Fish are undergoing rapid evolutionary changes due to intense harvesting, according to a new paper published on 2 August in Science (1). And as a result, becoming less resilient to overfishing, growing slower, and maturing much earlier, leading to reduced yields.
One hallmark of the Anthropocene — now recognised as the Earth’s current epoch — is the rapid change in the way humans have affected other species. In other words, human-induced environmental change has lead to marked evolution on decadal or even shorter time scales, explain Jørgensen and Enberg in an accompanying perspective entitled “Fishing for answers” (2).
“Most people think of evolution as a very slow process that unfolds over millennial time scales, but evolution can, in fact, happen very quickly,” says Dr Nina Overgaard Therkildsen lead author of the study and assistant professor of conservation genomics at Cornell University.
Fishing vessels understandably target the biggest fish with the highest value. Moreover, the smaller slow-growing fish that manage to allude fishing nets are, therefore, more likely to pass their genes on to the next generation. This is how “fishing can cause rapid evolutionary changes in growth rates and other traits”, explains Therkildsen.
And while changes in size are increasingly observed in wild catches, for the first time, scientists uncover the underlying genetic reasons for this. More specifically, they identified two ways in which these so-called phenotypic changes — differences in physical appearance — occur: large blocks of genes change in concert, dramatically shifting the frequencies of hundreds of genes all at the same time; and hundreds of individual genes were consistently modified across the genome between populations selected for fast and slow growth.
To do this, the team of researchers sequenced the full genome of almost 900 fish to examine the DNA-level changes. The samples were, in fact, from an influential lab-based study performed in 2002.
The prior experiment investigated changes in six populations of a small fish — no bigger than six inches or around 15 centimetres in length— called the Atlantic Silverside. In two populations, the largest individuals were removed; in another two populations, the smallest individuals were removed; and in the final two populations, the fishing was random with respect to size.
At the time, tools for studying the genomic basis of the rapid fisheries-induced evolution were unavailable. But fortunately, the researchers had enough foresight to store the samples in the freezer. Amazingly, the new analysis now shows that in addition to the observed visual changes, evolutionary changes in the remaining populations occurred after just four generations — resulting in a two-fold difference in sizes between groups.
The new findings clearly suggest that simply tracking growth rates is not enough, genetic changes must also be monitored. Therefore “conservationists who assess a population as threatened because of its small size should extend this concern to somewhat larger populations that were recently or are currently under strong selection”, write Jørgensen and Enberg.
(1) Therkildsen, N. O. et al. Contrasting genomic shifts underlie parallel phenotypic evolution in response to fishing. Science (2019). DOI: 10.1126/science.aaw7271
(2) Jørgensen, C. and Enberg, K. Fishing for answers. Science (2019). DOI: 10.1126/science.aay3158