Lect 10
BIOL468 & ECOOL198
UNIVERSITY OF LIVERPOOL
Interactions of fish, environment and fishing:
Considerations for Management
Dr R.T.Leah
School of Biological Sciences
Status : v2
Introduction
Fish populations are very important to human populations and yet they fluctuate greatly in
size. The problem of the prediction of the size of future fish stocks is at the very centre of
fishery science. Its investigation can be thought of as a very important part of a wider
subject area known as impact analysis which deals with prediction of the results of change.
For fisheries, such change can be brought about by both man-made and natural changes in
the environment or by the process of fishing itself. Our understanding is made more difficult
because the fish stocks consist of multi-species populations which are also undergoing the
processes of intra- and inter-specific competition at the same time as they are being
perturbed.
Thus to analyse the dynamics of fish stocks we must deal with two components which are
and must remain completely inter-linked:
a) An ecological entity that is undergoing biotic interactions
b) An economic entity that humans are exploiting.
Changing Fish Populations through time
Various mortality factors operate on fish populations, some of which may be density
dependent or independent. The natural mortality of fishes may be described as a density
dependent function of age (where the density independent component is modulated in a
density dependent way). It is expected that there will be considerable variability in mortality
at differing life stages. Within a cohort, factors such as food and temperature will govern
survival but since most exploited stocks of fish are multi-aged, these effects will be linked
through considerable periods of time.
Perturbations and change
The main point of understanding the dynamics of fish stocks is to enable predictions to be
made of changes in the size of fishable stocks as they are affected by the vagaries of the
environment. The aim is to put the competent authorities in the position of being able to
make rational management decisions to maximise the human benefit from a particular
fishery. Of course in some instances, because of past mismanagement, conservation may be
the primary aim.
Unfortunately, there is a fundamental problem in achieving this understanding because any
agent of change usually modifies the processes that regulate population abundance at the
same time as altering the abundance itself. These processes are in any case usually poorly
known or understood. Processes that affect population size are either density dependent or
independent - it is the density dependent processes that concern us in this section.
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Compensation/depensation
Processes that regulate population size can be either
or
Compensatory
Depensatory
Compensatory - processes that tend to increase mortality or decrease reproduction as
population size increases.
Depensatory Processes that tend to increase mortality or decrease reproduction as
population size decreases
Where depensatory processes occur at some life stage, opposing compensatory processes
must occur at another to cause persistence of the populations through time. The degree to
which additional stress can be offset by compensatory processes depends in part on the
levels of mortality already occurring in the system.
The processes are important because they control a population's response to a perturbation
such as that caused by fishing or pollution. The problem of prediction is exacerbated
because of its sensitivity to the nature and timing of the response.
For many years the aim of fisheries management was:
To obtain the maximum or optimum sustained yield of fish from the stocks being exploited.
This maximal, safe level of exploitation is equivalent to the removal of fishes equal to the
amount of fish flesh produced each year (the production) without reducing the standing crop
or biomass so that production in subsequent years is not affected.
When comparing data from different sources, a clear distinction has to be made between
biomass (stock of fish present at any one time), biological production (a rate ) and the catch
(yield). The yield represents only a proportion of the production and can range from virtually
all of the production in a fishpond to a very small proportion of the production in a water
where fishing conditions are difficult, or where there are few fishermen, or where natural
mortalities are great.
Biological Production
As very few estimates of biological production have yet been made in natural waters
(freshwater or sea), the yield (expressed as kg/Ha) is often used as an index of production
and in fisheries literature this yield or catch i.e. the proportion of the production cropped by
man, is often loosely termed the ‘production’.
Yield
Fisheries managers need to understand the causes of yield variability so that levels of harvest
can be set that are appropriate to the capacities of both the individual species and the fish
community as a whole. However, yields from individual fish stocks are notoriously variable
from year to year (although the aggregate catch of all fish species from a given region is
often more conservative).
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Productivity at all levels in the community is affected by abiotic environmental factors, but
their importance relative to biotic interactions, including the impact of fishing, is a matter of
dispute. Various fish yield indicators have been devised, most of them based on information
on the variation of abiotic factors. They are useful in developing the management of
scattered fisheries of small economic value which occur in various parts of the world where
complex indices which require detailed ecological process studies could not be justified or
practically carried out.
The use of such indices is reviewed and compared for freshwater and marine systems by
Kerr & Ryder (1988).
Fish Yield indicators in freshwater lakes
The preference in freshwater has been to rely on less data-intensive procedures appropriate
to large numbers of small, discrete fisheries. An indicator that has found general application
in many regions of the world, including African Lakes and reservoirs, is the Morphoedaphic index (MEI) (Ryder 1965, 1982; Ryder et.al., 1974). The impetus for the approach
came from the vast area in northern Ontario bounded by the Hudson and James Bays
containing a large number of lakes, many capable of supporting appreciable fisheries. A
method was required which would require little on-site sampling. Ryder used regression
analysis to explore the relationship of fish yield with several limnological variables that
appeared to have merit and found that the use of total dissolved solids ( an edaphic factor
associated with potential nutrient availability, (expressed as conductivities, µ mhos cm-2))
divided by the mean depth of lake (expressed in metres ), a morphometric factor known to be
associated with the fishery potential of lakes, accounted for much yield variability.
The morphoedaphic index is used to estimate aggregate catch of all species, independent of
typological considerations, the success of which process, demonstrates the relative
conservatism of fish production systems.
The Density-dependent response to fishing pressure:
The first step in the analysis of how some particular change in the reproductive or mortality
rates will effect a population is to determine what density dependent processes affect
population size. The important result is the balance between births and deaths.
It is a commonplace observation that populations of fish have been exploited for many years
and yet, unlike for many animal groups, the actual extermination of fish species by overexploitation is quite rare although there are many examples where they have become
'commercially extinct'.
The Blue Pike (Stizostedion vitreum glaucum) of the Great Lakes was the only over-fishing
extermination example that Goodyear (1980) could come up with although there were a lot
of near misses. In fact it is probable that a combination of factors were involved, even in this
case. More recent analyses (eg Chpt 15, Hart & Reynolds 2002) suggests that overfishing
was the third most important cause of extinction in the 34 spp becoming extinct since 1500.
Interestingly, all fish becoming extinct were freshwater and the most important causes were
habitat alteration and introduced species.
However, we can conclude that the ability of populations to compensate for loss processes is
somewhat limited.
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Temperature and Year Class Strength
Another major abiotic factor which affects the abundance of a year class of fish is
temperature. There is a very good explanation of how temperature and year class strength are
associated in smallmouth black bass (Mclean et al 1981). The investigation of the
physiological basis for the temperature effect showed that two sensitive periods in the early
life history of the species account for most of the variability in the ensuing recruitment. The
first period lasts from the egg stage to the time when larvae are actively foraging. The second
extends over the first winter of life - a time when the young bass must subsist on their stored
reserves of energy. The temperature effects are not independent. Eggs and larvae spawned
early in the season are vulnerable to storm-induced influxes of cold-water but bass spawned
later risk not growing to the size threshold required to carry them through the winter.
Overfishing
As a fishery develops, the catch per unit of effort inevitably declines as more fishermen
share the catch.
At what point does overfishing occur? - need to assess stock size and recruitment rates.
Overfishing may be one of three kinds
1
Growth overfishing when the young fish recruits to the fishery are caught before they
reach a reasonable size / age
2
recruitment overfishing - when the parent stock is so reduced that not enough young
are produced for the fishery to maintain itself
3
ecosystem overfishing - (not yet clearly defined) in a mixed fishery when the catch
decline through fishing of the original abundant stock is not compensated for by the
contemporary or subsequent increase of other exploited animals ie the transformation
of a relatively mature efficient system into an immature, inefficient system
open ocean
pelagic
continental shelves
African Lakes
floodplains
unmanaged
ponds
managed
ponds
0.01
0.1
1.0
10
100
1000
10000
Yield (kg ha-1 yr -1 )
Fig 1 Comparative fish yields from tropical ecosystems (from Lowe-McConnell)
Types of fish community and response to exploitation
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Communities which respond to great environmental change to very stable ones - pioneer /
mature (Margalef, 1968)
pelagic zones - fish with short life cycles (‘r-strategists’) - numbers fluctuate markedly with
nutrient supply, fast growing fishes with a high production/biomass ratio, mobile fishes with
relatively simple behaviour, lacking territoriality and complex breeding behaviour.
In the more mature communities, life cycles tend to be longer and learning processes more
important
In the pelagic community, trophic specialisation is generally for grabbing food as low as
possible in the trophic web (phytoplankton or detritus) or to feed on fishes which do this. In
reef and rock faunas, adaptive radiations have led to trophic specialisations to use the many
diverse food supplies.
These two types of fish community are likely to react very differently to exploitation and
other perturbations such as pollution.
Communities in which r-selection dominates with their very fast population turnovers and
already subject to natural fluctuations in numbers are likely to be able to recover very rapidly
if the perturbations are not too drastic and long lasting. The short life cycles
characteristically found in tropical waters mean that relatively few age groups are
represented in catches and responses to fishing pressures will be much more immediate than
in temperate seas where the average age of individuals caught is generally high.
The very diverse communities, which seem so ‘stable’ are likely to be much more easily
damaged and to take longer to recover. Once disrupted, the very complex web of
interrelationships will recover very slowly , or not at all.
Thus there is a particular danger to the complex communities of locations such as Coral
reefs, African Great Lakes etc
Changing fecundity
One way that population abundance affects the balance between births and deaths is by
altering the number of viable eggs per adult. The number of viable eggs usually declines as
population size increases. As population density increases, competition for food decreases
growth rates so that individuals size-age relationship becomes smaller. As for many fish
there is usually a direct relationship between size and the number of eggs. Thus the number
of eggs deposited by each female would decline as population density increases. ie. agespecific fecundity declines. This has been shown for sockeye, walleye, bluegill, perch, brook
trout pike and plaice.
A second and related effect is on age at first maturity. The age at first maturity is often
observed to decrease as fishing progresses thus increasing an individuals total lifetime egg
production potential.
Changing egg deposition rates
Population density effects the number of eggs deposited by females e.g. by crowding on
spawning grounds in Sockeye.
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It is possible that there is a pheromone type regulation in the Carps such as goldfish and the
common carp.
Growth and Predation
Another way that density dependent processes can be compensatory hinges on the
relationship between growth and predation. As individuals grow, they become too large for
certain size classes of predator ie they "grow through predation". Competition, leading to
low growth, exposes individuals to size-specific predation for longer periods. Thus the
fraction surviving is density dependent. Such size selective predation has been hypothesised
(eg Ricker 1954 & Beverton and Holt 1957) as one possible basis for the influence of stock
size on recruitment in fish populations. It is most powerful in its effects in the early lifehistory stages when most of the mortality occurs although quantitative evidence is hard to
obtain. For it to exert a compensatory effect, growth must be a function of population
density and predation must be size selective. One complication is defining what "growth"
actually is because if the small individuals are dying faster than larger ones this can create a
negative correlation between apparent population growth and density.
If such predators are not present, the existence of density dependent growth in early life
stages will not necessarily result in concurrent compensatory changes in mortality rate.
Density and predation
The above required no change in the behaviour of the predators, but the density of predators
or the intensity of their predation may be directly related to the density of their prey. It arises
from predator aggregation and selective feeding. Fishing mortality may behave in such a
compensatory way where fishermen aggregate in areas of high abundance and fish
specifically for the abundant species.
Cannibalism
This is perhaps the easiest form of compensatory mortality to observe although very large
samples may be needed to demonstrate its occurrence in the field. Even a small observed
incidence could have a powerful influence on population processes. In fact it can be very
difficult to observe e.g. Ricker (1954) showed that cannibalism could be the sole mechanism
of regulation when trout consumed only 3% of their own progeny.
Competition for spawning sites
The most obvious example of this is in salmonid fish in the competition for suitable redds.
When stocks are high late spawning fish cut into the early redds destroying many eggs.
There may also be 'forced' use of sub-optimal sites and there may be a greater intensity of
fungal infection etc with increasing density.
Agonistic Behaviour
Many fish exhibit agonistic behaviour between individuals and this has the effect of
decreasing survival of weaker fish at high densities. The territorial behaviour of salmonid
fish such as trout provides a good example of this. After the fry emerge they disperse and set
up territories. If they do not succeed they starve or go to relatively unsuitable habitat. The
influence of increased agonistic behaviour can be to make predation by other predators more
likely.
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Starvation and the 'critical period'
Conservation and Management of Freshwaters
Ricker (1954) postulated that one possible compensatory mechanism is death from
starvation or debilitation of younger stages of large broods resulting from competition for
food. It would only be compensatory if the food shortage is due to the predation of the fry
themselves. The idea that fish larvae went through a critical period of their lives when a
supply of food was particularly crucial to their survival was originally put forward by Hjort
in 1914. It seems that it could explain much of the variability in recruitment of some marine
fishes (May 1973) but it remains speculative.
Growth and susceptibility to stress
It is often found that the probability of overwinter survival increases with increasing size.
This might be a straight forward effect of size selective predation but in some instances it
appears to be connected with size related survival of the stresses involved in overwintering.
Thus if you get density dependent growth you get compensatory mortality.
Depensatory mechanisms
Density dependent mortality need not always be compensatory. For example if predators
consume a constant number of individuals, the proportion of a population will increase as
the population size decreases. This is depensatory. It has been shown in some salmon
(Johnson 1965) and the Yellow Perch (Forney 1971). Another process which can be
depensatory is the difficulty of finding a mate as population size decreases
Interactions of Fish and Fishing - Reading
For General Reading, see the 2nd volume of :
Hart, Paul J.B. and John D. Reynolds.(eds) 2002 Handbook of fish biology and fisheries
edited by Blackwell, Oxford.
Although old, Hart and Pitcher, 1982 Fisheries Ecology, Chapman and Hall is very readable
and still good.
See also Wootton, 1990 Ecology of Teleost fishes or the very detailed Chapter by Goodyear
referred to below
Beverton, R.J.H. and S.J. Holt 1957. On the dynamics of exploited fish populations. Fish.
Invest., Se. 2, no. 19
Forney, J.L. 1971 Development of dominant year classes in a yellow perch population.
Trans. Am. Fish. Soc. 100:304-305
Goodyear, C P 1980 Compensation in fish stocks (Chapter 11 in Biological monitoring of
fish. Eds - C H Hocutt & J R Stauffer Jr. Lexington Books, Lexington USA) p253-280
Available - Student Reprint Collection
Hjort, J. 1914. Fluctuations in the great fisheries of northern Europe viewed in the light of
biological research. Rapp. P.V.Reun., Cons. Perm.Intern.Explor.Mer. 20:1-228.
Johnson, W.E. 1965 On the mechanisms of self-regulation of population abundance in
Oncorhynchus nerka Mitt. Inst. Verein. Theor. Agnew. Limnol. 13:66-87
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Kerr & Ryder (1988). The Applicability of Fish Yield Indices in Freshwater and Marine
Ecosystems
Available - Student Reprint Collection
Lowe-Mconnell, R.H. 1987 Ecological studies in tropical fish communities Cambridge
Univ. Press, Camb. 382pp
ISBN/ISSN: 0521236010. Copy: only at PT E @PORT E DM183
May, R.C. 1973. Larval mortality in marine fishes and the critical period concept. In H.S.
Blaxter (ed.), The early life history of fish, pp 3-20. Berlin: Springer-Verlag.
Mclean et al 1981 Temperature and the Year-Class Strength of Smallmouth Bass
Available - Student Reprint Collection
Ricker, W.E. 1954. Stock and recruitment. J.Fish.Res.Board Can. 11:559-623
Ryder, R.A. 1982 The morpho-edaphic index - use, abuse and fundamental concepts. Trans.
Am. Fish. Soc. 111, 154-64
Papers Labeled: Available - Student Reprint Collection
Are available for loan from RTL
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Lect 10
BIOL468 & ECOOL198
Shifts in biotic communities
stressed by environmental
insults
Overharvesting
Invasion by non-native
species
Pollution
Eutrophication
Toxic wastes
Acid rain
FISH STOCKS
Impoundments
River
engineering
Afforestation
or
Deforestation
Habitat loss
Removal of riparian
vegetation
Land
Reclamation
Perturbations affecting the status of inland fisheries
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Land Drainage
Flood Alleviation
Management of Fisheries
BIOL468 & ECOL198
Habitat improvement and
restoration techniques
Management of
flow regimes
Reforestation
Fisheries
Management
Biomanipulation
RESTORATION &
IMPROVEMENT OF
FISH STOCKS
Do nothing
Treatment
Pollution
control
Legislation
Stocking and
introductions
Education
Introduction
species
of
new
To
act
predator
as
Techniques for rehabilitation of inland fisheries
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To 'fill vacant
niche'
Diversion
Legislation