Managing Evolving Fish Stocks Notes

Evolution causes vary across a wide range. Everything from introduction of a new species to a new food or energy source to frequency forest fires can influence species selection from generation to generation. In natural settings, these changes often introduce themselves slowly except in catastrophic cases, thereby giving species time to select for the appropriate attributes across hundreds of generations. Human interaction, however, depends on ideas and the techniques and technology selection and improvements occur many times in the one person’s lifetime. The evolution of techniques to exploit a resource, therefore, out strips the natural selection process creating a more powerful artificial selection.

Fisheries contribute to the food supply for people and ecosystems alike but due to highly efficient fishing catching tools, the contribution to people outweighs ecosystems. Fishing “has become the main source of mortality in many fish stocks, and may exceed natural mortality by more than 400%” (Jorgensen et al). At such rates human induced selection pressures outweigh natural selection, so much so that evolution can occur in a few generations rather than a few thousand years.

As Jorgensen et al predicts using the life-history theory, pressure on fisheries will push evolution toward earlier reproductive periods. Strong evidence for fishing created pressures comes from purposeful aquaculture modification and experiments finding evolution in generations. “[A]nalyses of fishers data spanning a few decades have detected widespread changes in maturity schedule that are unlikely to be explained by environmental influences along” (Jorgensen et al.). The issue at hand remains how fast such evolution will occur not whether it will.

The changing yield type for fisheries becomes an important factor for the markets and fishing dependent economies. Smaller fish or less desirable fish may result in reduced profits especially as fish numbers decrease. The fish evolution under these conditions may not recover or recover slowly to original sizes but equally strong pressures to push fish species to evolve in longer reproductive cycles will not exist. Without strong selection pressure to elongate reproductive cycles, populations will not return to their original order.

The evolutionary permanence of such changes will have indirect effects for the ecosystems via changing food web dynamics. The possibility for unforeseen effects via positive or negative feedbacks demands study on techniques to temper human fishing effects. Successful fishery management will benefit from considering known and unknown effects and ecological services created by fisheries.

Successful fishery management will use tools and models that take into account the artificial selection push from catching large fish.

Evolutionary Change # of Species # of Studies Change % (n)
Maturation at lower age 6 10 23-24 (1)
Maturation at smaller size 7 12 20-33 (3)
Lower PMRN midpoint 5 10 3-49 (13)
Reduced annual growth 6 6 15-33 (3)
Increased fecundity 3 4 5-100 (3)
Loss of genetic diversity 3 3 21-22 (2)

"Fisheries-induced evolution has been demonstrated in several species and studies, for some stocks (n) the magnitude of change could also be quantified. Analyzed time series covered between 13 and 125 years. PMRN, probabilistic maturation reaction norm (18). The documented evolutionary changes potentially affect fishery yield, recreational fishing experience, tourism revenue, trophic interactions, resilience to fishing, resilience to environmental fluctuations, and adaptability (e.g., to climate change)."

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