Industrial fishing has profoundly affected the diversity and abundance of fish populations. Scientists estimate that since preindustrial times the fishing industry has removed nearly half of the world's biomass of commercially caught species, and today their stocks are shrinking in almost every climate zone on the planet.
But it is not only the number of fish that suffers. Fishing, a global industry valued at $400 billion, is also affecting their molecular biology: both the sequences of their DNA and the way the body uses that DNA.
A study of wild fish in the Mediterranean, published in July, adds to a growing body of research on how fishing drives biological shifts. These changes, which unfold both over short spans of time and across generations, amount to a form of human-induced evolution. They can reduce genetic diversity in overfished populations.
Departures in the genetic code and in how it is expressed make it harder for fish populations to withstand the stresses of climate change and disease, and to maintain healthy interactions with other species in their ecosystems.
We are making populations less resilient to what lies ahead.
— Margarida Barcelo-Serra, a molecular ecologist at the Mediterranean Institute for Advanced Studies in Spain
Genetics and Epigenetics
Wild fish populations across the world's oceans are responding to fishing in evolutionary terms, and the scientific literature bears this out. Craig Primmer, an evolutionary genomicist at the University of Helsinki in Finland, has studied fishing-induced evolution for more than a decade. His research has shown that, through artificial selection, wild zebrafish can become smaller and less inclined to explore within a few generations, the same changes seen in commercially fished species. Because of the fish's own habits and the rules set by fishery managers, individuals with certain traits, such as large and dominant ones, are more likely to end up in the catch.
Kwi Young Han, an evolutionary biologist at the Helmholtz Centre for Ocean Research in Germany, reached similar conclusions. He found that over a 25-year observation period, the body length of eastern Baltic cod in his sample shrank by 48%. At the same time, the fish's genetics changed as well: they began to grow more slowly.
Exactly how fishing affects fish genetics, and to what extent it drives evolution, is still not fully clear, Barcelo-Serra says. Over the past quarter century, however, epigenetics has flourished: the study of short-term, reversible changes in how genes work, which can be examined within a single generation. It is a promising tool for investigating changes brought about by the environment. Some epigenetic markers can be inherited from parents, though the mechanisms behind this are not yet fully understood.
If genetics is about which genes we have, epigenetics is about which of them are switched on, and how. Janay Fox, an evolutionary biologist at the Moffitt Cancer Center in Florida, explains that if you think of the genetic code as a book, epigenetic changes do not alter the words on the page. They are more like sticky notes telling the reader to skip certain pages or, conversely, to pay special attention to them.
Epigenetics, Fox says, "opens up the possibility for life experience to influence evolution, both within a single lifetime and across generations."
A Study in the Balearic Islands
Barcelo-Serra and her colleagues set out to assess the link between fishing and genetic and epigenetic changes in the razorfish, or pearly razorfish, a culinary delicacy in Spain's Balearic Islands. The authorities strictly enforce a ban on catching it in the off-season, yet according to the researchers, recreational anglers take as much as 60% of the local population in just the first few days after the season opens.
In July of this year, they published the results of a study comparing razorfish populations off three locations in the Balearic Islands. Each location had two neighboring sites: one in a recreational fishing area and the other in a marine reserve, where all fishing is banned.
The idea that populations of the same species can differ epigenetically depending on their environment, for instance in a hatchery versus in the wild, is not new. What is new about this study, Primmer says, is that both sites in each pair were in geographically similar conditions. The only variable was whether fishing took place or not. The team found that the epigenetic markers of fish from the fished areas and from the neighboring reserves did indeed differ.
The researchers examined a mechanism called DNA methylation, which acts as a kind of switch for genes. Methylation can turn a gene off, partly dampen its effect or leave it fully active. Environmental conditions can trigger the attachment of a particular chemical group, a methyl group, to DNA, which switches a given gene off. Conversely, removing those groups, a process called demethylation, turns it back on. In the study, the genes in question were responsible for immune responses, melanin production, nervous system development, protein synthesis and other functions.
In the case of the razorfish, the researchers found that fishing appears to have a marked effect on the DNA methylation of certain genes. Some genes were more heavily methylated at the fished sites, others in the marine reserve.
Many factors can influence DNA methylation patterns, especially an organism's age. And fishing can remove the oldest (and largest) fish from a population. But the team used a statistical model to correct for this factor, and the patterns held.
The differences in methylation were not dramatic, and not all of the genes studied showed a difference depending on the level of protection. That does not make the results insignificant, however. Even a small change in methylation patterns, Janay Fox says, can have major consequences for an organism's behavior or traits.
She also stresses that the study was conducted in an area of recreational, not large-scale industrial, fishing. In her view, though, this could mean that the epigenetic effect of commercial fishing would be even stronger.
The genetic results were less conclusive than the epigenetic ones. Even so, the marine reserves did, on the whole, have greater genetic diversity than the fished populations. Barcelo-Serra says this may be linked to fishing's effect on age structure: when many fish of the same age, such as larger and older individuals, are removed from a population, genetic diversity declines.
The study is correlational and cannot rule out the many other factors that can cause biological variation. But epigenetic plasticity, the capacity of genetic material to produce different behaviors or physical traits under environmental pressure, speaks to our influence on the animals that become our food. Just not in quite the same way as traditional evolution.
"What we see is that when fishing causes genetic shifts, they are very hard to reverse," Fox says. "But if the changes are plastic, perhaps that offers hope that if we can reduce the pressure or stress that individuals are under, those changes could be reversed more quickly."
The Cost of Industrial Fishing
Many Indigenous communities use traditional fishing methods that take the long-term sustainability of populations into account: catching only as much as they need to feed themselves, for example, using small traps and forgoing large vessels. Industrial fishing, however, took a different path, one that has ultimately depleted fish biomass in oceans and rivers around the world.
Gavin Gibbons, chief strategy officer at the National Fisheries Institute, told reporters by email: "All food production has consequences. Feeding people requires land, water, energy and other natural resources, and that can affect ecosystems, biodiversity, climate, soil and species abundance, but it is necessary for human survival. Whether it is agriculture, fishing or other food systems, no method of food production comes without an environmental impact. While feeding a growing planet is difficult, completely eliminating the impacts of food production is virtually impossible."
Craig Primmer believes it is important to continue research into the genetic evolution and epigenetics associated with commercial fishing. In his view, this may be the best hope for preserving the very possibility of fishing. "The more we know about the biology of species and the influence of external factors, whether fishing, climate change, pollution or anything else, the better we will be able to keep populations and stocks healthy and viable," he says.
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