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NW Fishletter #387, Nov. 5, 2018

[4] From Anchovies To Zooplankton: Impacts Of North Pacific 'Warm Blob' Linger

Imagine eating jellyfish instead of shrimp. Anchovies can. So can sardines and smelt.

That's what they were apparently forced to do a few years ago, when the Pacific Ocean warmed beyond anything scientists had seen before, and the preferred foods of these forage fish became scarce.

Their feeding situation is also a window into the conditions that young Columbia River salmon faced as they tried to make the major, energy-intensive transition from fresh water to salt water from 2014 through 2016.

A recent study found that, in the midst of that oceanic heat wave known as the Warm Blob, forage fish ate far more gelatinous zooplankton (like tiny jellyfish) and far fewer euphausiids (such as krill), decapods (such as shrimp and crabs) and copepods (tiny crustaceans that feed directly on phytoplankton cells) than they did in years when the ocean's surface temperature was cool, or even average.

The study, published in Marine Ecology Progress Series, is yet another indication of the far-ranging impacts of the anomalous Blob. A high pressure system that stalled over the North Pacific in 2014 and 2015, the Blob was followed by a strong El Niño in 2016, causing warm ocean-water conditions that lasted longer, spread farther and went deeper than previously recorded in the Pacific.

Although conditions in the North Pacific are starting to return to normal, poor salmon runs continue to plague the region. Last month, the Washington Department of Fish and Wildlife took the rare step of closing most of the Columbia River to salmon fishing. An agency official said the closure is at least partly due to poor ocean conditions in 2015, and noted that Chinook runs ranging from California to Alaska are depressed.

On Sept. 25, the U.S. Commerce secretary declared a commercial fishery disaster for salmon between 2015 and 2017, and will distribute $20 million to commercial fisheries in Washington, Oregon and California.

The study looked at several species of forage fish and the contents of their bellies in 2015 and 2016--at the height of the Warm Blob--and compared those to their diets in 2011 and 2012, which had cooler-than-average ocean conditions; and to 2000 and 2002, which were average. Forage fish included northern anchovy, Pacific sardine, surf smelt, whitebait smelt, Pacific herring and jack mackerel.

Scientists found that forage fish ate much greater quantities of gelatinous material during the warm years, and also determined that in general, the forage fish were smaller and weighed significantly less than those in average or cold-water years.

"We were seeing much skinnier fish than we have in the past," said Richard Brodeur, lead author of the study and a research fisheries biologist with NOAA's Northwest Fisheries Science Center based in Newport, Ore.

According to the study, "Forage fish play a central role in the transfer of energy from lower to higher trophic levels." And, it surmises, "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 condition."

The impact of these findings on salmon is twofold, said Bill Tweit, Columbia River fishery coordinator for the Washington Department of Fish and Wildlife. "First, if you look at when juvenile Chinook and steelhead swim into the ocean, they're about the same size as forage fish. Essentially, they are forage fish," Tweit said.

So, he continued, if anchovies and smelt were having difficulties finding krill or other energy-packed foods during the Blob, so were salmon. That means their first year in the ocean, when salmon were already stressed from the journey downstream and the change to a marine environment, they weren't getting high-calorie foods to help them survive the transition, he explained.

"Secondly, as they grow into predators of forage fish the next year, there are still forage fish to eat, but those have a lower caloric content," Tweit said. So again, the salmon that survived the first blow were hit with another during their second and perhaps their third and fourth years in the ocean.

Tweit noted that some salmon--when the northern California Current ecosystem is not productive--are able to escape some of the hazards of poor ocean conditions by traveling north, to Alaska. "Unfortunately, this time, in addition to the California Current ecosystem, the Gulf of Alaska was equally or maybe more affected," he said.

The study hypothesizes that the warming altered the prey available to forage fish, which led to the change in their diets. That premise corresponds to what's known about the availability of prey from plankton and trawl sampling in 2015 and 2016, which found several order-of-magnitude decreases in euphausiid abundances--or krill--and a marked increase in salps, pyrosomes and other gelatinous organisms in 2015 and 2016.

Pyrosomes are colonial gelatinous plankton previously considered to be tropical or subtropical. They first started appearing in the northern Pacific off the coast of Washington and Oregon in 2014, and by the summer of 2017, appeared in unprecedented numbers along the entire West Coast, reaching the western Gulf of Alaska, according to a recent article in Ecology. The article concludes that pyrosomes could become more permanent residents of the northern Pacific, with "the potential to restructure energy flows."

Brodeur said pyrosomes are not the gelatinous material found in the stomachs of forage fish in his study, although some new data is showing that juvenile salmon, and even adult salmon, are eating these low-energy pyrosomes.

Brodeur also co-authored a study published in Marine Ecology Progress Series in February 2017, demonstrating the Warm Blob's impact on Chinook salmon, noting that high percentages had empty stomachs in 2015, with fish weighing 17.6 percent less than the same length fish in a cold-water year.

In the recent forage fish study, the authors note, "Although gelatinous material was detected in the stomachs of all forage species except jack mackerel and whitebait smelt during the earlier average years, and to a lesser extent, in the cool years, the levels of occurrence increased dramatically in 2015 and 2016. In fact, we detected at least some gelatinous material in all of the Pacific sardine, whitebait and surf smelt examined in 2016."

Brodeur said although scientists have studied the diets of both forage fish and salmon and know what they like to eat, there's much left to discover about the links between different levels of the food web, and how changes in one part of the food web may affect another.

He said while populations of krill appear to be recovering after the Warm Blob, pyrosomes--which only appeared in northern Pacific waters in 2014--are continuing to flourish. "They're a tropical species," he said. "They shouldn't be here, but they may be established permanently in our waters."

Tweit said that it makes sense to him that the ocean hasn't snapped back to normal or pre-Blob conditions. "That was a tremendous amount of thermal energy, and that thermal energy doesn't just wander away. It's gone somewhere. Just because we're not seeing it at or near the surface doesn't mean it's not there," he said. "We still don't truly understand how that much thermal energy got concentrated."

And because Chinook spend two to four years in the ocean before making their long journey home to spawn, they are among the most impacted by this long-lasting maritime warming. Tweit said he doesn't have much hope Chinook stocks will come back next year.

"We're concerned about next year," he said. "We've looked at the jack counts for summer Chinook already, and they look very poor--as poor as we've seen in a long time. We haven't gotten a read out on fall Chinook, but we're not expecting any dramatic turnaround."

Tweit's concern extends farther than the Columbia River Basin. He said poor Chinook runs are being seen from the Yukon River in Alaska to the Sacramento River in California, and most rivers in between.

"That's unusual," he noted. "Usually, when they're doing poorly in one, they're doing well in another. I've never seen a situation like this, where Chinook have been in this much trouble throughout their entire range." -K.C. Mehaffey

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Publisher/Editor-in-Chief: Mark Ohrenschall, Editor: K.C. Mehaffey
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