Exploring human induced evolution in fish

Dr. Mikko Heino. University of Bergen, Norway.Recently scientists have been grappling with a frightening prospect: that the activity of fishing is actually driving evolutionary changes in fish. Like all groundbreaking scientific ideas, the notion of fisheries induced evolution has generated debate among scientists and faces many unanswered questions.

The journal Evolutionary Applications just released a special issue titled Toward Darwinian Fisheries Management devoted entirely to this topic. The issue includes 13 articles written by ecologists and fisheries scientists exploring the different facets of the topic in great detail. I strongly recommend that anyone with an interest in marine conservation or evolutionary biology check this out.

We interviewed Drs. Mikko Heino and Christian Jørgensen - evolutionary ecologists from University of Bergen, Norway and leading experts on the issue of fisheries induced evolution. Together with Erin Dunlop and Dr. Christian Jørgensen. University of Bergen, Norway.Katja Enberg, they were guest editors for the special issue, Toward Darwinian Fisheries Management.

1) When did people start waking up to the reality that fisheries might be causing evolutionary changes in fish, and what prompted this realization?

Heino: The first suggestion that fishing may drive evolution is more than a century old, and there has been sporadic interest ever since. Interest started to increase in late 1980s, but the discussion remained rather academic because empirical evidence was very inconclusive and models were too rough. The real wake-up happened after year 2000. Several things fell into place around that time: methods to analyze data from wild fish stocks were improved and started to suggest that evolutionary changes are commonplace, fast evolution was demonstrated in tank experiments, and more realistic models were developed. This happened against the background of fisheries management entering the era of ecosystem approach, and growing recognition among academics that contemporary evolution is commonplace.

Jørgensen: It is worth mentioning the pioneers of the early work from the 1980s and early 1990s, especially the theoretical work by Richard Law and the combination of field studies, experiments and theory developed by Adriaan Rijnsdorp. Their central papers made fishing-induced evolution impossible to ignore and in many ways gave birth to the field.

2) How did you become interested in the topic?

Heino: My PhD was about life history theory, and I wanted to include something that had more applied flavour to it. I knew about the seminal theoretical work by Richard Law from York, England, and decided to start developing those ideas further. While Richard's work ten years earlier had relatively little following, this time the time was ripe and things started to gather momentum.

3) Can you walk us through the mechanism by which the actions of fisheries can cause evolutionary changes in fish?

Heino:  In livestock breeding, breeders select the individuals with most desirable characteristics to breed, so that "good" genes get more frequent in future generations. Fishing works in directly opposite way: fishermen try to catch fish with most desirable characteristics (read: big fish), and by so doing, make less likely that fish with "good" genes contribute to future generations.

There are also more hidden, yet no less important, facets of fisheries selection. Importantly, increasing overall mortality is also selective because it favors "fast" life histories: there is no point in investing resources to late life if it is very unlikely that you survive so long. Among other things, this favors early sexual maturation. By maturing early, an individual makes it more likely that it leaves at least some progeny. Late maturation allows a fish to grow large and be very fecund once mature, but this pays off only if it survives to maturity.

4) Why should we care about evolutionary changes in fish? What are the  economic and ecological consequences?

Heino: There are two simple reasons why evolutionary changes in fish can be economically disadvantageous. When fish start reproduce earlier and invest more to reproduction instead of growing, potential fisheries yield can decline; moreover, big fish are more valuable per kg than small fish - there is a loss of quantity as well as quality. Smaller fish may also be less resilient to fluctuations in their environment. At the same time, one should acknowledge that evolutionary changes may allow fish populations to tolerate higher levels of fishing. We know little about ecological consequences, but when life histories are truncated, it is possible that food webs get simpler, and the ability of fish to fully utilize their habitat is compromised (e.g., small fish cannot economically migrate as far as a big fish).

Jørgensen: It is important to realize also that current fisheries management is based on how the fish stocks have behaved in the past. When evolution changes life history traits and other traits, then population dynamics and recruitment also change, in the end with implications for quotas and yield. By evolving, the stocks basically bring fisheries management to uncharted territory, where current knowledge has a lower ability to predict consequences of harvesting or of environmental change. To me, the logical consequence in this increased uncertainty in the biological system is to heed the precautionary principle, and therefore relax the pressure exploitation and other human interventions put on these stocks.

5) What are some things that fisheries managers and policy makers can do to address this issue?

Heino: The simplest thing is to keep harvest pressure at moderate levels. This makes sense anyway because this limits also ecological impacts of fishing. In case of overfished fish population, reduced fishing will also make it possible to increase yield and catch fish with less effort, which means fewer emissions and lower costs. In addition, one may need to rethink how minimum size limits are set. Traditionally, minimum size limits are set to protect juveniles so that more individuals have a chance to reproduce at least once. This may have an unwanted side effect by amplifying the benefits of early maturation at small sizes: fish that mature under the size limit are advantaged, whereas those that had tendency to mature at larger sizes are disadvantaged.

Jørgensen: Fisheries institutions have already done a great job by collecting long-term datasets as part of routine monitoring, stock assessments, and to keep track of the catch. These dataseries have had tremendous value in seeing the longer-term perspective, but probably there is much more data lying around that can be analyzed in new ways or at least made available for analysis. It is also important that these timeseries are continued. Although contemporary evolution is lightning fast compared to the geological time-scale, data covering two to three decades can still be short when one attempts to separate any effect of evolution from environmental fluctuations in sea temperature, density dependence, prey abundance, or predators.

6) In terms of our scientific understanding of this topic, can you identify some of the major gaps where scientists disagree or we just don't have enough information?

Heino: The majority of scientists agree that fishing is driving evolution, but not everybody agree that evolutionary changes can be so fast that we should care. There is also controversy about the strength of evidence for evolutionary changes in wild fish stocks: some argue that we can only speak about evolution once it is proven that the changes are genetic through molecular genetics methods. Others, including us, argue that evolution is the most parsimonious explanation for changes we have observed. Also the precautionary principle obliges us to take evolutionary changes seriously while (eagerly) waiting for a definite proof. However, if fisheries managers acknowledge that fisheries-induced evolution warrants attention, the best way forward is not obvious.

Jørgensen: Evolution is creative and although we can predict changes in a few select traits over short time-spans, it only needs a quick look at the baffling diversity in a forest or at a coral reef to understand that there is no way to predict the full breadth of evolutionary responses. Research has so far concentrated on a few characteristics, mostly life history traits related to growth and reproduction. But experiments have for example shown that behaviour may also evolve, which poses important challenges because behaviour is very difficult to study in the wild and mostly we don't have quantitative observations from before fishing started, nor time-series. The creativity of evolution needs to be matched with creativity among the scientists that study these effects.

7) Can you give a quick summary of how some of the studies in Toward  Darwinian Fisheries Management address these gaps?

Heino: In the introduction to this special issue, we highlight that evidence for evolutionary changes in fish comes from many sources. While no single source of evidence is conclusive, taken it all together makes the case already quite strong and should not be dismissed by scientists and by fisheries managers showing prudent foresight. Hutchings gives some suggestions how evolutionary management objectives could be expressed in language familiar to fisheries managers, and in papers by Okamoto and others and Jørgensen and others, some alternative management scenarios are assessed from an evolutionary perspective.

The paper by Enberg and others uses a relatively realistic model of cod-like fish to demonstrate that fisheries-induced evolution is fast, leading to marked changes in the life history after some decades. Moreover, it shows that if fishing were stopped, the evolutionary recovery would be very slow.

Jørgensen: Experiments have been tremendously important to the field because they provide proof of concept and allow a rich diversity of evolving traits to be assessed. Conover and Baumann give an overview of what has been done on the experimental side, and also point to how experimental research can continue to play such a pivotal role.

Sharpe et al. performed a meta-analysis on empirical evidence from the wild, in total 143 time series. They concluded that changes in maturation traits are correlated with the fishing intensity, thus strengthening the interpretation that fishing is the driver of change in these traits. They also concluded that there probably is a genetic change underlying the change in maturation traits.

I also find it exciting that so many different theoretical models are now used to study fishing-induced evolution. These models differ in their structure, in their assumptions, and in their focus. Mostly the results agree, but other times there are interesting differences between them. This diversity of approaches makes it in my opinion much more likely that we will be able, some day, to paint a more complete picture of what is going on in the oceans. Perhaps it will also be possible to turn that evolutionary change into something that ends up being good for the fish, the ecosystems, and for humans.

 --Reviewed by Rob Goldstein

Dunlop, E., Enberg, K., Jørgensen, C., & Heino, M. (2009). Toward Darwinian fisheries management Evolutionary Applications, 2 (3), 245-259 DOI: 10.1111/j.1752-4571.2009.00087.x