Tuesday, November 22, 2011

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.

11 comments:

  1. I had no idea that any of this was happening. i knew that some species around the world were becoming extinct because of humans and them messing with nature too much, but I did not know the extent of it. This opens my eyes and makes me wonder about all other animals that humans farm for eating. I am strongly against this which I why I became a vegetarian and do not eat any meat or any seafood. It is a small thing, but it is the least that I can do. I hope more people take notice to things like this and change what is happening to preserve nature and the ecosystems. It makes me sad to think that the world is falling apart due to humans when they seem to have the cranial capacity and the genius to prevent these things from happening and make the world an even better place.

    I was wondering how many places this has happened to, not just fish but all animals because I feel like it would be a large number. I was also wondering how many places have been saved from this happening due to studies like this bringing light to the topic so that more and more people will discuss it and be aware of it.

    More studies like this must be done, and it should be in the news more in order for the public to be aware of it. I think this was a great topic, and I learned a lot about the problem in general and how to fix it. I will be telling my friends about it, so they can be on the watch out as well.

    Thank you very much for this post, I quite enjoyed it.

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  2. It’s very interesting to see a how some of the concepts we’ve been discussing can be used to explain real world situations such as this. The unnatural selection pressures created by humans clearly impact the world directly, by depleting the population of interest, and indirectly, by causing an imbalance to be filled by the predator’s prey and other organisms by consequence. The story reminds me of the growing concern regarding commercial fishing enterprises and the growing popularity of fish farming that has been discussed on NPR, mainly the pros and cons regarding both fields. Clearly this post points out modern concerns in commercial fishing, but do those concerns outweigh questionable farmed fish that have excess antibiotics and other chemicals used to maintain their health for mass production.

    I was wondering how effective these fishing regulations truly are, do fisherman really stay out of the no fishing zones and are the zones a closely monitored, changing entity based off of the movements of the fish? (Mainly since they have to be close enough to fished areas to ensure genetic diversity and crossing between the two areas.)

    When considering the other possibility of offering a minimum-maximum size, I was wondering if such a regulation’s selection pressure was considered? Specifically, would the size be a truly reliable manner with which to deem whether the fish had reproduced yet or not. I just feel like this type of regulation would be counterproductive in attempting to preserve diversity.

    The post was very interesting, thank you.

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  3. It seems that learning form this situation is the most beneficially outcome. Due to the environmental factors, that are predominantly out of our hands, it seems that this population will not recover in a period of time to benefit the current generation inhabiting earth. By understanding how we have influenced this fish population, other fisheries can establish practices to ensure that this does not happen, or is limited.
    Selecting fish of only a certain size seems like a smart practice. By releasing small fish it gives them a chance to breed, and by releasing large fish, who presumably got to that size due to "good genes", it allows them to continue to spread their genetic material. This leaves selection of only 'medium' sized fish.

    It seems that it would be difficult to establish marine safe zones that are of a size that would facilitate population growth AND fishing. It seems that enforcing these zones would be nearly impossible, due to how large they would need to be. The costs associated with enforcing these zones would be immense, and who pays. I don't think too many Americans would want their tax dollars going to something such as this. Additionally, fish don't remain in one place all the time. This fact would cause these safe zones to be enormous.

    I think the fact of the matter is that humans are on the verge of overpopulation in the world given our current farming and agricultural practices. The question is how to maintain food stocks and still feed everyone. A question that I don't have an answer to.

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  4. It seems as though human impact is drastically directing selection on extreme quantitative traits so much that we are maximizing, or in the case of fish, minimizing quantitative traits. With combined efforts of overfishing, and as we seen in class the effects of fisheries on both fish size and egg volume, we are not only pushing fish species to extinction, but forcing macroevolution (assuming they don't die first).

    While the line between microevolution and macroevolution can be vague, I would still like to posit that the millions of years it takes for us to observe massive size changes in other species, say the Irish elk to modern Cervidae, is occurring in approximately 49 years. I make the assertion this is macroevolution because we are seeing a shift in an entire species for the trait of size/sexual maturity. If it were a microevolutionary case, we would most likely observe a more stabilizing selection-like trend with a fluctuating mean size with ample phenotypic variance still existing. This phenomenon, while I agree is sad, is still exciting due to its wonderful corroboration of Darwin's 4 postulates.

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  5. I enjoyed the humor that was put in the beginning of this blog. Its nice to see that not all scientific postings being so straight forward and monotonous. I also liked the use of the cat and fish tank example. Most people learn new concepts or have concepts more clearly understood through examples such as the cat and the fish tank. Also giving examples of how much the 1.6 million tons of fish was with a visual representation gives people a way to grasp the enormity of the number.

    I was wondering why you did not use any of Mark Hixon's papers in your post? I ask because He states exactly the same findings as you do and He is so famous at Oregon State University with his work on fisheries. Especially his work on marine reserves and how they protect the big fat fertile female fish which have the most eggs with the best genes.

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  6. I really enjoyed this article. It reminded me of the Baelev experiment we discussed in class where selection on friendliness to humans indirectly selected for other traits. Here, it seems, selection for size resulted in faster development and therefore reproduction.
    I was curious whether, instead of just letting these possibly irreversibly changed ecosystems try to readjust, actual selection pressure could be applied. If for some amount of generations, pressure for increased size (by fishing for only small fish) was applied, wouldn't these populations have to increase in size, thereby reestablishing their dominance?
    Thanks for the good read.

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  7. Good post, this is the kind of information that I think every seafood-eater should be subjected to, in order to allow them to make informed choices! Overfishing is a huge issue - mostly stemming from the false and age-old belief that the ocean and its supply are endless. As of late, fishermen and seafood companies are seeing a decline in catch and stocks, which undoubtedly exist as a by-product of their own actions. What I'd like to address here is the importance of marine protected areas! They exist often, and promote an increase in fish stock numbers. Marine protected areas (MPAs) strive to protect specifically the areas in which species aggregate and spawn, thus allowing reproduction in a safe habitat. The consequence of this safe reproduction is a phenomenon called the spillover effect, which allows fish from the MPA to disperse into nearby areas, increasing the population. Since fishermen are not allowed to take from MPAs, this spillover result produces an effective fishing area surrounding the MPA and an effective breeding area/nursery ground inside the MPA; a system which often works out. A commonly environmentally friendly alternative to eating wild fish is buying from aquaculture. While certain aquaculture tactics result in destruction of important ecosystems (shrimp aquaculture is very destructive to mangroves), most aquaculture management is aware of the potential damage their business is capable of. As a marine biology major, I've taken classes focused on these problems and how we can fix them, so I hope I can offer options to some of you who are feeling greedy for eating fish! Also, Monterey Bay Aquariums website has a 3-fold printout of environmentally friendly vs. environmentally detrimental seafood options... http://www.montereybayaquarium.org/cr/cr_seafoodwatch/content/media/MBA_SeafoodWatch_WestCoastGuide.pdf so check it out!

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  8. I want to play the devil's advocate here. These species are responding to a selection pressure, so that they can survive more effectively in the environment (granted, an environment we created). Humans exist in every part of the globe, and exert selective pressures on just about every species in existence. If we "prevent" them from evolving in response to us, aren't we preventing adaptation and their hopes of survival? We seem to have this attitude that we can "wall off" nature and allow it to exist in a pre-human state, and that this is our best conservation strategy. But look around, what are the most successful species? Cows, goats, sheep, dogs, cats, etc. Domesticated species. We bring the same 10 species around the world wherever we go and devote vast tracts of land to them. These species will survive for a long time because of it. So, instead of walling off species and not letting them evolve to our presence, what if we tried to domesticate and utilize MORE species. It would be cool if in Australia you could have had a pet Tasmanian Tiger rather than a dog, we should be eating Pronghorn, Elk, Bison and Mule Deer in the western states, I think the Black Bear is well on its way to domestication...Domestication is a path to survival for many species. The reason we have only a few species that live well with humans is because they got a head start. If we allow other species to catch up, we have a real chance at preserving some deep branches on the tree of life. It may not appeal to our romantic tradition of wild and free landscapes; but at some point we have to get real. We have made the world an human-based habitat, and we don't plan on relinquishing it any time soon. Species must evolve to tolerate our presence and we should help them do that.

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  9. I think this is a very interesting example of a selection pressure put a population by humans. Thinking about a relevant issue today though the lens of our coursework helps reinforce the material. I wonder if practices to prevent the example of the Northern Cod happening in other fisheries could effectively be applied to sustainable fisheries management.
    As someone mentioned above there is a question of how closely people will adhere to weight limits or boundaries, there will always be cheaters, right? What would be the punishment to deter the same practices that have depleted the northern cod fishery?
    The concept of a zero sum ecosystem is an interesting one to me. The idea that once one organism has left a niche or has a drop in population, other organisms will fill the space and use the newly available resources. I think it could be applied to more than just an ocean ecosystem.

    The comment the Joseh just made is an interesting proposition. The directed and intentional domestication of a wider range of species in hopes to make them better adapted to the new human-dominant landscape to make a pass at preservation of biodiversity.
    I think this would have a better chance at working in animals than plants, as instead of the 10 or so vertebrate animals we have domesticated and brought to all corners of the world. We as a species we have hundreds of plant that we encourage and cultivate for food; add to that the thousands of weeds that we've unintentionally brought along with them and you have a scenario already full of species already adapted to a human influenced landscape and at the ready to gobble up any resource and fill any niche available.
    Although an interesting proposition, one would have to take the costs and payoff of a society or government taking on such a project. The reason we have these cast of 10 or so domestic animals that we trek along is because it's easy. Balev has proven that domestication can be rapid in animals so if possible (and kinda cool to think about) I don't know if it could actually come into action.

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  10. I really liked this article. I think the thought experiment you wrote is very well informative and puts the situation into perspective. I also think that it is a great idea to learn from this, but I do have to agree with Josef in that i think we would be better off trying to domesticate them. We are a society that is "our way our the highway" and with the selection preference you have shown, it does not look like the cod are going to adapt. But by domesticating them we could increase their fitness (though me personally would like to see other fish domesticated rather than cod). However i do think it is unrealistic to think that we can domesticate all these organisms whom we are forcing evolution upon.

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  11. Everything about this article was great. I personally am very interested in trophic cascades like this one. I first heard about this topic in ichthyology, when talking about fisheries. I was wondering how the other stocks are doing since Atlantic cod are on both sides of the Atlantic. If the other stock is doing well, could there be a way to collect eggs or to transplant some fish with the genes to spawn later in life to the west side of the Atlantic?

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