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Warm Ocean Temperatures Off NW Coast Forced Forage Fish To Eat Less Energy Rich Food
Posted on Friday, September 28, 2018 (PST)

Warm ocean temperatures in 2015 and 2016 changed the food supply used by forage fish, leaving those fish to feed more on less energy rich gelatinous zooplankton, according to a recent report.


In average temperature (2002 and 2004) and cooler temperature years (2011 and 2012) forage fish feed on plankton, such as tiny crustaceans called copepods and euphausiids, but in recent warm years, they consumed a higher proportion of the low-energy gelatinous plankton.


Plankton and trawling samples collected during these three periods show a general shift in the food web from a system dominated by crustaceans to one with more gelatinous organisms, according to the study and a NOAA Fisheries summation of the study written by Michael Milstein, NOAA spokesman.


Forage fish are consumed by, among other predators, salmon and whales. The study looked at the gut content of such forage fish as northern anchovy, Pacific sardine, jack mackerel, Pacific herring, surf smelt and whitebait smelt off the Washington and Oregon coasts, the study says.


“Effects of warming ocean conditions on feeding ecology of small pelagic fishes in a coastal upwelling ecosystem: a shift to gelatinous food sources,” was published online in the journal Marine Ecology Progress Series (


Its authors are Richard D. Brodeur, research fisheries biologist, and Mary E. Hunsicker, research ecologist, both with NOAA Fisheries’ Northwest Fisheries Science Center, Ashley Hann, University of North Carolina, and Todd W. Miller, NOAA’s Alaska Fisheries Science Center.


Euphausiids, decapods, and copepods were the main prey items of forage fish for most of the years examined in this study, it says. However, gelatinous zooplankton was consumed in much higher quantities in warm years compared to cold years.


“This shift in prey availability was also seen in plankton and trawl surveys in recent years and suggests that changing ocean conditions are likely to affect the type and quality of prey available to forage fish,” the study says. “Although gelatinous zooplankton are generally not believed to be suitable prey for most fishes due to their low energy content, some forage fishes may utilize this prey in the absence of more preferred prey resources during anomalously warm ocean conditions.”


A direct result of feeding on the less-preferred diet was that during the warm years, the fish were smaller and in poorer body condition, likely because they took in less productive food and grew more slowly, according to the authors.


“For a given size, they weren’t as well fed,” said Brodeur. “They just weren’t getting their preferred food, so they did not have the same amount of energy to grow and sustain themselves.”


Scientists are still unraveling the effects of the anomalously warm ocean conditions that dominated the eastern Pacific from 2014 into 2016, and became known as the “warm blob,” leading to persistent high temperatures and low plankton production from the California Coast into the Gulf of Alaska, Millstein wrote.


Still, it’s not clear how these smaller and leaner forage fish impacted the food chain, such as up the chain to their predators, salmon and whales.


However, several species of birds that typically feed on forage fish experienced die-offs along the West Coast, Brodeur observed. In addition, California sea lion numbers declined as mothers struggled to find sufficient food for their young.


“We need a better understanding of the linkages between forage fish and their predators so that we can anticipate and mitigate the ecological impacts of warming events, such as the warm blob, which are expected to increase in frequency and intensity,” Hunsicker said.


“It wasn’t just these fish that were having a hard time,” Brodeur added. “We saw negative effects of the warm blob on multiple trophic levels throughout the California Current.”


More information on ocean conditions is at the NWFSC’s Ocean Ecosystem Indicators page at


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