We can’t manipulate entire stocks of Bluefin Tuna in massive experiments. But we can use guppies to get the same answer.

But first, a quiz: How does overfishing affect fish populations? Obviously, more commercial catch reduces the total biomass and number of individuals in a stock—that’s a given. But commercial over-harvest has another, more subtle effect.

Bigger fish = more meat = more money. If you’re making money from fishing, you’re after the big ones (think Wicked Tuna). But all that targeting of the largest of individuals can take a toll on the genetic structure of populations.

Each year of size-selective harvest adds to a slow-but-steady pattern of killing fishes that 1) grow older, 2) grow larger, and 3) have the most eggs. By killing these individuals and allowing others to live and reproductive, this strategy promotes the proliferation of individuals that 1) grow to smaller maximum size and 2) reproduce earlier in life 3) with fewer eggs. This trend leads to a stock with ever-decreasing size of individuals, lower reproductive potential for stock recovery, and lower yield.

Although this is a good idea that seems to fit observations, actually testing this theory with experiments on commercially exploited stocks is impossible because evolution works at the time-step of generations. Evolutionary change comes quick in rapidly-reproducing species (e.g. bacteria), but much slower in organisms that may take 10 or more years to mature.

That’s where the guppies come in. And by guppies, I mean any livebearing fish of the family Poeciliidae. Guppies have several life history characteristics that make them great models for testing evolutionary hypotheses: rapid generation time, relatively low fecundity, and complex mating strategies.

In some cases, natural experiments provide insight into how size-specific morality shapes the evolutionary trajectory of fish populations. For instance, a massive amount of research has been devoted to studying populations of guppies in Trinidad.  In parts of some rivers, guppies occur in relatively predator-free environments. In other parts, guppies co-occur with several predator species. Areas with and without predators may often be separated by only ten or so meters: waterfalls prevent upstream migration of predatory species. In this case, predators present an ecological analogy to fishing pressure. After all, death is death—whether from a purse seine or from the jaws a fish.

Where they live with predators, guppies have exhibit a life history geared toward ‘living fast and dying young’. Compared to guppies in predator-free habitats, they mature and reproduce at earlier ages, reproduce more often, and give birth to more young per litter. This life history strategy is in-line with predicted effects of fishing on life histories of exploited stocks of fish. Moreover, stocks may take longer than expected to rebound after fishing moratoria because the opposite life-history strategy is not selected for.

In the fisheries world, we often see topics as being unrelated. Stock assessment scientists have little contact with nongame biologists, and evolutionary ecologists rarely interact with fisheries economists. But this type of research is a great example of what we can learn by looking just beyond our immediate discipline.

Further reading:

Jorgensen, C., K. Enberg, E. S. Dunlop, R. Arlinghaus, D. S. Boukal, K. Brander, B. Ernande, A. Gardmark, F. Johnston, and S. Matsumura. 2007. Ecology-Managing evolving fish stocks. Science 318:1247-1248.

Reznick, D. N. and C. K. Ghalambor. 2005. Can commercial fishing cause evolution? Answers from guppies (Poecilia reticulata). Canadian Journal of Fisheries and Aquatic Sciences 62:791-801.

Ricker, W. 1981. Changes in the average size and average age of Pacific salmon. Canadian Journal of Fisheries and Aquatic Sciences 38:1636-1656.

by Brandon Peoples