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 (https://www.int-res.com/abstracts/meps/spf/av3/
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 https://www.nwfsc.noaa.gov/research/divisions/fe/estuarine/oeip/b-latest-updates.cfm.