A school of sardines in the Pacific. (Photo: Monterey Bay Aquarium)

Forage fish feed on less nutritious organisms owing to ocean warming

A new study finds that anchovy, sardine, herring and other forage fish are consuming less nutritious organisms because unusually warm temperatures across the eastern Pacific Ocean in recent years drained the food web of many energy-rich plankton they used to feed on.

To reach this conclusion, a group of scientists from NOAA Fisheries and the University of North Carolina compared the stomach contents of fish collected during recent unusually warm years such as 2015 and 2016 with samples collected during relatively cool years such as 2010 and 2011.

In both warm and cool years the fish preyed on plankton such as tiny crustaceans called copepods and euphausiids, but in the recent warm years, they consumed much a much higher proportion of the low-energy gelatinous plankton.

These researchers explain that plankton and trawling samples collected during the same period showed a general shift in the food web from a system dominated by crustaceans to gelatinous organisms.

The study, published in in the journal Marine Ecology Progress Series, opened a window on the way changing ocean conditions reverberate through the ecosystem, with the scientists concluding that, “some forage fishes may utilize this prey in the absence of more preferred prey resources during anomalously warm ocean conditions.”

According to the researchers, 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.

“For a given size, they weren’t as well fed,” said Richard Brodeur, research fisheries biologist with NOAA Fisheries’ Northwest Fisheries Science Center. “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.

“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,” said Mary Hunsicker, co-author and research ecologist with the Northwest Fisheries Science Center.

“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.”
 
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