Honey, We Shrunk the Fish: Overfishing, Evolution, and the Threat of Permanent Ecosystem Change

Let’s say you are a big, healthy Atlantic cod off the coast of Newfoundland in the year 1985 and you are in your sexual prime. Your sap has risen and you and several thousand other of your slimy kin are piled onto a rocky reef, perhaps listening to Marvin Gaye, and getting it on in a frenzied orgy of spawning. There are tons of high quality forage fish to eat; the million eggs you just laid or fertilized are ready to carry your genes; things are going great. You might even be smoking a cigarette and sporting a self-satisfied smirk as you think about what a lovely thing it is to be a cod. Then a net the size of a football field sweeps you and your lovers away, your food source eats most of your eggs, and your great-great grandchildren all turn out to be runts and teen parents. This is your story.

The collapse of the cod fishery off the coasts of Newfoundland and Labrador (hereafter Northern cod) may be one of the sadder stories in recent environmental history. In 1962, the first year in which good data are available, there were an estimated 1.6 million tons of spawning cod off the northeast Canadian coast (Hutchings and Myers 1994). 1.6 million tons. It would take 212 Yankee stadiums with a 317 pound sumo wrestler crammed in every seat to equal that mass of fish. It might not be surprising that almost everyone thought that it would be impossible to overharvest such a population. Alas, almost everyone turned out to be wrong. Catch rates plummeted throughout the late sixties until the early nineties along with a sustained decrease in the amount of spawning cod biomass (Figure 1). By the time Canadian resource managers finally placed a moratorium on the fishery in 1992, only 22,000 tons of spawning fish remained, a paltry figure that equates to less than three stadiums worth of fish. Optimists who believed that time would heal all, and that cod populations would certainly recover have also turned out to be wrong. The processes of quantitative evolution may be part of the reason why.

A Thought Experiment

To understand how fisheries can change the genetic makeup of a population over time, let’s perform a thought experiment. You’ve got a very large aquarium of genetically and phenotypically diverse fish in a room with a very clever cat. The cat non-selectively catches a certain proportion of the fish in the tank. Those fish that the cat eats before they have a chance to spawn have a fitness of zero, and those fish that spawn before eaten have a fitness greater than zero. As the experiment progresses, it isn’t difficult to see that the fish population will be selected to breed at a younger age, since the penalty for slow maturation will often be death without hope of passing on genetic material. The change in your fish population may occur faster than you might expect, given that it is estimated that about 20-30% of variation in life history traits of fish is heritable (Kuparinen and Merilä 2007). What may be even more surprising is that all of your fish are gradually becoming smaller, even though your cat is being completely non-selective in which fish it eats. This is because the growth of fish declines when a fish becomes sexually mature since energy is being shunted from growth toward reproduction (Conover, Munch, and Arnott 2009). The outcome? Over time all you’ve got is a tank full of small, sexually precocious fish.

We Are the Cat (But Worse)

The parallels between the thought experiment above and fisheries are obvious. If fishing pressure is intense enough, and little gene flow occurs among populations, and if fish populations have sufficient genetic variation, then early maturation and small size will eventually dominate a population. The problem with this parallel is that fishermen, unlike the cat, are highly selective in the fish they target. Fisheries most often target large individuals, meaning that an overt directional selective pressure is being placed on fish relating to size, in addition to the pressures nudging fish toward early reproduction (Kuparinen and Merilä 2007). In the case of the Northern cod fishery, both intensive selective pressure on size and inadvertent selection on breeding age appear to have occurred. Age and size at first breeding was seen to plummet before the closure of the fishery in 1992, and as expected in a scenario of a genetic change in the population, these parameters have not recovered since. It appears that the selective pressures of overfishing have changed Northern cod into a small, early breeding fish. Add the pressures of a changing ecosystem, and you have an even greater problem.

Permanent Ecosystem Change?

Marine ecosystems are often thought of as a zero sum units. When large quantities of a top predator, such as cod, are removed, other sources of biomass will fill the vacuum and may radically alter the structure of the ecological community (Frank et al. 2005). The prey species of northern cod, including Atlantic herring and crab species, have skyrocketed over the same time period that cod have collapsed, and large scale changes in the plankton community have also occurred (Frank et al. 2005). These changes may actually be inhibiting the recovery of cod stocks through several mechanisms. In a curious case of predator-prey role reversal, burgeoning herring populations are thought to be dining on cod eggs and juveniles, causing undue mortality on cod well before they are able to spawn (Swain and Sinclair 2000). Changes in phytoplankton and shrimp abundance may also have led to decreased foraging efficiency, and thus slower growth for cod. These radical changes are occurring in an environment where cod are also hamstringed by genetic shifts toward early reproduction and slow growth. The frightening upshot is that Northern cod maybe be permanently relegated to only a bit role in the ecosystem in which it was once the lead player.

Evolution and the Path to Prevention

It is unclear whether the Northern cod population will ever recover to its former level, but the lessons learned from its story are being applied to other areas in hopes of preventing further fisheries collapses. One option that is currently de rigueur is the establishment of marine protected areas, where little or no fishing occurs. The idea is simple. If fishing has a tendency to create genetic changes in breeding age and size in populations, then sources of fresh genes that have not been altered by fishing selection can counteract these shifts through genetic mixing. For these protected areas to work however, they must be large enough to maintain genetic diversity and close enough to harvested populations to ensure sufficient interbreeding. Another option is to establish minimum and maximum size length limits to both ensure that individuals are always able to breed at least once before harvest, and to maintain the genetic variability within the largest members of a population. Though it might be too little for the Northern cod fishery, change is not too late for other marine areas. Our disregard for evolutionary impacts of fishing in Northern cod can at least be partially amended by our application of evolutionary concepts toward maintaining genetic diversity in other harvested fish species across the globe. We’ve learned that 212 Yankee stadiums worth of fish is easier to lose than you’d think. With well-considered fisheries practices in the future, it is not something that we should lose again.

Conover, D.O., S.B. Munch, and S.A. Arnott. 2009. “Reversal of evolutionary downsizing caused by selective harvest of large fish.” Proceedings of the Royal Society B: Biological Sciences 276(1664):2015–2020.

Frank, K.T., B. Petrie, J.S. Choi, and W.C. Leggett. 2005. “Trophic cascades in a formerly cod-dominated ecosystem.” Science 308(5728):1621.

Hutchings, J.A., and R.A. Myers. 1994. “What can be learned from the collapse of a renewable resource? Atlantic cod, Gadus morhua, of Newfoundland and Labrador.” Canadian Journal of Fisheries and Aquatic Sciences 51(9):2126–2146.

Kuparinen, A., and J. Merilä. 2007. “Detecting and managing fisheries-induced evolution.” Trends in Ecology & Evolution 22(12):652–659.

Swain, DP, and AF Sinclair. 2000. “Pelagic fishes and the cod recruitment dilemma in the Northwest Atlantic.” Canadian Journal of Fisheries and Aquatic Sciences 57(7):1321–1325.