North Atlantic cod populations haven't just decreased in number during the last century — their genetic diversity has plunged, too, which could ultimately impede cod from traveling long distances, according to a new study published in the June 26 issue of Science Advances.
In particular, overfishing, climate change or a combination of factors may have led to the loss of a closely-linked group of genes — a "supergene" — associated with migratory behavior. Absence of this supergene may interfere with the cod's ecological niche and could make populations more vulnerable to future collapse.
Researchers investigated genetic variation in Northern cod around the Canadian province of Newfoundland and Labrador, which have experienced one of the steepest population declines of any marine species. The rise of massive super-trawler fishing in the mid-20th century brought consumption to an unsustainable peak, and in 1992 the fishery collapsed.
For the first time, the researchers found evidence in cod DNA matching the population changes undergone by the species.
"We find that the genetic diversity in Northern cod drops during the last century and that these changes over time seem to differ between genetic groups," said Tony Kess, a scientist at Fisheries and Oceans Canada and the first author of the study. "Genetic reconstructions indicate both groups collapsed substantially since the 1970s, and this timing coincides with increased [fishery] exploitation."
The team analyzed the genomic impact of overfishing on Northern cod stock around Newfoundland and Labrador, sampling thousands of DNA variations across the Northwest Atlantic range. They identified a group of closely-linked neighboring genes inherited as a group by some members of the population — a chromosomal rearrangement that functions as a "supergene."
By comparing DNA variations with data on the tendency of particular North Atlantic cod populations to migrate or remain in one location, the team determined that the supergene is associated with migratory behavior. While this finding was previously reported in smaller scale studies in Labrador and Norway, this study expanded it across the entire Northwest Atlantic range.
Cod populations containing more individuals with the supergene also contain more fish that seasonally migrate or swim vast distances to find new habitats, while populations with fewer individuals containing the supergene tended, on the whole, to stay put within a given area, the researchers found. They suggest the supergene may equip cod with higher-functioning swim bladders and greater muscular efficiency required to migrate long distances.
However, these migratory superpowers come with a catch. The researchers found that populations with individuals with two copies of the supergene were hit harder over time, with evidence of decline from the mid-1900s onward. Fish with only one copy of the supergene and those lacking it altogether had a period of population productivity between 1900-1970, corresponding with an increase in cod, followed by decline that matches the cod stock collapse in the 1990s.
"What our work seems to suggest is that individuals within a population may not all respond equally to disturbances such as harvest or climate change," said Kess. "Differences in responses between individuals will ultimately shape the genetic diversity and abundance of the population over time, with potential consequences for how populations look and behave."
The researchers fear that continued decline of cod populations due to overharvesting, climate change and other factors may lead to the loss of the supergene, limiting the cod's ability to migrate. This could change the cod's widespread distribution, altering marine ecosystems throughout the Northwest Atlantic. Since the presence of diverse DNA within a species provides a protective buffer against threats such as disease, loss of the supergene could also increase the cod's risk of future extinction.
"Ultimately, the maintenance of this biological diversity within populations and species is expected to promote stability and persistence over time and will likely be important to successful management and conservation of exploited marine species," said Kess.
[Credit for associated image: NOAA]