A diet high in low-fat food may have kept Alaska Pollock born in 2013 from gaining the weight they needed to survive over the winter, a new NOAA Fisheries study concludes.
According to biologist Jesse Lamb of the Alaska Fisheries Science Center (AFSC), research results point to poor diet as a contributing factor.
Still there is probably not just one answer, said Lamb, who led the study, which was released in early July, with colleague David Kimmel.
“Cannibalism and wind-driven transport to inferior habitat likely also played a role,” Lamb said. “With that combination, the 2013-year class had the deck stacked against them.”
The study was prompted by observations that while the number of juvenile Alaska Pollock in the Gulf of Alaska that summer was the largest on record, that a year later most of those fish were gone.
The Alaska Pollock fisheries in the Gulf of Alaska and Bering Sea are the largest by weight in the United States and second largest in the world. Alaska Pollock fisheries are recognized as well-managed and sustainable, so understanding how many juveniles will become adults available to the commercial fishery, and why, is vital to their sustainable management.
AFSC biologists have been studying the early lie stages of Alaska Pollock and other fish in the Gulf of Alaska since 1979. Their 2013 Gulf of Alaska survey found the abundance of juvenile Pollock was about six times higher than any on record sine 2000, but surveys in subsequent years showed that very few of these fish survived beyond their first year.
To determine whether diet played a role in the decline of Pollock abundance, Lamb and Kimmel compared juvenile Pollock abundance, size, condition and diet composition in relation to available prey across the Gulf of Alaska study area.
They found that this unusually large proportion of Pollock found in the southwest region of the Gulf of Alaska during that period ate more pteropods and larvaceans, which are small and lower in fat than the relatively larger and fat rich euphausiids and copepods.
“A fish needs to eat a lot of larvaceans to equal one copepod,” noted Lamb. “We think their relatively poor diet may have prevented them from bulking up enough to survive the winter.”
Lamb suggests that diet was just one of a combination of factors that led to the failure of the 2013-year class.
Another recent NOAA Fisheries study found that wind-driven displacement of juvenile Pollock was stronger in 2013 than in any other of the six years of the study. Wind-driven currents carried a large segment of the population from spawning grounds around Kodiak Island to the southwest area of the Gulf. Lamb said researchers have observed over the years that the more fish end up in the southwest area of the Gulf, the less likely a year class is to be successful.
Fish that remain around Kodiak Island seem to be better off,” Lamb said.
He also speculated that a lot of the juvenile Pollock in the southwest area may have competed for euphausiid and copepod prey.
Cannibalism by the very large 2012-year class also may have reduced the 2013-year class, researchers said.