What is fisheries-induced evolution?

Written by George Brett

Since the concept of shifting baseline syndrome was first outlined in 1995, particular attention has been paid to the historical data regarding the size and levels of fish landings. Given the difficult nature of establishing accurate records, researchers have often turned to alternative means of gathering the data instead. One such method is to look at old pictures of past fish landings to help establish a trend or baseline. However, upon inspecting such pictures a worrying phenomenon presents itself – one of an overall decrease in the size of fish over time. This trend of decreasing size is universal amongst commercially fished species and is seen throughout historical records globally. This phenomenon is representative of a process known as ‘fisheries-induced evolution’ (FIE), a process by which anthropogenic fishing efforts become the dominant selective force on fished populations.

industrial fishing gear
Modern fishing technologies and an increased demand for seafood is not only causing overfishing, but also forcing genetic shifts in populations

Fishing efforts are a selective, non-random process by nature. Fishing gear, location and intensity are all designed in a way to achieve maximum sustainable yield and thus maximise profits. International fishing efforts have massively increased since the 1950’s and as a result, so too has the technology and efficiency of fishing gear. Such proliferations have left their mark on global fish populations. The effects are now so evident that some scientists have gone as far to say that fisheries are ‘a large scale experiment on life-history evolution’.

Causes & consequences

Fishing has always been a selective process, hobby fisherman regularly discard small fish and keep larger ‘trophy fish’ and the same applies to international large-scale fishing operations. This results in a concept of ‘fishing down the food web’, a process by which the largest (often predatory) fish are depleted first in an eco-system. Once this stock of larger fish is depleted, fishing efforts systematically move down onto the next largest species until all that remains are small fish and invertebrates. Given the hyper-efficient nature of international fishing it is easy to see how ‘fishing down the food web’ will over time have significant impacts on the populations of different species.

FIE diagram
This diagram represents the general trend in decreasing size of commercial species over time

In addition to fishing the largest species, large-scale fishing operations also target the largest individuals within a species. Practices that permanently remove the largest individuals of a species are also creating a strong selection force in which only smaller individuals will survive and reproduce, over time coming to dominate the population with their ‘small’ genotype. In short, the continual targeting of large individuals over time has reduced genetic diversity in targeted species and actively selected for smaller genotypes to proliferate. So not only are larger species disappearing from our oceans, so too are larger individuals.

As much of modern fishing equipment is species specific and designed to capture as much of a target species as possible, they can also exert a selection bias which can work in favour of non-target species as well. As these populations are not targeted and the fishing gear is not as effective on them, such species may proliferate artificially. The term ‘artificial’ is appropriate here as the anthropogenic selection pressures (in this case fishing) often prove to be stronger than the ‘natural’ selection pressures and therefore shape the surrounding eco-system to a greater degree. As fishing efforts on their rivals continues, these non-target species will continue to benefit from their absence and as a result make the recovery of target species much harder.

modern fishing
Modern fishing methods are so efficient they are not only influencing target species, but non-target fish as well

This issue of genetic proliferation by proxy poses many uncertainties for fish populations. While the current state of fished marine eco-systems may be beneficial to these non-target species, if fishing efforts were to change (a very real possibility given the mounting pressure from environmental groups on governments to enforce stricter regulations) then these non-target species may find themselves at sudden risk of decline. This has led to the paradoxical point where if we reduce fishing activities on some species, then others may suffer as a result.

Evidence of FIE

As previously discussed, there is plenty of observational data to draw upon when it comes to making a case for the concept of fisheries induced evolution. This anecdotal evidence from pictures, catch records and even hobby fisherman presents some compelling evidence.

Photographs provide an excellent insight into historical fishing practices, including the size of their catch

However this evidence fails to answer a fundamental question that concerns the concept of fisheries induced evolution. This question concerns time. Since Darwin published ‘On the origin of species’ in 1859 it has always been assumed that evolution occurs over vast swathes of biological/geological time. Given that we have only recently transitioned into the Anthropocene (evolutionarily speaking) and that global fishing has only become the large scale operation it is today over the last 60 years, the validity of FIE has been called into question on grounds that there simply has not been enough time for significant genetic shifts to take place at the population level.

To address this issue we can look at two important experiments from the last two decades, one set in a laboratory environment and another based on data gathered from the field.

The first study was conducted on Atlantic salmon, Salmo salar, by Solberg et al in 2013, which found that if you subject a population to enough selection pressures a significant change in genotype can be achieved in a very short time frame. During this laboratory experiment a group of Atlantic salmon were subjected to ‘stress conditions’ (reduced water level, twice a day) while the control group was subject to normal ‘hatchery’ conditions. Significant differences in growth rates were identified between the groups initially, with the control group growing at a faster rate. However, the researchers actively selected individuals from the stress group who displayed a lower response to stress and started again with these individuals. After repeating this process over 10 generations a fully domesticated population was created, showing a 300% increase in growth rates under stressful conditions. Such a significant change in genotype over such a small period of time (only 10 generations) shows that – given the right conditions – populations can display a greater level of plasticity than previously thought and that adaptation does not necessarily require the long periods of geological time often assumed.

size selection
The decisions fishermen make regarding the size of individuals they keep and discard can have profound impacts on the overall population

The second study from the field, conducted by Cassoff, Campana and Myklevoll in 2007, provides even more evidence of this. They investigated the effects of overfishing on varying populations of porbeagle, Lamna nasus, a type of mackerel shark that had suffered a fisheries induced population decline of more than 80%. Comparing reproductive data and various vertebrae of individuals from the early 60’s all the way through to 2004, the researchers found significant differences in both age and length of maturity, with newer populations reaching maturity much earlier. Males now reach maturity two years earlier than their 1960’s counterparts and some populations of females mature as much as five years earlier. Such significant changes in species age/length at maturity have occurred over a relatively brief time of 50 years and unlike the previous laboratory example involving Atlantic salmon, these changes can be directly attributed to fishing pressure.

To sum up

The answer to the question ‘what is fisheries induced evolution?’ can be summarised in a single sentence – ‘Fisheries induced evolution is the process by which large-scale fishing activities exert enough selective pressure on fish populations as to be the primary driver of fish evolution, adaptation and eco-system change’.  By comparing and contrasting historical fishing records beside anecdotal evidence and then combining them with experimental evidence from the lab and the field, we are able to build a full-bodied base of evidence which substantiates the claim that large scale fishing activity has had a profound effect on fish populations globally over a short temporal scale. Understanding the causes and effects of fisheries induced evolution will allow researchers, conservationists, and fisheries alike to form robust management plans and conservation models which factor in this phenomenon, better securing the future of the planet’s fish populations.

full net
FIE is a worrying problem, but there is still time to let nature take back control

George is an integrated masters student studying Marine Biology with Zoology at Bangor university in Wales. He has a passion for all things sharks and reefs, with a keen interest in ethology. You can follow him on Twitter @gbrett96 or contact him directly via email at g_brett_96@hotmail.com.

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