Genetic diversity of marine fisheries resources
Possible impacts of fishing


Fisheries Research Centre
Ministry of Agriculture and Fisheries
Wellington, New Zealand

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ISBN 92-5-103631-4

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Food and Agriculture Organization of the United Nations Rome, 1994
© FAO 1994


Conserving biodiversity in the ocean has been more neglected than that on land, yet the ocean is rich in genetic, species and ecosystem diversity. Fishery resources are an important subset of the world's biodiversity. They are affected by human activities including fishing, aquaculture and other development sectors. The present paper is a general review on genetic diversity of marine fishery resources with particular emphasis on the impact of fishing. Understanding the impact is an important prerequisite in our effort to develop the strategy for prevention of possible loss in biodiversity.

The paper was written by Dr. Peter J. Smith of New Zealand, who has been working in fisheries with a strong interest in biodiversity. The author is grateful to Dr Devin Bartley, FAO Fisheries Department for the helpful comments on a draft of this paper and also to Dr Purwito Martosubroto for his assistance. The views expressed in this paper are solely those of the author and do not necessarily represent the views of FAO.


FAO Fisheries Department
FAO Regional Fishery Officers
Directors of Fisheries
Regional and International Fisheries Organizations

Smith, P.J.
Genetic diversity of marine fisheries resources: possible impacts of fishing.
FAO Fisheries Technical Paper. No. 344. Rome, FAO. 1994. 53p.
This report reviews the evidence for the genetic impact of fishing on marine fisheries resources. The most widely used method for measuring genetic diversity in natural populations has been protein electrophoresis; marine teleosts have levels of genetic diversity ranging from 0.0 to 18% and marine invertebrates from 0.4 to 32%. Genetic studies have shown that populations of marine species are less differentiated than freshwater species, experience temporal genetic changes, can be changed locally by pollution, and contain cryptic species.
Genetic changes in populations occur through selection or drift. In natural populations fishing is a major source of mortality and is non random with respect to age and size of individuals. A common observation in heavily exploited teleost fisheries has been a decline in the age and/or size at sexual maturity. Size selective fishing would favour early maturity. However growth rate in some fishes is density dependent and increases when the stock is reduced; faster growth rates lead to a reduction in the age or size at onset of sexual maturity. Thus it is not possible to determine if the observed changes are genetic or compensatory in response to reduced stock density.
Genetic drift is unlikely to be a major factor influencing levels of genetic diversity in many marine fisheries, except for some populations, e.g. giant clams, which have been reduced to near extinction levels. Some rare and endangered freshwater fishes show low levels of genetic diversity. There is no evidence for loss of genetic diversity in collapsed stocks of pelagic species. While the stocks have collapsed from a commercial perspective most have maintained large population sizes at their lowest state.
The use of hatcheries to produce seed for aquaculture and enhancement could lead to loss of genetic diversity in natural populations through escape of farm stock or inappropriate choice of broodstock.
Experimental studies are required to determine the heritability and the response to selection of life history characters of exploited species, and to determine if relaxation of fishing pressure allows the recovery of “fast growing” and “late maturing” genes or gene complexes in populations. Also it would be desirable to monitor levels of genetic diversity in recently exploited or highly exploited species. A combination of experimental and field studies would permit a more rigorous testing of genetic changes in exploited populations.
If genetic changes are demonstrated in exploited species then changes to management would be needed to conserve natural levels of diversity.

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Measuring genetic diversity
Levels of genetic diversity
Genetic differentiation in marine populations
Temporal genetic changes
Cryptic species
Pollution induced genetic changes
Life history characters
Evidence for genetic changes due to selective fishing
a. Selection for early maturity in cod
b. Selection for early maturity in haddock
c. Selection for early maturity in flatfish
d. Selection for smaller size at maturity in rock lobster
e. Lowering the age of sex change in shrimp
f. Selection for late spawning in herring
g. Selection for smaller size and reduced age in Pacific salmon
h. Selection for early maturity in Atlantic salmon
i. Disruptive selection in Pacific salmon
j. Selection for early maturation in lake fishes
k. Selection for morph type and reduced size in whitefish
l. Loss of genetic diversity in orange roughy
m. Increase in genetic diversity in sockeye salmon
Experimental studies on selection of aquatic organisms
a. Selection for slower growth rate in Orechromis mossambicus
b. Selection for age and size at maturity in guppies
c. Selection for size in Daphnia magna
Modelling studies on selective effects of fishing
Genetics of collapsed populations
Genetic changes due to selection
Genetic changes due to drift