Predator vs prey

The predator-prey mess in the ocean/Alaska Department of Fish and Game


A news analysis

The Bristol Bay sockeye salmon season is coming off another record year, and crab numbers in the Bering Sea are crashing.

The Alaska Department of Fish and Game Monday announced it was closing the crab fisheries there because of stocks have fallen below the  “regulatory threshold for opening a fishery.”

Bycatch in trawl fisheries working far offshore are being blamed by many,  but the situation is nowhere near that simple, according to fisheries scientists, and all those Bay sockeye, which grow fat feeding in the Bering Sea, are somewhere in the mix.

The question isn’t whether they have played a role in the crab decline, but how big of a role they have played.

It’s complicated.

Tiny prey

Larvae of the crustaceans that comprise the order Decapoda – crabs, shrimp, prawns and lobsters – are well-known prey for young salmon.

 Scientists studying sockeye in the nearshore waters of Kodiak in the Gulf of Alaska in 2001 reported that the “dominant prey of sockeye salmon (there) were decapod larvae.”

There are, however, great variations in what juvenile salmon prey on in the North Pacific Ocean from region to region and year to year, and nobody knows exactly the relationship between crab larvae and sockeye smolt in the Bering Sea at the moment.

Still,  scientists from the National Marine Fisheries Service’s Alaska Fisheries Science Center in Seattle who looked specifically at the possible influences of juvenile sockeye on crab populations way back in 1994 concluded that even low levels of crab consumption by juvenile sockeye could prove a factor in crab declines.

A predator-prey model they built indicated that if crab “zoea (the larval form of crustaceans) were estimated to comprise 1 percent of the diet and were available to the juvenile sockeye salmon for 30 days..the results of the bioenergetic model indicate that smolt-induced mortality can be
large at high levels of juvenile abundance and low levels of zoea crab.”

“At high and average levels of sockeye and average zoea abundance,” they wrote, “the impact is not great at a 1 percent consumption rate. However, increasing the consumption rate a few percent at this level could reduce survival and subsequent” survival rates for young crab.

Today, sockeye levels in the Bering Sea are best described as above high and zoea abundance below low.

Winners and losers

When the Fisheries Science Center model was built in 1994, sockeye returns to the Bay averaged under 50 million per year. The five-year average for the first years of the 1990s was 47.3 million, according to University of Washington School of Fisheries data, or about 73 percent of the latest five-year average.

Sockeye salmon populations were then thought to be booming, but they were nothing compared to what has been seen in recent times. The return of 79 million this year was the biggest ever, according to the Alaska Department of Fish and Game and it followed on a string of banner years:

Just over 66.1 million sockeye in 2021; 58.2 million in 2020; 56.5 million in 2019, at that time the fourth largest return in the fishery’s 100-year history; and 62.3 million in 2108, the largest return in Bay history as of that time.

The record was bested by the return of 2021 and dwarfed by the run of 2022, which helped push the five-year average return to 64.8 million, a staggering 49 percent above the 20-year average of 43.6 million from 2002 to 2021 and nearly double the average run size of 27.5 million from  1990 to 2010.

But the number of adults returning is just part of the equation.

Studies of sockeye salmon smolts indicate only three to 10 percent of those that go to sea survive to spawn.

Using the larger figure, one can calculate that it would require about 500 million smolt to produce the 50 million return of sockeye in 1994. To get the 79 million return this year, about 790 million smolt would need to go to sea and start feeding.

That’s a 58 percent increase in the number of hungry little mouths factored into the 1994 study, and if more than 1 percent of their diet comprised larval crab, well, it’s easy to envision that sockeye smolt collapsing the crab population.

This is how things work in the old battle between predator and prey even if the predators (sockeye smolt) and prey (crab larvae) are tiny compared to the moose and wolves which have dominated the discussions of predator-prey relationships in the 49th state.

But wait, there’s more. All of this is now being compounded by other environmental factors.

The waters of the Bering Sea have also warmed and the “cold pool” of water there has shrunk. Both of these influence the survival of salmon and crabs.

“It’s all got to be related,” as Milo Atkinson, a fisheries scientist at the University of Alaska Fairbanks observes. “Warmer ocean and land, loss of the cold pool, etc. Hard to say exactly how though. Even harder to predict what’ll happen next year.”

Or in the years after.

The sea miners

Against this complicated backdrop, it is easier to just blame the trawlers, the big boats which mine the sea with nets and which all real Alaskans hate given that they are part of an offshore fishery worth more than a half billion dollars that hauls most of the profit back to the port of Seattle.

And never mind that a good-sized part of the rest of the Alaska fishing fleet is based there.

National Fishermen magazine puts employment in the Bering Sea pollock fleet, the biggest segment of the trawl industry, at an estimated 30,000 workers and notes the U.S. Department of Commerce reported the value of pollock hauled aboard trawlers in 2019 “added up to $532.7 million for a harvest of more than 1.4 million metric tons.”

That so-called “ex-vessel value” is the lowest way of valuing the catch. First wholesale prices typically triple or quadruple value, and value just keeps going up until the fish hit the retail market. 

The largest single-species fishery in the water off Alaska, the pollock fishery accounts for 57 percent of the volume of the Alaska seafood harvest and other typical trawl-caught species –  mackerel, cod and various flatfish – comprise another 25 percent of the total harvest, according to a report prepared for the Alaska Seafood Marketing Institute. 

Salmon account for 14 percent, and crab but 1 percent.

Trawl-caught Alaska fish are, ironically, the Alaska seafood most Americans eat while being pound-for-pound the fishery least valuable for Alaskans, presenting fishery managers with a head-on clash between the national interest and the state interest.

When the trawlers were owned and operated by foreigners in the 1960s and 1970s, the conflict between offshore fisheries and nearshore fisheries was easily resolved. Congress passed the Magnuson-Stevens Fishery Conservation and Management Act that created a fisheries conservation zone extending 200 miles offshore from the U.S. coastline, and then banned the Japanese, Russians and other non-American trawlers from fishing there.

American-owned trawlers eventually moved in, however, and Alsakans are again crying foul even though the bycatch rates in the existing fishery are a fraction of what they were in the bad, old days of foreign trawling.













4 replies »

  1. Craig, Thanks for keeping us thinking about stocks and fisheries important to Alaskans. There are lots of factors affecting the health and abundance of fish and shellfish stocks and lots of scientific assessments of what is known, or not known, about their causes and effects. Readers, and managers, are lulled into thinking that environmental and biological interrelationships are so complicated that we really don’t know what limits fish and shellfish production, why stocks decline, or how to manage for abundance.

    I suggest the key to healthy stocks and sustainable fisheries is simply to maintain natural environmental conditions and natural distributions and abundances of spawners. Managers have little control over predator-prey relationships, or, climate change, ocean acidification, the Aleutian low, Russian hatchery salmon, etc. Unfortunately, managers are also challenged by industry pressures to open fisheries in the face of these non-management uncertainties.

    I suggest that Bristol Bay sockeye salmon populations and stocks are healthy because natural watershed conditions persist and spawners have been naturally distributed and abundant. Stock-recruit relationships clearly show that Bristol Bay sockeye returns and harvests are positively related to the distribution and abundance of spawners.

    Likewise, I suggest that Bering Sea crab stocks have declined (again) because overfishing has compromised the natural distribution and abundance of spawners. Fisheries management is an experiment. Just because we want to harvest more crab than the populations and stocks can sustain doesn’t mean we should harvest more crab.

    • Ben: I’d largely agree. But there’s no denying BB sockeye have gotten a big boost from global warming. Or, crazy as it sounds, that we could harvest them at rates designed to reduce future returns to historic levels. Would be interesting to see what that experiment would mean ecosystem-wide. Hard to predict. It could simply boost Russian pink returns on the other side of the Bering Sea.

  2. The state of Alaska needs to bring back its state income tax. If not, stop whining about the money made by out of state workers packing the money out of Alaska.

    • The issue isn’t with individual workers. The issue is with business. A whole lot of business spins around the trawl fleet.

      As for an income tax, it would bring in pitifully little from seafood workers in general, and it might well prove illegal to hit offshore works with a state tax. Until they enter state waters at three miles from shore, they’re technically not working in the state.

      P.S. Alaskans love to whine. Especially in the winter when it is cold and dark.

Leave a Reply