In recent decades, many commercially harvested fish have grown more slowly and matured earlier, which can result in lower yields and reduced resilience to over-exploitation.
Scientists have long suspected that the rapid evolutionary change in fish is caused by intense harvesting pressure. Now, for the first time, scientists have unveiled the genomic changes that bring about the evolution induced by fishing – changes that had previously been invisible to researchers, according to a study published in Science, on 2 August.
"Most people think of evolution as a very slow process that takes place over millennial timescales, but in reality the evolution can take place very quickly," the lead author said. Nina Overgaard Therkildsen, Cornell assistant professor of conservation genomics in the Department of Natural Resources.
In heavily exploited fish stocks, fishing is almost always aimed at larger individuals. "The slower-growing fish will be smaller and will better escape the nets, with a greater chance of transmitting their genes to later generations. In this way, fishing can cause rapid evolutionary changes in growth rates and other characteristics, "said Therkildsen." We see many indications of this effect in wild fish stocks, but no one knew what the genetic changes were at the base. "
Therkildsen and his colleagues took advantage of an influential experiment published in 2002. Six populations of silversides from the Atlantic, a fish growing no larger than 6 inches in length, had been subjected to intense harvesting in the laboratory. In two populations, larger individuals were removed; in two other populations, the smaller individuals were removed; and in the last two populations, fishing was random with respect to size.
After only four generations, these different collection schemes had led to the evolution of an almost double difference in the size of the adults among the groups. Therkildsen and his team have sequenced the entire genome of nearly 900 of these fish to examine the DNA changes responsible for these surprising changes.
The team has identified hundreds of different genes across the genome that have changed consistently across selected populations for rapid and slow growth. They also observed large linked blocks of genes that changed in concert, drastically shifting the frequencies of hundreds of genes simultaneously.
Surprisingly, however, these major changes occurred only in some populations, according to the new document. This means that in this experiment there were more genomic solutions for fish to enlarge or reduce them.
"Some of these changes are easier to cancel than others, so to predict the impacts of the evolution induced by fishing, it is not enough to keep track of growth rates alone, we need to monitor changes at the genomic level", has affirmed Therkildsen.
When the experiment was originally conducted almost two decades ago by co-authors David Conover, professor of biology at the University of Oregon and Stephan Munch of the National Marine Fisheries Service, tools to study the genomic basis of rapid evolution induced by fishing noted that they were not available. Fortunately, Conover and Munch had the foresight to keep the samples in a freezer, now making it possible to return – armed with modern DNA sequencing tools – and reveal the underlying genomic changes.
Research like this can assess human impacts and improve the understanding of the humanity of "the speed, consequences and reversibility of complex adaptations as we continue to sculpt the evolutionary trajectories of the species that surround us," Therkildsen said.
The good news for the silversides of the Atlantic is that fishing selection has been able to tap into the large reservoir of genetic variation that exists across the natural range of this species from Florida to Canada, Therkildsen said: "That bank Genetics has fueled a rapid adaptation of strong fishing pressure in the face. Similar responses may occur in response to climate-induced changes in other species with great genetic variability. "
In addition to Conover and Munch, the collaborators of "Contrasting genomic displacements at the base of the parallel phenotypic evolution in response to fishing"It included the former postdoctoral researcher Cornell Aryn P. Wilder, now researcher at the San Diego Zoo Conservation Research Institute; Hannes Baumann, University of Connecticut; and Stephen R. Palumbi, University of Stanford This work was funded by the National Science Foundation.
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