
waters due to declining fish stocks. Facing ever smaller fish stocks abundances, fishermen had to
increase the catching power of the fleet in order to maintain profitable fisheries. Thus, fishing
effort was increased to counteract the effects of declining fish stocks.1
The main argument in the analyses referred to above is based on qualitative observations
from fishermen complaining about declining fish stocks and government officials examining the
state of the fishing industry. While these analyses focus on an interesting question – human
responses to changes in the marine ecosystems – they suffer from a lack of quantitative data sets
to support the strong assertions. None of these historians have attempted to measure the scale of
1
Margret Beattie Bougue, Fishing the Great Lakes: an environmental history, 1783-1933, 2000. Robb Robinson,
The English fishing industry, 1790-1914: a case study of the Yorkshire coast, 1984. Robb Robinson, Trawling: the
rise and fall of the British trawl fishery 1996. Michael Stuart Haines, Britain's distant water fishing industry, 18301914 : a study in technological change, 1998. Sean T. Cadigan & J.A. Hutchings ‘Nineteenth-Century Expansion
for the Newfoundland Fishery of Atlantic Cod: An Exploration of Underlying Causes’ in Poul Holm, Tim D. Smith
& David J. Starkey (ed.) Exploited Seas: New Directions forMarine Environmental History, 2002, p. 31-65.
Malcolm Gray, The Fishing Industries of Scotland 1790-1914, 1978. John G. Butcher: The closing of the frontier: a
history of the marine fisheries of southeast Asia, 2004.
1
changes in stock abundances. In this regard, fisheries historians can learn from marine scientists’
methodology. Fisheries biologists routinely use the concept of catch rates (catch per unit of
effort, CPUE). This is a powerful analytical tool which can lend methodological coherence to a
historical analysis of the fluctuations of fisheries. The article examines how to apply the CPUE
concept in an historical analysis, and what are the data requirements for and pitfalls of such an
approach. The article argues that CPUE can be used by historians for at least two purposes.
Firstly, CPUE is a useful indicator to assess the performance of fisheries. Secondly, it reveals
changes in the abundance of the fishermen’s target species. These assertions will be illustrated
with an example from nineteenth century Swedish North Sea fisheries for ling, cod and tusk.
Catchability
A CPUE analysis requires quantitative data sets for commercial catches and fishing effort.
Basically, the data sets should provide answers to the following three questions:
1. How much was caught?
2. How large was the fishing effort?
3. Where did fishing take place?
Indeed, such data sets are available for many historical fisheries, even centuries back in time.
With the advent of national fisheries statistics in Europe and North America in the nineteenth
century, data sets of this type were published annually by fisheries inspectors. For some
fisheries, catch rates can be derived from these sources.
In fisheries biology, the concept of CPUE has been used for more than a century. 2 In 1900
the British marine scientist Walter Garstang applied the catch-rate approach to an analysis of the
British fisheries data sets. Garstang standardised fishing effort into a common unit of effort in
order to determine if demersal stocks, i.e. bottom living fish such as cod and haddock, were in
decline. Garstang concluded that indeed a decline in the abundance of demersal stocks exploited
by British fishermen had taken place in the late nineteenth century, which was witnessed by a
decline in CPUE. The calculations were hampered by the fact that British vessels differed in
terms of efficiency (i.e. catching power). However, Garstang took these differences into
consideration when he calculated the total fishing effort of the British fleet. For instance he
estimated that a steam vessel equipped with a beam trawl was four times as efficient as a sailing
2
Tim D. Smith, Scaling Fisheries, 1994.
2
smack with a trawl.3 Through this sophisticated use of standardization Garstang managed to
establish CPUE as a measure of fish abundance which is still widely used a century later.
Strangely, however, references to CPUE studies have rarely been made by historians, and few if
any historians have made independent CPUE analyses.4 We have found ample historical data
sets that lend themselves to this sort of analysis, and aim in this paper to demonstrate the
potential by a Swedish example study. We hope to demonstrate the analytical richness and rigour
which may be brought to fisheries and marine environmental history by this quantitative
approach.
Within modern marine biology, CPUE is routinely used to calculate stock abundances, and
the standardisation of fishing effort is a major research issue. The basic assumption underlying
many CPUE analyses is that the catch rate is proportional to the abundance of the fish stock.
This means that CPUE is linearly related to stock abundance: the higher the stock abundance, the
larger the CPUE. When a fish stock declines, CPUE follows suit. Accordingly, CPUE is a
relative index of stock abundance, and stock fluctuations can be monitored from time series of
CPUE.
The fish catches depend on fishing effort, fish density and probability of catch.
Mathematically, this is stated in the equation below:
C = qEN
Where C is catch, E is fishing effort, N is fish density, and q is the catchability coefficient. The
catchability coefficient is a measurement of the probability of catch. Effectively, the catchability
q is the proportion of the stock caught for a given fishing effort. Thus the catchability reflects the
efficiency of the fishing gear. The equation can be rearranged in order to isolate the catch rate:
C/E = qN
Accordingly, the equation shows the relationship between fish density, catchability and catch
rates.5
Walter Garstang, ‘The Impoverishment of the Sea – a critical summary of the experimental and statistical evidence
bearing upon the alleged depletion of the trawling grounds’ in Journal of the Marine Biological Association, no. 6,
1900, p. 1-69.
4
See Robinson and Haines op. cit. They base their analyses mainly on Garstang’s observations from 1900. See also
Poul Holm, Kystfolk: Kontakter og sammenhænge over Kattegat og Skagerrak ca. 1550-1914, 1991, p. 198-202 for
the calculation of catch rates in the Skagerrak mackerel fisheries, 1890-1914.
5
Maunder & Punt, p. 141-43; Hilborn & Walters, p. 176-7.
3
3
Ideally, catchability is stable over an extended period of time. Hence, the catch rate is
proportional to abundance and changes according to stock fluctuations. Catchability may also
change, however, due to gear innovation.6 If more efficient fishing gear is employed, the
catchability of the stock increases. Catchability will also change if the fishing fleet changes
fishing strategy.
‘Changes in q can occur when a fleet moves from one part of a stock to another
… or where a fishery is developing and moving into areas which have not been
previously fished.’7
Thus, spatial changes in the fishery can invalidate the assumption of proportionality between
catch rate and stock size. A group of Norwegian fisheries biologists also warn of such problems:
‘…if the fleet substantially expand the area of fishing due to catch rate
reductions, misleading estimates of stock sizes might be produced.’8
In this case, catch rates are maintained by the fishermen only because the fishermen have
expanded the area fished. Hence, if spatial expansions of fishing effort are not accounted for in
an assessment of stock size, they may conceal a major stock decline. Accordingly, spatial
changes within the fishery should be accounted for, when any historical CPUE data sets are
analysed.
A recent example of the problems with interpretation of CPUE data comes from the
Newfoundland cod fishery, which collapsed in the early 1990s. Before the collapse of the stock,
catch rates gave a misleading, overly positive impression of the state of the stock, because
changes in catchability were not taken into account. The fishermen maintained high catch rates
despite a significant decline of the stock. This was due to the fishermen’s improved ability to
locate the fish, or in technical terms, due to increased catchability. The fisheries biologists relied
too heavily on the catch rate data sets, which caused them to overestimate the stock size.9 Thus,
for fisheries biologists trying to estimate fish abundances, commercial data sets may cause
problems. Basically, the commercial data sets may be biased. Fisheries biologists Ray Hilborn
and Carl Walters put it this way:
6
Simon Jennings, Michel J. Kaiser & John D. Reynolds, Marine Fisheries Ecology, 2001, p. 86-8, 131.
ICES CM 2001/ACFM:23, p. 29.
8
Odd Aksel Bergstad & Nils Roar Hareide (ed.), Ling, blue ling and tusk of the north-east Atlantic, ’Fisken og
Havet’ no. 15, 1996, p. 88.
9
Jeffrey A. Hutchings & Ransom A. Myers, ‘What Can Be Learned from the Collapse of a Renewable Resource?
Atlantic Cod, Gadus morhua, of Newfoundland and Labrador’ in Canadian Journal of fisheries and aquatic
sciences, vol. 51, 1994, p. 2140-2.
7
4
‘One of the major problems with using commercial catch and effort data to
estimate stock distribution and abundance is that the fishermen go where the fish
are.’10
Catch and effort data are biased because fishing effort is not randomly distributed in the sea. The
fishermen concentrate their fishing effort where they know stock densities to be high.
Consequently, the data are not always a random sample of the fish stocks.
Figure 1 adopted from Hilborn and Walters shows three possible relationships between catch
rates and stock abundances. The linear model assumes proportionality between catch rate and
stock abundance. This is the ideal situation and occurs in fisheries where the search for fish is
random.
Figure 1. Hilborn and Walters: Three possible relationships between catch rates and stock abundance. 11
The other two models show non-linear relationships between CPUE and abundance. In the
case of hyperstability, catch rates do not decline as rapidly as the stock. If the catch rates are
taken at face value, hyperstability can result in an overestimation of the abundance of the stock,
as happened in the case of the Newfoundland cod fishery in the 1980s. For schooling fish, like
herring and mackerel, catch rates do not necessarily decrease in spite of declining stock
abundances. The densities of the fish schools remain high even if the stocks are depleted, so the
10
Ray Hilborn & Carl Walters, Quantitative Fisheries Stock Assessment: Choice, Dynamics and Uncertainty, 1992,
p. 162.
11
Hilborn & Walters, op. cit., p. 176.
5
fishermen can maintain high catch rates. Rather, the decline of the stock results in a decrease of
the total number of schools and a spatial contraction of the stock.12 This article, however, will
focus only on the CPUE approach in regard to demersal, i.e. bottom living, species such as cod,
haddock, ling and tusk, which do not observe a strong schooling pattern.
Hyperdepletion occurs if only part of a stock is vulnerable to fishing. As this part of the stock
is depleted, catch rates decline rapidly even though the overall stock size remains high.
Consequently, the catch rates overemphasize the decline of the stock. Hilborn and Walters
mention some invertebrates as examples of hyperdepletion.13
The problems caused by hyperstability and hyperdepletion are probably less serious in
historical than modern analyses, because fishermen’s search for fish was much more random
prior to the mid-twentieth century due to the absence of sonars and other fish-finding
instruments. Nevertheless, it is relevant to discuss how these problems are commonly dealt with
by fisheries biologists. Hilborn and Walters suggest that the principles of spatial stratification
should be applied to commercial catch and effort data,14 and that the data sets should distinguish
between the fishing grounds. By stratifying the fishing effort and catches by fishing ground,
spatial shifts within the fisheries can be detected. Consequently, the question ‘Where were the
fish caught?’ becomes highly important. Hence, historical catch rates should be spatially
disaggregated.
One final point should be added to the above observations. In a multi-species fishery, it is
important to know exactly which species the fishermen were targeting. If a change of target
species took place, a literal interpretation of time series of CPUE could lead to invalid
conclusions on stock abundance changes. Focusing on data sets from a modern fleet of longliners, a group of Norwegian fisheries biologists says:
‘If cod and haddock are target species and ling and tusk occur as bycatch,
consistently underestimates of CPUE of ling and tusk is expected.’15
In this particular case, only the catch-rate data sets for cod and haddock are useful for the
fisheries biologists. For the historian analysing historical CPUE data, it is equally important to
12
Hilborn & Walters, op. cit., p. 175-6; Pierre Fréon & Ole Arve Misund, Dynamics of pelagic fish distribution and
behaviour: Effects on fisheries and stock assessment, 1999, p. 174-250
13
Hilborn & Walters, op. cit., p. 176.
14
Hilborn & Walters, op. cit., p. 177-8.
15
Bergstad & Hareide, op. cit., p. 88.
6
identify the fishermen’s species preferences, before the CPUE data sets are analysed. Only
CPUE time series for target species should be used in the analysis of changing stock abundances.
Standardising fishing effort
CPUE is defined as the catch divided by a standardised unit of fishing effort. Potentially,
uncertainties pertain to both catch and effort data sets, but most difficulties stem from the effort
data. The basic challenge for the researcher is to standardise fishing effort, and this is a major
research topic within marine biology. Recently, a volume of the scientific journal Fisheries
Research was dedicated to this issue, addressing methodological problems in regard to CPUE
analyses.16 Stated simply, the question is how to convert the fishing effort from fleets of different
fishing vessels, equipped with different gears and engines, into a standardised unit of effort?
When time series of CPUE are used to monitor changing stock abundances, one should keep
track of changes of the fishing fleet’s catching power, and thus of any changes in catchability.
Information on fishing technology, i.e. the gear and vessels employed, are at the core of this
issue. Presently, the catching power of the European fishing fleets is estimated to increase by
several percent annually. In the nineteenth and early twentieth centuries, the annual changes may
have been smaller, but fisheries historians should keep such changes in mind, when analysing
long-term changes in stock abundances. Below is a brief list of some of the parameters that
should be taken into account when fishing effort is standardised.
The type and size of the fishing gear:

Number of long-lines per vessel

Length of long-lines

Number, size and shape of hooks

Bait types

Size of trawl

Size of nets

Mesh sizes
The attributes of the fishing vessels:
16

Tonnage

Crew size
Fisheries Research 70, 2004.
7

Horse power
The actual definition of CPUE depends on which specific fishery is analysed. For instance, in
the long-line fisheries fishing effort can be measured as the number of baited hooks put in the
water in a given time period. Accordingly, CPUE is defined as the catch per baited hook per time
period. In bottom trawl fisheries, fishing effort may be defined as the number of hours trawling
with a gear of a particular size (length of beam trawl and mesh size) by a vessel with a given
engine power.17
As part of the standardisation of fishing effort, time budgets for the fishing operations should
be established. The actual time spent fishing is an important parameter in the calculation of total
fishing effort. Fisheries biologists Hilborn and Walters distinguish between different phases of a
fishing trip. The time from the vessel leaves port until it returns to port is referred to as total
time.18 This is split into four phases as shown in the equation below:
Total time = Travel time + Search time + Setting time + Handling time
Travelling time is defined as the time spent sailing to and from the fishing grounds, whereas
search time is spent on finding the right spot to set the gear. Setting time is defined by Hilborn
and Walters as the time it takes the fishermen to set the gear. Finally, handling time refers to the
time the fishermen spent processing and curing the fish onboard the fishing vessels. Ideally, the
travel and handling time should be discarded from the calculation of fishing effort, because this
time does not reflect the actual fishing effort. On the other hand, search and setting times are
important to the calculation of the abundance of the stock, and as such they should be accounted
from in the estimate of the total units of fishing effort.
Ideally, time budgets should be established for each single fishing vessel, but historical data
sets seldom allow for such detailed analyses.19 If no such data are available, more rough
estimates should be examined. Are there any indications of changes in the operation of the
fisheries at sea? Does anybody – either the fisheries inspectors or the fishermen – say that more
or less time was spend fishing? If indeed such changes did take place, a literal interpretation of
catch rate data sets will produce invalid conclusions. Rather, the calculation of units of fishing
17
See Anders Fernö and Steiner Olsen (ed.), Marine Fish Behaviour in Capture and Abundance Estimation, 1994
for more information on standardization of fishing effort:
18
Hilborn & Walters, op. cit., p. 122-5.
19
The Dutch North Sea herring fisheries are a notable except. For these fisheries very detailed and consistent
sources are preserved. See Bo Poulsen, Historical Exploitation of North Sea herring stocks - an environmental
history of the Dutch herring fisheries, c. 1600-1860. PhD dissertation, University of Southern Denmark, 2005.
8
effort should account for changes in the time budgets. For example, a twenty percent increase in
the fishing season, will probably lead to a similar increase in fishing effort. For any historical
analysis, it is important to take the season length into account.
Defining CPUE for nineteenth century Swedish North Sea fisheries
Applying CPUE methodology to fisheries history, means asking questions hitherto considered
irrelevant to maritime history. Many questions are technical and not interesting per se, but they
are relevant as stepping stones towards the broader goal of understanding the dynamics of
fisheries. For historians examining the impact of changing fish stock abundances on the fisheries,
the crucial question is if catching power of the fishing fleet changed? Specifically, how much
time was spent fishing each year and on which particular grounds? How did technological
changes, such as the introduction of new hooks, smaller net meshes or stronger vessel engines,
influence the catching power of the fleet?
The following section will give an example of how to translate the above questions into a
historical research strategy. The example is from the highly important North Sea fisheries, which
expanded strongly during the latter half of the nineteenth and early twentieth centuries.
Specifically the example comes from the Swedish long-line fisheries for ling and cod, which
underwent an expansion both in spatial and technological terms during this time period. To
understand the dynamics of the expansion, a CPUE approach is very useful. In this regard, two
interesting questions require a quantitative analysis. Firstly, how did demersal stock abundances
fluctuate? Secondly, how did the fishermen react to such stock fluctuations? Or to rephrase the
two questions into one hypothesis: Did declining coastal stocks force the fishermen to sail farther
and introduce more efficient gear?
Swedish fisheries statistics were published annually from 1859 onwards, but information on
catches and fishing effort is available from at least the early 1840s. For the fifteen year period
between 1872 and 1886 CPUE data sets with spatial reference are available for the individual
fishing vessels. The structure of the data set in shown in Table 1. For the earlier and later periods
from the early 1840s to the outbreak of World War One, only less detailed data sets are
available. Nevertheless, these data sets are also useful for a CPUE analysis. Based on this
information, it is feasible to assess the fishermen’s long-term successes and failures, which is
relevant for the discussion of the driving forces of fisheries. In order to undertake such an
analysis, it is necessary to examine the changes in the catching power of the fleets. When was
9
new fishing technology introduced into the fisheries and how did that change the catchability? In
other words, catchability changes and spatial changes in the fishery should be identified before
the quantitative data sets can be properly used in a CPUE analysis. Fortunately, relevant
information both on fishing technology and spatial changes is available from many sources from
the 1840s to 1914.
Table 1. An example of Swedish catch and effort statistics from 1883.20
Fishing
Name of
Fishing
community vessel
Tonnage Crew ground Catches
Ling
Cod Tusk etc.
Mollösund Wilhelm
18.76
12 Jäderen
4,480
510
700
…
Catch
value
Landing
harbour
Göteborg &
5,810 Mollösund
The Swedish long-line fisheries for ling, cod and tusk took place at least from the sixteenth
centuries to the early 1950s and the last Swedish long-liner withdrew from the fishery only
recently.21 The Swedish fishermen were based in the county of Bohuslän, to the north of the city
of Göteborg. In the 1840s, fishing took place with sailing vessels in the Skagerrak and northeastern North Sea. Two types of sailing vessels were employed in the fishery – bankskutor and
sjöbåtar. A bankskuta had a crew of at least twelve men, and a keel of 14 metres. Each
bankskuta spent at least two to three weeks per trip, and often longer due to adverse weather
conditions.22 Curing took place ashore, and the vessels stayed at sea for so long that the catches
often deteriorated onboard. Frequently the catches were partly rotten when they were landed.
The bankskutor fished mainly in the north eastern part of the North Sea, i.e. to the south west of
Cape Lindesnäs, Norway.23
The second type of vessel was the sjöbåt. Originally, the sjöbåtar were undecked, a fact
which prevented the fishermen on these vessels from following the bankskutor to the grounds to
the south west of Lindesnäs. Accordingly, in the 1840s the sjöbåtar fished closer to Bohuslän, in
the Skagerrak.
An important distinction was made between between liggare and beseglare. When the
bankskutor arrived on the grounds, they anchored and the long-lines were set and retrieved from
20
Berättelse öfver Göteborgs och Bohus läns hafsfisken 1883
Olof Hasslöf, Svenska Västkustfiskarna: Studier i en yrkesgrupps näringsliv och sociala kultur, 1949; Lennart
Dalén, Den bohuslänska fiskelägesbygden, 1941.
22
G.U. Ekström, Praktisk Afhandling om lämpligaste sättet att fiska Sill, Torsk, Långa, Makrillm Hummer och
Ostron, 1845, p. 38, 43, 93-4. Axel Em. Holmberg, Bohusläns Historia och Beskrifning, 1845, vol. X, p. XX
23
Ekström, op. cit., p. 40.
21
10
dinghies. They were called liggare, a word which referred to the fact that the vessels anchored
during the fishing operations. The sjöbåtar on the other hand were called beseglare, because they
could keep their sails set during the fishing time, and the long-lines could be set from the
sjöbåtar.24
During the 1850s and 1860s, the bankskutor were replaced by large, decked sjöbåtar. Being
easier to manoeuvre, the sjöbåtar became the mainstay of the Swedish long-line fleet until the
early 1890s.25 The sjöbåtar could be used to set and retrieve the long-lines, thus partly rendering
the dinghies superfluous. Nevertheless, in some fishing villages the practice of setting and
retrieving the lines from dinghies was maintained.26
For the Swedish fisheries, the 1870s and 1880s were a period of technological stability, but
significant fleet changes took place during the 1890s. Swedish fishermen bought sailing smacks
from Britain which had become superfluous due to the introduction of steam ships in the British
fishing fleets. By 1903 87 sailing smacks were employed in the Swedish long-line fisheries, and
this was the majority of the fleet. The former British vessels enabled the Swedish fishermen to
expand the fishery to distant grounds off Shetland.27
To assess changing stock abundances it is important to know the catching power of the old
and new vessels. How efficient were the new vessels in comparison with old ones? Did they
increase catching power by 20% or 40%? Some rough estimates may be available from fisheries
inspectors’ reports. Indeed, in the Swedish case fisheries inspector Gerhard von Yhlen said that
the different fishing practices employed by beseglare and liggare did influence how much was
caught.28 Unfortunately, he did not specifically state how much. Neither did he compare the
catching power of the different vessels. This is unfortunate for the long-term estimates of
abundance changes in North Sea ling and cod from the 1840s to 1914. However, this problem is
not relevant for the assessment of abundance changes during the period 1872-86, for which very
detailed data sets are available. The main technological changes in the Swedish fisheries took
place during earlier and later periods.
24
Göteborgs och Bohusläns Hushållningssällskaps Qvartalsskrift juli 1868, p. 58.
Ekström, op. cit., p. 93-4.
26
Hasslöf, Bankskutor och sjöbåtar i Bohuslän, 1939, p. 14-5; Johan Hjort, Norges fiskeriet I: Norsk Havfiske,
1905, p. 222.
27
René Taudal Poulsen, An Environmental History of North Sea ling and cod fisheries, 1840-1914, PhD
dissertation, University of Southern Denmark, 2005.
28
Göteborgs och Bohusläns Hushållningssällskaps Qvartalsskrift Qvartalsskrift juli 1868, p. 58.
25
11
Fishing gear has an important bearing on the overall catching power of a fishing fleet.
Accordingly, changes in gear should be carefully examined. In the Swedish case, long-lines
remained the only important catching technology employed during the whole period. The
fishermen made a distinction between koljebackor and storbackor. The former were used to
catch bait for the latter. The fishermen used bivalves (musslor) as bait for the koljebackor.
Haddock caught with the koljebackor was used as bait for the storbackor.29 For the assessment of
fishing effort in the ling and cod fishery, only the fishery with the storbackor is relevant.
Usually, the storbackor were set during the evening or night and retrieved the following
morning or day. In the 1850s and 1860s, the setting and retrieval of the long-lines took place
from dinghies, while the fishing vessels anchored. One fisherman estimated that it could take ten
hours to retrieve the lines.30 The men who stayed onboard the vessel spent time baiting hooks for
the next set.31 The time spent fishing depended on the season. In the summer, the long-lines
would sometimes be in the water for only three hours, while they would stay in the sea the whole
night during the spring and fall.32
While the basic gear was stable during the period 1840-1914, the number of hooks and
accordingly the fleets’ catching power may have increased. Evidence of such a change is not
available, however. In fact, very little information is preserved regarding the number of hooks
employed in the fishery. Lack of information on the number of hooks should not necessarily be
interpreted as proof that no such increase took place. A gradual increase or for that matter a
decrease could have taken place, without anybody writing about it. The lack of information on
the number of hooks employed in the fishery increases the uncertainty of the conclusions drawn
from the CPUE data sets.
In conclusion, fishing technology changed significantly in the Swedish long-line fisheries
twice between 1840 and 1914. These changes took place in the 1850-60s and 1890s and must
have increased the catching power of the fleet, and thus the catchability. Given the sources at
hand it is impossible to estimate the changes quantitatively. For the assessment of the detailed
data sets from 1872-86 the technological changes are not relevant, however, since they took
place at an earlier and later time. The years 1872-86 were a period of technological stability.
Göteborg museums’ archive, Göteborg (GMA): GMA 5233:9; GMA 431:1-2.
GMA 431:3.
31
GMA 2797:3; GMA 431:3; GMA 5233:5-9; GMA 2064:1; GMA 443:3.
32
GMA 2153:3.
29
30
12
Hyperstability is a common problem in regard to CPUE time series. Any historian analysing
stock fluctuations from CPUE data sets should doubt the assumption of linearity between catch
rates and stock abundances. Indeed, it is crucial to address this premise when CPUE time series
are used to assess stock fluctuations. In this regard, relevant information is often available in
publications by marine scientists, such as the annual work group reports from the International
Council for the Exploration of the Sea (ICES). This is also the case for long-line fisheries for
North Sea ling and tusk.33 Marine biologists presently use long-line data sets for the monitoring
of ling and tusk fluctuations. A recent study by the Norwegian fisheries research institute on the
biology of the deep sea stocks of ling and tusk concluded that catch rates from long-liners are a
useful index of stock abundance.34 Similarly, the International Council for the Exploration of the
Sea (ICES) working group on the assessment of deep-sea species recently concluded that
abundance changes can be inferred from time series of fishing effort and catch rates.35
Furthermore, information from nineteenth century Swedish fishermen indicates that the search
for fish was much more random than it is today. The Swedes did not have modern fish finding
equipment at hand and the fishing grounds were located by the Swedes by means of lead lines
with blobs of tallow to retrieve bottom samples.36
How can CPUE be defined in the Swedish long-line fishery? Ideally, CPUE should be
defined as the catch per number of baited hooks set per given time period, but this is impossible
in the Swedish case. A coarser substitute – the annual catch per fisherman – is suggested. While
the annual catch per man is a far cry from modern definitions of CPUE, the feasibility of this
definition depends on the questions asked by the historians. Firstly, does the annual catch per
man reveal the successes and failures of the fishermen? Secondly, does the annual catch per man
properly reflect changing stock abundances?
The answer to the first question is definitely affirmative. The annual catch per man clearly
shows the performance of the fishermen regardless of changes in fishing technology. Indeed, the
concerns about catchability changes are irrelevant for the analysis of the successes and failures
of the fishery and the annual catch per man can be used as a proxy for fishermen’s performance
in any historical fishery.
33
ICES WGDEEP, www.ices.dk
Bergstad & Hareide, op. cit., p. 106.
35
ICES CM 2000/ACFM:8, p. 2, 11; ICES CM 2004/ACFM:15, p. 2-3, 48-51, 57.
36
Ekström, op. cit., p. 33.
34
13
The answer to the second question is conditional. It depends on the fishing technology
employed. The annual catch per man only reflects changes in stock abundances, during periods
with stable fishing technology. In the Swedish long-line fishery, this was the case between 1872
and 1886. Accordingly, the annual catch per man is a feasible proxy for stock abundances of ling
and cod during the 1870s and 1880s. When catching power and catchability changed, as it did in
the 1850s, 1860s and 1890s, the annual catch per man is not a proper index of stock abundance.
The Swedish fishermen of the 1890s were definitely more efficient than their predecessors had
been in the 1850s.
Time budgets for the nineteenth century Swedish fisheries
The above definition of CPUE assumes that the time spent fishing was consistent over time.
Before the annual catch per man is accepted as a proper index of stock abundance, it is crucial to
ask if the time spent fishing – i.e. search time + setting time – changed. Any changes in search or
setting time or season length will influence the fishing effort, and should be accounted for in the
CPUE analysis. For the Swedish long-line fisheries detailed time budgets are impossible to
calculate. However, more rough indications of the time spent fishing can be inferred from many
sources.
The time spent by the vessels travelling to and from the fishing grounds should be
disregarded in the calculation of fishing effort. The sailing vessels depended on favourable
weather, and considerable time could be used on trips to and from the grounds. In one instance,
referred to by Swedish etnographer Olof Hasslöf, the fishermen used a dinghy to tow the fishing
vessel most of the way from the Skagerrak to Bohuslän, which was a considerable distance. With
no wind and catches deteriorating in the hull of the vessel, the fishermen had no other choice.37
The travelling time depended on the location of the home port, the fishing ground and the
landing port. For the Swedish North Sea and Skagerrak vessels it mattered little from where in
Bohuslän they came. The distances to the fishing grounds from any of the coastal communities
in Bohuslän were almost the same. In contrast, travelling time from the fishing grounds to the
potential markets in Norway, Great Britain and Sweden differed considerably. It is thus highly
relevant to know where the vessels landed their catches. The Swedish fisheries inspectors’
annual reports provide information on landing harbours. Between 1872 and 1886 each vessel’s
37
Hasslöf, 1949, op. cit., p. 328.
14
landing port was stated specifically and after 1886 the landing harbours were stated for each
village’s fleet.
To the nineteenth century Swedish fishermen, travelling time was also an important issue. In
1863 Swedish fisheries inspector Uggla emphasized:
‘... the advantage of the fish market adjacent to the rich fishing bank, in particular
the Storeggen off Norway’s north west coast, where the most attractive fishery
takes place. It saves the time-consuming trips with the catches during the fishing
season. ...’38
Hence, proximity to the fish market was an important factor and in part explains the rise of the
Swedish fishery in the 1860s on the Storeggen ground several hundred miles away from
Bohuslän, off the western coast of Norway. The Storeggen fishermen mainly landed their fish in
Ålesund, Norway, where they were paid the same prices for their catches as they were at home.39
Also the North Sea fishermen started landing fresh fish in Norway in the mid-1860s.40 Fisheries
inspector von Yhlen estimated that ca. 38 percent of the total catches were landed in Norway in
1867.41 Also in the 1870s large quantities of fish caught in the north eastern North Sea were
landed in Norway.42 In 1876 von Yhlen estimated that at least 25 percent of the total catches
were landed in Ålesund or other Norwegian harbours.43 The fishermen were relatively flexible as
regards choice of landing harbour. In 1880 von Yhlen said that some catches from the north
eastern North Sea were sold in Norway, but most were landed in Bohuslän.44 In contrast, many
catches were landed in Norway the following year.45
The archives of the fishing company Göteborgs och Bohus Läns AB för Storsjöfiske also
illuminate this question. In the late 1850s, the company attempted to increase the catching power
of the fishing fleet by introducing a system of carriers that could transport fish from the fishing
vessels on the grounds to the ports. Hence, the fishing vessels could stay longer on the grounds.
For the fishing season from March to September 1857, it was estimated that each boat undertook
38
Göteborgs och Bohusläns Hushållningssällskaps Handlingar 1863, p. 15.
Göteborgs och Bohusläns Hushållningssällskaps Handlingar 1863, p. 49; Göteborgs och Bohusläns
Hushållningssällskaps Handlingar 1864, p. 107.
40
Göteborgs och Bohusläns Hushållningssällskaps Handlingar 1865, p. 1; Göteborgs och Bohusläns
Hushållningssällskaps Qvartalsskrift juli 1868, p. 60; Göteborgs och Bohusläns Hushållningssällskaps
Qvartalsskrift juli 1872, p. 46.
41
Göteborgs och Bohusläns Hushållningssällskaps Qvartalsskrift juli 1868, p. 61.
42
Berättelse öfver Bohusläns hafsfiske 1872, p. 2.
43
Berättelse öfver Göteborgs och Bohus läns hafsfisken 1876, p. 2.
44
Berättelse öfver Göteborgs och Bohus läns hafsfisken 1880, p. 1.
45
Berättelse öfver Göteborgs och Bohus läns hafsfisken 1881, p. 1.
39
15
7 trips, most of which were destined to the fishing grounds in the north-eastern North Sea.
Around 8 days were spent fishing on each trip. Hence, only 2 out of 6.5 months were actually
spent on the fishing grounds. During the other time the boats sailed back and forth between the
grounds and the ports.46
This information is confirmed by fisheries inspector Uggla in the first annual report from
1859. Since curing of the fish took place only ashore, the quality of the fish was very low.
According to Uggla, the low quality was partly due a spatial expansion of the fishery during the
1850s, which involved more travelling time. Uggla advised improvements of the handling of the
fish, and commented favourably on a new carrier system introduced by the fishing company
Göteborgs Aktiebolag för Storsjöfiske. The fishing season was from March to mid-September,
and Uggla remarked the waste of time sailing to and from the fishing grounds. Uggla estimated
that two thirds of the season was wasted for that purpose. Uggla also estimated that on average
the fisherman spent 10-14 days on a trip home from the north-eastern part of the North Sea and
vice versa. Due to adverse weather, trips could even take much longer.47
The attempts at establishing a carrier system were unsuccessful, and travelling time remained
high. The issue reappeared in an article in the fisheries journal Bohuslänsk Fiskeritidsskrift in the
mid 1880s. Its author, M. Rubenson, noted that large parts of the fishing season from March to
the end of August or early September were wasted due to the long travelling times. Only 25 to 30
days were spent fishing, he said.48
In the fisheries inspector’s annual reports from the 1890s and 1900s estimates were made of
the number of trips by fishing ground. Figure 1 shows the minimum and maximum number of
trips undertaken per vessel per year from 1890 to 1912. The frequency of trips depended on
which fishing grounds were visited. The closer to home fishing took place, the more trips were
undertaken. One vessel fishing in the Skagerrak could make up to twenty trips per year, whereas
the Shetland vessels usually made between two and five. Except for an increase in the minimum
number of trips per year to the Skagerrak grounds, no major time trends are evident from the
figure. The data in Figure 1 can also be compared to information from fisheries inspector
Gerhard von Yhlen, dating from 1880. In 1880 fisheries were still taking place on the Storeggen
ground. The vessels fishing here made only one trip between their home and the grounds per
Landsarkivet i Göteborg (GLA), A 274, Göteborgs och Bohus Läns AB för Storsjöfiske, ‘Anteckningar om Fisket
i Bohuslän samlad av C.B.’
47
Göteborgs och Bohusläns Hushållningssällskaps Handlingar 1859, p. 5-6.
48
Bohuslänsk Fiskeritidsskrift, II, 1885, p. 358.
46
16
year, since catches were landed in nearby Ålesund. The vessels in the north-eastern North Sea
made four or five trips a year, partly landing their catches in Norway and partly in Sweden. 49
This is in agreement with the data in Figure 1. Thus, no marked changes in the frequency of trips
seemed to take place from 1880 to the 1910s.
25
20
Shetland Lowest
Shetland Highest
Skagerrak Lowest
Skagerrak Highest
North eastern North Sea Lowest
North eastern North Sea Highest
15
10
5
0
1890
1892
1894
1896
1898
1900
1902
1904
1906
1908
1910
1912
Figure 1. Minimum and maximum number of trips per vessel per fishing season, by fishing ground.50
As described earlier, the Swedish long-line fishermen usually brought with them bivalves to
use as bait for the small long-lines, the koljebackor. The koljebackor were used mainly to catch
haddock which were cut to pieces and used as bait for the large long-lines, the storbackor. The
time spent with the koljebackor should be disregarded in the catch rate calculations for the main
target species ling and cod. However, it is impossible to estimate how much time was devoted to
the setting of the gear. By assuming that this parameter was stable, the problem can be
disregarded however. If there were no changes in the setting time, this aspect did not influence
the catch rates significantly.
49
Gerhard von Yhlen, Die Seefischerei an der Westküste Schwedens, 1880, p. 39-40.
Berättelse öfver Göteborgs och Bohus läns hafsfisken, 1890-1904/05; Berättelse öfver Göteborgs och Bohus läns
fisken, 1904/05 -1905/06; Berättelse öfver Göteborgs och Bohus läns fisken, 1906/07-1912/13; Göteborgs och
Bohusläns Hushållningssällskaps Publikationer 1912, IIIA.
50
17
The fourth parameter in Hilborn and Walters’ equation is handling time. This concept refers
to the onboard handling of the catches. If curing took place onboard the vessels, less time could
be spent on the actual catching operations. Onboard curing of the fish meant higher quality of the
fish, since the deterioration of the catches was halted. In the long term from 1840 to 1914, an
increase in the practise of onboard curing took place, but this development was by no means
linear. Since handling time did change during the period, the total time spent fishing must have
changed, thus influencing the total fishing effort. This time trend is highly relevant to the
standardisation of fishing effort. The question is how much it influences the catch rate
calculations. To answer this question, an examination of the evidence regarding onboard curing
practices is required.
To contemporaries the issue of handling time vs. setting time was a real one. On the one
hand, onboard curing took time which the fishermen could have used fishing instead. On the
other hand, the vessels that landed the catches uncured had to travel more frequently between
their home ports and the fishing grounds in order to avoid deteriorating catches.51 Uggla, the
fisheries inspector, estimated that on balance onboard curing would leave the fishermen more
fishing time, since they did not have to undertake the long trips between port and the grounds so
often.52 Based on this information, however, it is difficult to estimate precisely the time spent
fishing. The discussion of the onboard curing issue appeared frequently in the fisheries
inspector’s reports, and the inspectors advocated a change of the handling practices.
One of the main goals of the fishing company Göteborgs och Bohus Läns AB för
Storsjöfiske was to inspire the Bohuslän fishermen to onboard curing of catches. By 1860, the
goal had still not been achieved.53 In 1863, Uggla, however, was satisfied that salting was
increasingly being practiced onboard the long-line vessels. While 7 vessels had done so in 1862,
the number had increased to 22 the following year. Uggla noted that the Bohuslän fishermen
were beginning to imitate the new practices of onboard curing.54 However, the practice of
onboard curing was on the decline by the mid-1860s. By that time Göteborgs och Bohus Läns
AB för Storsjöfiske and one other fishing company which was set up to improve the handling
practices had failed. Prices paid in the mid 1860s were better for fish salted ashore than they
51
Göteborgs och Bohusläns Hushållningssällskaps Handlingar 1863, p. 52.
Göteborgs och Bohusläns Hushållningssällskaps Handlingar 1864, p. 106.
53
Göteborgs och Bohusläns Hushållningssällskaps Handlingar 1860, p. 47.
54
Göteborgs och Bohusläns Hushållningssällskaps Handlingar 1863, p. 15, 46, 48.
52
18
were for fish cured onboard. The market preferences had not changed in favour of the fish cured
onboard.55
In the late 1860s von Yhlen reported that the fishermen increasingly brought with them salt
for onboard curing.56 During the early 1870s von Yhlen’s annual published reports distinguished
between fish cured onboard and fresh landings. Unfortunately, the data set covers a very short
period and no major trends can be seen from it. In 1880, von Yhlen said that the vessels fishing
in the Skagerrak did not salt their fish at sea.57 This was confirmed by an article in the journal
Bohusläns Fiskeritidsskrift from 1885.58
The issue reappeared in fisheries inspector Malm’s reports from the late 1880s.59 One year he
advised the fishermen to initiate a system to reward those fishermen who salted the fish at sea.60
From this proposal it can be inferred that the practice of onboard curing was not common at the
time. However, the issue was not discussed in the fisheries inspectors’ reports from the 1890s
onwards which indicates that gradually onboard curing was being accepted by the fishermen and
the market. This development is also supported by other evidence. When fisheries historians
Haneson and Rencke published a book on the history of the Bohuslän fisheries in 1923, they said
that the long-line fishermen returned to port with only the most recent catches uncured. Most of
the catches were cured at sea.61 This was also confirmed by the Johan Hjort, a leading
Norwegian fisheries biologist, writing about the Swedish long-line fisheries in 1905.62
In conclusion, the long-term trend reveals an increase in onboard curing practices. The
development was not linear, however. While it was increasingly accepted in the early 1860s, the
practise soon declined. Still in the 1880s onboard curing was uncommon, but the issue gradually
disappeared from the fisheries inspectors’ agendas, which indicates that it had become more
common by the turn of the century. The most detailed data set on catch rates covers the period
1872-86 and there is no indication that a major change in curing practices took place then. It is
fair, therefore, to assume that the handling time was constant during these years.
55
Göteborgs och Bohusläns Hushållningssällskaps Handlingar 1864, p. 109; Göteborgs och Bohusläns
Hushållningssällskaps Handlingar 1865, p. 18 & Bil 1, p. 1-2; Göteborg och Bohusläns Hushållningssällskaps
Qvartalsskrift, juli 1867, p. 13, 46.
56
Göteborg och Bohusläns Hushållningssällskaps Qvartalsskrift juli 1869, p. 6-7.
57
Von Yhlen, op. cit., p. 35.
58
Bohuslänsk Fiskeritidsskrift, II, 1885, p. 358.
59
Berättelse öfver Göteborgs och Bohus läns hafsfisken 1886-87, p. 5.
60
Berättelse öfver Göteborgs och Bohus läns hafsfisken 1888-89, p. 5.
61
Valdemar Haneson och Karl Rencke, Bohusfisket, 1923, p. 291.
62
Hjort, op. cit., p. 221..
19
An additional parameter to consider in the time budget is the season length. A lengthening of
the fishing season would cause the total fishing effort to increase. On the other hand, there is a
risk of overestimating fishing effort if a shortened fishing season is not accounted for.
In 1845 the fishing season started in March or April depending on the weather and ended in
September.63 The fisheries inspectors’ reports from the 1860s and 1870s did not mention the
fishing season, and this is indicative of a stable season length. Unfortunately, fisheries inspectors
von Yhlen’s and Malm’s annual reports are not informative with regard to season length between
1872 and 1886, which is the period with the most detailed data set. The lack of information on
season length between 1872 and 1886 may be interpreted as an indication of stability. It is
reasonable to assume an overall stable season length between 1872 and 1886. In 1880, von
Yhlen said that the long-line fishing season was from March or April to September.64 This is in
agreement with the earlier information.
In the 1860s and 1870s, the distant fishery for cod on the Storeggen ground off Ålesund had
a longer fishing season than the other long-line fisheries. In order to make it to the ground in
February, the fishermen left home earlier than the North Sea and Skagerrak fishermen. In 1880
von Yhlen estimated that the skrejd (very large cod) fishermen’s seasons were one to two months
longer than other long-line fishermen’s seasons.65 When the Swedish fishermen left Storeggen
for good in the early 1880s, they postponed the first fishing trip of the year to the spring like the
other Swedish fishermen.66
In the late 1880s the mackerel sector of the Bohuslän fishing industry expanded. Taking
place in the Skagerrak and the North Sea, the mackerel fisheries were typically pursued by longline fishermen using their traditional vessels. From 1887 this group of fishermen shortened the
long-line season by shifting to the mackerel fishery in early August and September.67 In
conclusion, the most important change was the shortening of the long-line season from 1887
onwards due to the initiation of the mackerel fisheries.
What are the implications for the standardisation of fishing effort? One long-line fisherman’s
work in one fishing season starting in March and ending in late August or early September is
equalled to one unit of effort. This measurement has a number of uncertainties attached to it. At
63
Ekström, op. cit., p. 35, 39.
Von Yhlen, op. cit., p. 34.
65
Berättelse öfver Göteborgs och Bohus läns hafsfisken 1880, p. 1.
66
Berättelse öfver Göteborgs och Bohus läns hafsfisken 1888-89, p. 3.
67
GLA, Göteborgs och Bohus Läns Länsstyrelse, Landskansliet, DVb:16, Malm, Femårsberättelse 1886-90. The
same conclusion in Femårsberättelse 1891-95; Berättelse 1888-89, p. 2; Berättelse 1892-93, p. 3.
64
20
least two parameters relating to the handling time and the season length changed during the
period from 1840 to 1914.
With regard to the handling time, the fishermen gradually converted to the curing of catches
at sea. This took time which could otherwise have been spent fishing. On the other hand, the
fishermen could make less frequent trips between the grounds and the ports since catches did not
deteriorate quickly after curing. It is impossible to estimate exactly how this development
influenced the catching power of the individual fishermen. It is certain, however, that the
development was not a linear one.
Obviously, the above changes should be kept in mind when the long-term trends in catch
rates are interpreted. They add uncertainty to the analysis. With regard to the more detailed data
set for the period 1872-86, it is safe to assume that no significant changes took place. Therefore,
the catch rates between 1872 and 1886 are immediately comparable. In conclusion, CPUE is
defined as annual catch per fishermen and one year is defined as 5 or 6 months fishing season,
from March to August or early September.
From 1872 to 1886 CPUE for ling and cod fluctuated significantly and on a decadal scale
catch rate changes played an important role in regard to the fishermen’s spatial shifts. Thus,
ecology was an important driver of change in the development of the Swedish fishery. Based on
the historical CPUE data set we were able to calculate total abundances of ling and cod in the
Skagerrak and north eastern North Sea in 1872. Ling abundances exceeded cod abundances
significantly at the time. This differs from the modern state of the ecosystem, and shows that
major shifts have taken place in the North Sea ecosystem on a centennial scale.68
Conclusion
Being top-predators in the oceans, fishermen have always depended on the abundance of prey.
Changes in the marine environment, and in particular fish stock fluctuations, have an important
bearing on the development of fisheries. In any analysis of the dynamics of fisheries, ecological
aspects should be taken into account and CPUE is a powerful means to that end.
This article proposed a CPUE approach for the analysis of two specific but related questions.
Firstly, CPUE is a relative index of stock abundance, and time series of CPUE may reveal
changing stock abundances many centuries back in time. Therefore, the CPUE approach may be
René Taudal Poulsen, Andrew B. Cooper, Brian MacKenzie & Poul Holm, ’The importance of historical baselines
to fisheries management - An abundance estimate of ling (Molva molva) and cod (Gadus morhua) in the Skagerrak
and the northeastern North Sea, 1872’ submitted to Fisheries Research.
68
21
used to identify historical changes in the ecological basis of the fisheries. This approach offers
new insights into historical development, by asking to what extent changes in fisheries were
caused by ecological changes. Secondly, CPUE is a measurement of the fishermen’s successes
and failures. By calculating the annual catch of fish per man a simple, but powerful tool is made
available both for temporal and spatial comparisons of fishermen’s performance. Did fisheries
decline because of declining catch rates? Did the fishermen introduce more efficient gear
because catch rates declined? For instance, calculations of the annual catch per fisherman will
now allow for a comparison of the productivity of North Sea cod fisheries from the sixteenth
century to the twentieth century. Such comparison has never been attempted by fisheries
historians before.
The calculation of the annual catch per man is straightforward and does not pose any major
challenges to fisheries historians. It is strong proxy for the success and failure of the fisheries.
Data sets on the catch, the location of the fishery, the number of men employed and the season
length are all that is needed for such an analysis.
The assessment of changing stock abundances requires detailed data sets, which may only be
available for some historical fisheries. For such an assessment very detailed data sets and a
thorough analysis of fishing power and catchability changes are required. In the Swedish case
study presented in this article this analysis is only feasible for the period 1872-86.
An interdisciplinary study of the dynamics of fisheries, combining history and marine
science, is no easy task. Indeed, such a study poses new challenges to historians both in terms of
methodology and data quality. However, the application of methodology from marine science
within an historical analysis offers encouraging possibilities for new, stimulating insights on the
driving forces of history. The interdisciplinary challenges encountered by historians are well
worthwhile overcoming. In fact, CPUE is a powerful tool, which will increase fisheries and
marine environmental historians’ understanding of the underlying causes of the fluctuations of
fisheries, and ultimately allow for a better understanding of how human societies have adapted to
ecological changes.
22