APPLYING CPUE AS MANAGEMENT TOOL
by
Luc van Hoof and Pavel Salz
Agricultural Economics Research Institute LEI
Fisheries Division
Burg. Patijnlaan 19
The Hague
The Netherlands
1
Discussion paper: Applying CPUE as management tool
ABSTRACT
The catch per unit of fishing effort (CPUE) is a much used ecological measure for the density of
stock. At the same time it is a proper indicator for the state of affairs in the fishery being a
measure for physical and financial productivity: gross proceeds are related to the catch and cost
of production are related to the fishing effort.
This discussion papers examines the potential for the CPUE as a relatively easy to establish
management indicator which links fishing effort and stock levels. The paper examines the
applicability of CPUE as a management tool among others by focussing on the several
composing parts of the indicator.
A sample of Dutch beam trawler data for flatfish species (e.g. sole and plaice) for the 1992-1999
period is used as a reference for the CPUE measure. In the final analyses it will be examined
whether CPUE can be used as a ‘real time’ indicator useful to both fishers and policy makers.
1. Introduction
The Catch per Unit of Effort (CPUE) is already a standard tool among biologists to determine
developments in fish stocks and among economists as an indicator for the efficiency of the fishing
operation. The relative simplicity of the measure renders it a potential tool for continuous fisheries
operation monitoring.
As point of departure we will take a simple fishery model in which the realised catch can be
defined as:
C=fqB
(1)
In which:
C = Catch
f = effort
q = catchability coefficient
B = stock biomass1
Catch can be measured in numbers of fish caught or in the yield by weight. Effort can be
measured in for example size of the fleet, or time spent at sea fishing with a particular gear. The
dimensions of the units will be further examined in section 3.
Using (1) the catch per unit of fishing effort (CPUE) in its basic form can be defined as the total
catch divided by the total fishing effort in a given period, or in formula2.
U = C/f
(2)
In which:
U = Catch per unit effort
For example the catch expressed in kg fish, the effort as sea time and the CPUE expressed in kg
per unit of time spent at sea.
Also the CPUE can be related to the stock biomass:
U =qB
(3)
1
B refers to biomass in kg or tons. The also often applied quantity N refers to numbers. Yield by weight can be found
using Y =  N with Y being yield by weight and  being weight coefficient.
2
Cf. Gulland (1983)
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Discussion paper: Applying CPUE as management tool
Several definitions can be used for both the catch and the effort depending on the requirements
of the analysis. For example the catch can be specified as catch in kg or as proceeds in monetary
units. The effort can be defined for example as time spent fishing times engine capacity (Hp *
days fished) or energy usage per time unit (for example litres of fuel per hour) .
The CPUE can be used as both a status indicator as well as a process indicator. As state
indicator the CPUE (e.g. amount of kg of sole caught per hour fishing) is used as an absolute
measure.
Ū=qN
(4)
As a process indicator CPUE is used as a relative measure to determine changes over time.
C t/f t= α(C t+1/f t+1)
(5)
With t and t+1 indicating consecutive time periods and α being an indicator of change related to
the stock development:
q tN t = α (q t+1N t+1)
(6)
If we assume the catchability coefficient to be constant over time than CPUE development is
directly related to stock development.
2. Context of CPUE
According to a.o. Hilborn and Walters:
‘Increasing catches during fishery development are generally accompanied by (1) increase in fish density in areas were
fishing effort is initially concentrated, (2) movement of effort into less preferred fishing areas, and eventually(3) a decrease
in indices of fishing success, such as the catch per unit of fishing effort.’
A healthy level of stock abundance (going hand in hand with a level of stock density 3) can be
related to a specific level of the CPUE.4 For policymakers in first instance a policy
recommendation could be that a certain fishery stays open as long as the realised CPUE value
remains above this predetermined value. As soon as the CPUE falls below this level the fishery
(area) is partly or completely closed. 5
In order to establish this, a reliable image of CPUE and its development needs to be established.
Concerning the catch in the Netherlands, logbooks of the commercial fleet provide reliable data
on the amount of fish by species and market grade landed after a trip. In order to keep the fishery
open the fleet has an interest to keep the CPUE above the established level and hence to land as
much of the caught fish as possible.
To establish a proper image of the effort is somewhat more complicated. Firstly CPUE’s differ
depending on the ship size and fishing method applied. Secondly the unit of time measurement
(e.g. days at sea or actual time spent fishing) needs to be established.
3
Fish density Dt = Nt A-1 With Dt the number (or weight) of fish per unit of surface. Substituting in (4) results in C = f q D A
or the catch realised depends on the fishing effort, the gear and species specific catchability coefficient, the density of the
fish and the area fished.
4
The desired level of stock abundance can be represented by a range of values around a central desired stock level. The
population density will vary according to species, area and season.
5
This method can be compared to the Norwegian system of closing areas to reduce discards of juvenile fish. (Cf.
Buisman et al, 2000.)
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Discussion paper: Applying CPUE as management tool
For the ‘active’ forms of fishing (e.g. trawl and beam trawl) a proper time measurement could be
the total time (hours) spent at sea. This measure can be easily established by way of registration
of time of departure and arrival. Steaming to the fishing grounds and time spent searching for fish
are activities belonging to the totality of the fishing effort. More time spent on either of this
activities indicate a lesser stock abundance or at least lesser stock abundance in nearby fishing
areas.
A suitable relation between the stock abundance and the CPUE has to be further refined
considering factors such as spatial distribution of the stock (and hence spatial differentiation of
the CPUE) and catch composition in terms of age classes.
3. Merit of CPUE indicators
As discussed above, several definitions of specific CPUE indicators can be used. The merit of the
specific indicators can be determined by the usefulness of the composition of the indicator in
terms of measure of catch and measure of fishing effort.
Catch in kg is of course a very relevant measure for the catch recognised both by the industry
and by policymakers. However, a distinction needs to be made between the actual catch realised
and the amount of catch landed. The actual landings have a bearing on the economy,
determining the proceeds of the operation as well as the price level in the market. On the other
hand the actual catch (including discards) is important for the biological analysis of stock
abundance.
Proceeds in monetary terms is a relevant economical measure for the catch. However, proceeds
depend on prices and quantities and are therefore not directly related to the actual biomass.
Partly price effects (inflation) can be removed by using constant prices.
In addition prices can be used as weights in aggregating multiple species catches. Using fixed
prices to sum catches composed of several species will reflect the relative (economic) importance
of the individual species. By using fixed prices price effects are eliminated.
Using prices provides interesting opportunities for stock assessment. Assume for a basic year
prices for market grades of a species are known as well as the size composition of the landings
and of the stock. By relating age classes to market grades and using the given (fixed) prices it is
possible to determine the biomass in tonnes and to establish the monetary value of the stock.
The prices are used as weighing coefficients per age class to establish the total stock value. By
applying the constant prices to alternating stock compositions one can establish a time series of a
single indicator providing a measure of the qualitative (economical) development of the stock.
In addition it can be mentioned that next to current market prices also economical (shadow)
prices can be used. The difference between actual prices and shadow prices can be interpreted
as an indicator for the degree of market failure.
Hp-days are a common indicator for the fishing effort in the beam trawl operation. However, some
remarks can be made as to the accuracy of the measure:
 The official registration of the engine capacity is not always reliable. Measuring Hp is difficult
and the actual application of the engine capacity (e.g. full throttle) during the fishing operation
is not known.
 Registration of the days at sea should be measured in hours to avoid definition problems
between calendar days and periods of 24-hours.
 Empirical research shows that the average productivity per Hp-day declines with increasing
engine capacity. This being congruent with the law of diminishing marginal returns.
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Discussion paper: Applying CPUE as management tool
Measuring effort in terms of hours actually fished (given engine power and fishing gear used)
seems to be a more objective measure circumventing the problem of registration of days spent at
sea and the part thereof spent steaming and searching for fish. However data on actual time
spent fishing are generally not available. In addition, time spent steaming to the fishing grounds
and time spent searching for fish are activities directly related to the effort involved in obtaining a
specific catch.
Costs per time unit as indicator for effort is a relevant economic indicator. In contrast to physical
effort measures (Hp-days, hours fished) costs can account for the quality aspects of inputs. Also
shadow prices can be used to compare actual and desired input deployment. Question is, which
cost categories to be included in the measure, distinguishing between:
 Proceeds dependent costs such as crew remuneration;
 Effort independent costs such as capital costs (depreciation and interest);
 Effort dependent costs such as fuel, gear and maintenance.
 Catch dependent costs such as costs of landings such as auction costs and costs of
unloading.
Costs independent of the level of activity do not qualify as effort indicator. Also result oriented
costs are not eligible. Hence, the most appropriate effort related costs indicator are the data on
fuel, gear and maintenance.
A last measure can be energy usage per unit of time. Both the actual fuel use as well as the catch
can be expressed in terms of energy. For example both catch and amount of fuel used can in
principle be expressed in their relative energy contents in joule. Although a proper conceptual
approach the practical implication renders this measure quite impractical.
4. CPUE and the Dutch beam trawl fisheries
In the Netherlands a systematic survey of costs and earnings of fishing vessels were started in
1948. The studies are implemented by the department of Fisheries of the Agricultural Economics
Research Institute. The figures are obtained from voluntary participation by vessel owners; LEI
visits vessel owners and their accountants to collect data directly from the accounts.
At the moment the following areas are covered :
- economic results of the cutter fisheries
- economic results of the mussel culture
- financial position of fishing enterprises
The cutter fisheries research is based on the costs and earnings figures of some 120 vessels,
ranging in size from some 80 to 4000 Hp, out of a total fleet of 400 vessels in 2000. The sample
is divided into twelve Hp-groups and into two geographical areas: North of the port of
Scheveningen (including the port of Urk on Lake IJsselmeer) and south. These groups include
the beam-trawl fishery, (pair)trawling on cod, whiting and herring and the shrimp fisheries.
For the period 1992 – 1999 a group of vessels has been selected that had beam-trawling for
flatfish as main activity. Data on fishing effort (Hp * sea days) catch of sole and plaice and costs
(represented by the level of fuel costs) were used for different CPUE assessments. Stock
abundance for sole and plaice was measured in total biomass based on ICES data.
In figures 1 and 2 the development of stock and catch per unit of effort in kg/hp day is presented.
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Discussion paper: Applying CPUE as management tool
Figure 1: Sole stock and CPUE development 1992-1999 Beam trawl
120
100
80
60
40
20
0
0,3
0,25
0,2
0,15
0,1
0,05
0
kg/hp-day
Kt
Sole stock and CPUE
1992 1993 1994 1995 1996 1997 1998 1999
Sole stock Kt
CPUE kg sole/hp-day
Figure 2: Plaice stock and CPUE development 1992-1999 Beam trawl
500
400
300
200
100
0
1
0,8
0,6
0,4
0,2
0
kg/hp-day
Kt
Plaice stock and CPUE
1992 1993 1994 1995 1996 1997 1998 1999
Plaice stock Kt
CPUE kg plaice/hp-day
In table 1 the correlation coefficient between several CPUE measures and stock levels are
presented.
Table 1: Correlation coefficient between selected CPUE measures and stock levels
Sole stock
t
Plaice stock
t
Sole stock
t-1
Plaice stock
t-1
Sole stock
t-2
Plaice stock
t-2
Plaice
0,79
Sole
0,69
CPUE value current prices /hp-day
Plaice
0,20
Sole
0,03
CPUE value current prices /fuel costs
0,65
0,32
0,89
0,88
0,87
0,68
0,82
0,68
0,92
0,68
0,34
0,30
0,10
0,10
0,29
0,19
-0,17
0,16
0,11
0,23
Plaice
0,45
Sole
0,12
CPUE value constant prices/hp day
Plaice
0.59
Sole
0.44
0,20
0,01
0,48
0,32
0,42
0,12
0,06
0,31
0,25
0,22
0.64
0.56
0.55
0.53
0.71
0.58
0.35
0.61
0.61
0.68
CPUE kg /hp-day
In the table the CPUE is defined for plaice and sole respectively as total catch per species per
amount of Hp-days spent fishing, value of the catch of the particular species in current prices per
number of Hp-days and by amount spent on fuel and as value of the catch by species in constant
1992 prices per number of Hp-days. The index t indicates a time period; t-1 the period prior to
period t and t-2 the period prior to period t-1.
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Discussion paper: Applying CPUE as management tool
It becomes clear that the actual catch in kg per unit of effort has the highest correlation with stock
abundance. Value measures have a relative low rate of correlation. This can be expected since
the above values are based on prices. When the amount of fish landed decreases prices increase
and hence proceeds are much more stable than stock or catch levels. In constant prices a higher
rate of correlation between stock levels and value of the catch is found.
From the table above one can conclude that the CPUE in kg per unit of effort for sole strongly
relates to the sole stock of period t-1. For plaice the strongest correlation is found between the
realised catches and the plaice stock level in period t-2. This implies that the current CPUE is an
indicator for future stocks.
The correlation found can be interpreted two ways. If we take the correlation between the CPUE
of kg sole per Hp-day and the sole stock as an example. The realised CPUE is an indicator for
the sole stock in period t-1. This implies that the current CPUE for sole is in indicator for expected
stock levels for the next period.
Vice versa, a stock assessment for sole is an indicator for the CPUE one period from the period
of assessment. For plaice this time-period between CPUE and stock levels is two periods.
In table 2 a summary of the regression analysis and the analysis of variation (ANOVA) statistics
are presented. For the analysis we have used the CPUE and the stock levels in period T and for
sole and plaice respectively t-1 and t-2.
Table 2: Regression statistics and ANOVA for selected CPUE – stock level relations
Multiple
R
0.65
Stock plaice period
t
And
kg plaice/hp-days
Regression
Residual
Total
Intercept
X variable
Stock plaice period
t– 2
and
kg plaice/hp-days
Regression
Residual
Total
Intercept
X variable
Stock sole period
t
and
kg sole/hp-days
Regression
Residual
Total
Intercept
X variable
Stock sole period
t-1
and
kg sole/hp-days
Regression
Residual
Total
Intercept
X variable
0.93
R
Square
0.42
0.86
Adjusted
R Square
0.32
SS
MS
F
0.014
0.019
0.033
0.014
0.003
4.306
0.48
0.78
Pvalue
0.197
8.86E-07
1.30
2.08
0.24
0.08
0.029
0.001
0.25
6.67E-07
5.45
5.96
0.0016
0.0001
0.12
1.34E-06
2.68
2.35
0.037
0.057
0.09
1.55E-06
3.52
4.56
0.013
0.004
35.540
0.39
0.005
0.005
0.009
0.88
T
Stat
0.83
0.029
0.005
0.033
0.69
Coefficient
0.005
0.001
5.52
0.74
0.008
0.002
0.009
0.008
0.0004
20.80
5. Debate topics
The Dutch beam trawl data analysis shows that a relevant correlation between the CPUE and
stock abundance can be established. However, the basic assumption that there is a simple linear
relationship between CPUE and stock size needs further attention 6. This applies also to the
assumption of the catchability coefficient remaining constant over time.
6
Cf. Gunderson, 1993
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Discussion paper: Applying CPUE as management tool
One of the characteristics of a valid indicator is that it is a rather simple and easy to measure
gauge for a more complex phenomenon. From the Dutch beam trawl data indication is that CPUE
can play a role as indicator for the development of the stock. The CPUE next to having a relation
with present stock levels also has a relation with future stock abundance.
Vice versa stock levels influence CPUE so by establishing present stock levels it is possible to
predict and set future desired CPUE levels. Also from the Dutch beam trawl data indication is that
a relation can be made visible between the value gained per unit of effort (hp-days) and the stock
abundance and stock level development.
In this assessment a partial analysis on sole and plaice has been implemented. Remaining issue
is whether such a partial analysis in a multi-species fisheries has validity for the total of the
operation. Stock developments of all species exploited in the fisheries will influence the effort
allocation and results of the undertaking. Also can there be an effect on the total CPUE
development related to the partial stock development across different species.
Multi-species fishery A first issue that needs to be dealt with is the topic of multi-species
fisheries. Can a relation be established between the aggregated (total) catch of a vessel during a
single time period and the aggregated available stock and which weights can be used to
aggregate distinct species of fish.
In addition, a question is whether for the CPUE a partial CPUE can be calculated based upon the
total aggregated catch by weight. Catch composition in terms of species and in terms of length
distribution is an important contributor to realised catches.
Catchability coefficient q. In the literature it has been documented that the catchability coefficient
changes overtime (cf. Gunderson 1993). Apparently q is inversely related to population
abundance. According to Gunderson this can be attributed to the fact that fishers allocate their
effort depending on the stock abundance. If the CPUE-measure was to be used to set
precautionary levels of stock abundance than it can be concluded that there is a tendency to
over-estimate low levels of stock.
Effort allocation. Fishers allocate their fishing effort over time and space. Allocation patterns also
change depending on the stock abundance hence influencing the CPUE. However, by including
steaming and search time when calculating the CPUE the actual total effort to obtain a specific
catch is more realistically represented.
Using a cost measure to represent effort the quality of inputs used is more accurately reflected.
Steaming to the fishing grounds and spending time to locate fish are important costs to obtain a
specific catch. Spending more time on either locating fish or steaming to more faraway fishing
grounds result in a depressed CPUE reflecting the fact of relative fish scarcity.
In this analysis sole and plaice have been used as target species. By managing on effort
allocation in a multi-species fisheries the fishers optimise their returns.
CPUE and stock need a degree of spatial and time definition. In addition insight into the effort
allocation of fishers needs to be obtained. But by establishing fleet independent stock
assessments and by obtaining the commercial fleet’s CPUE data the stock and catch
development can be more accurately interpreted.
If one was not to use CPUE as a direct estimate for stock abundance but that CPUE development
over time was used as an indicator for stock development over time it could attribute to improved
management policies. In this way the CPUE is an indicator of interest with potential as a
management tool because:
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Discussion paper: Applying CPUE as management tool



CPUE is a relative simple indicator which can be established ‘above the water’. With modern
technology monitoring in ‘real time’ could be established.
By using CPUE the motivation for underreporting catches and manipulating catch
declarations is reduced.
The information is derived from the commercial fleet itself which enhances the dialogue
between the industry and the policymakers.
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Discussion paper: Applying CPUE as management tool
Literature
Andersen, J.L. 1999. A review of the basic biological and economic approaches to fishing effort.
SJIFI Working paper No. 12/1999, Danish Institute of Agricultural and Fisheries Economics.
Beverton, R.J.H., Holt, S.J., 1957. On the Dynamics of Exploited Fish Populations. Chapman &
Hall, London.
Buisman, F.C. et al, 2001, Economic aspects of discarding (in print).
Gulland, J.A. 1983. Fish stock assessment: A Manual of Basic Methods. John Wiley and Sons,
New York.
Gunderson, D.R. 1993. Surveys of Fisheries Resources. John Wiley and Sons, New York.
Hilborn, R., Walters, C.J., 1992. Quantitative fisheries stock assessment. Chapman & Hall, New
York.
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