Scientists often assume that in the vast oceans, fish can travel far and wide and, as a result, different populations of a species would mix freely. However, according to a new study led by the University of Connecticut (UConn), this does not appear to be the case for a vital forage fish called the sand lance, a species of small schooling fish that is impressively rich in lipids, which makes them a significant food source for over 70 different species ranging from whales and sharks to seabirds.
The experts were interested to see if sand lance constitute a massive, homogeneous population, or whether there are genetically distinct groups among them. These are crucial questions to ask when it comes to the conservation and sustainable management of this species, since, due to climate change, the regions where sand lance live are warming faster than many other places on Earth.
By sequencing and analyzing nearly 300 samples from a variety of locations across this species’ range through a technique called “low-coverage whole genomic sequencing,” the scientists found an area on the Scotian Shelf (off the coast of Nova Scotia), where a genetic break appeared to occur between two different sand lance populations.
The genomic analyses helped distinguish two distinct groups, one north and one south of the divide, with parts of their genomes diverging quite dramatically. Without clear physical barriers – such as a mountain range, for instance – separating the sand lance populations, the existence of these genetic differences represents a scientific conundrum. However, according to the researchers, the answer may lie in the movement of ocean currents.
“When fish from the north reproduce and drift south, they are genetically less adapted to warmer southern waters, even if it’s five or six degrees warmer in the winter, they are just not surviving,” said study senior author Hannes Baumann, an associate professor of Marine Sciences at UConn.
“These populations may be linked by the ocean currents, but the realized connectivity is basically zero. Example after example shows that the ocean is not as homogeneous a place as expected, and there are all kinds of things that prevent that constant mixing. We found another striking example of that.”
Knowing that there are genetic and ecological barriers on the Scotian Shelf is important because, due to climate change, this barrier could move north and create problems, especially for the southern sand lance populations. Fortunately, the fact that there are only two genetically distinct populations, instead of smaller population clusters, could make their management and conservation less challenging.
“We want to make sure that this fish is productive and resilient, despite climate change, so we should make sure these areas where they are occurring are protected. These decisions should include experts to ensure if there’s an area that is very critical to sand lance, that any disturbance is temporary,” Baumann concluded.
The study is published in the ICES Journal of Marine Science.
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