Size declines in Alaska salmon/Nature Communications

Alaska’s decades-long bonanza of pink salmon has now been fingered as one of the likely suspects in a roller-coaster decline in size of four other species of Pacific salmon.

After examining a dozen reasons why chum, coho, Chinook and sockeye salmon have been shrinking, researchers who published in the peer-reviewed Nature Communications on Wednesday said “the only consistently negative effect across all species was that of Alaskan pink salmon abundance, although this effect was weak in most species.

“(But) intriguingly, the shared acceleration of size declines post-2000 occurred during a period of unusually high (though variable) pink salmon abundance in Alaska, suggesting high pink salmon abundances could be accelerating or exacerbating size declines. Our results provide further evidence that wild and hatchery-enhanced pink salmon abundance in the North Pacific has reached such high levels that they appear to be exerting an influence on ecosystem structure and function.”

That influence is also generally negative, concluded the team of scientists from the University of California, the University of Alaska, Canada’s McGill and Simon Fraser universities, Virginia Polytechnic Institute and State University, the Washington Department of Fish and Wildlife, the Alaska Department of Fish and Game, and GKV & Sons, an independent consultancy.

“Compared to salmon maturing before 1990, the reduced size of adult salmon after 2010 has potentially resulted in substantial losses to ecosystems and people,” they wrote. “For Chinook salmon, we estimated average per-fish reductions in egg production (of) minus 16 percent, nutrient transport (of) minus 28 percent, fisheries value (of) minus 21 percent, and meals for rural people (of) minus 26 percent. Downsizing of organisms is a global concern, and current trends may pose substantial risks for nature and people.”

Burden of riches

Driven largely by catches of pinks – or the fish Alaskans often call “humpies” because of the humpbacked shape of males on the spawning grounds – the 49th state has seen record and steadily increasing salmon harvests since the 1980s when the decadal, all-species average hit 122.4 million. 

Coming in the wake of the cold-water years of the North Pacific that reduced the average annual harvest to 47.1 million for the 1950s, ’60s and ’70s, the ’80s marked what some thought was destined to be a highwater mark.

They were wrong – with Alaska wild runs boosted by hatchery production (Alaskan’s call it fish ranching rather than fish farming and sell the ranched fish as “wild caught) – fishermen just kept mining the ocean for more scaly siver:

• 157.5 million per year on average in the ’90s.
• 167.4 million per year on average in the 2000s.
• And about 180 million per year on average in the 2010s.

The state record harvest came in 2013 when commercial fishermen caught more than 280 million salmon.  Pink salmon comprised about 80 percent of the commercial catch that year, according to Fish and Game data,  and more than 40 percent of those 219 million pinks were hatchery fish.

Pinks are the smallest of the Pacific salmon.  A short-lived species with two distinct populations – odd-year fish and even-year fish – Alaska humpies return in significantly greater numbers in the odd-numbered years than in the even-numbered years.

Commercial fishermen caught more than 129 million of the two- to five-pound salmon last year when they made up 62 percent of the statewide harvest of almost 207 million salmon of all species, according to the Alaska Department of Fish and Game.

The pink harvest this year appears unlikely to reach half that, and the all species catch could be under 110 million. That would mark it as the lowest harvest since 1988, according to Fish and Game data, and this despite another banner year for prized sockeye salmon in Bristol Bay.

Away from the big bay at the southern edge of the Bering Sea, there are some indications the odd-year productivity of pinks could be extending the natural oscillation between even- and odd-year fish to other salmon species even though most others spend years at sea.

Pinks, on the other hand, go to sea in the spring of one year, eat their way west then south and eventually east and north as they ride the currents of the North Pacific, and return to spawn and die as adults only two years after their parents buried eggs in Alaska gravels.

Disruption

Already forced to adjust to warming waters and shifts in prey species as the climate shifts, chum, coho, Chinook and sockeye salmon appear to face a compounding problem in the competition for food with the exploding population of pinks, the researchers said.

Scientists Greg Ruggerone from Seattle and James Irvine from British Columbia Canada, have calculated there are now more salmon in the Pacific than at any time in recorded human history, and most of them are pinks.

The two scientists have for some time suspected that food competition with pinks is stunting the growth of other salmon, and the new research appears to support that suspicion.

For salmon, the researchers added, few traits “are as profoundly important as body size, given its role in reproductive fitness, physiology, demography, predator–prey dynamics, and value for human use…(but) understanding the causes of body size declines is daunting given the influence of numerous, potentially interacting factors.”

To start to sort this complex picture, the team started by looking at 60 years of data on 12.5 million fish that had been measured by biologists across Alaska dating back to before Statehood.

What they found was that Chinook – or king salmon as Alaskans more often call them – started shrinking in the 1980s and have been in a steep decline since the start of the new millennium. The other three species of salmon, on the other hand, declined in the 1980s into the 1990s, spiked upward as the new millennium approached, and have been nose-diving ever since.

“Changing age structure (younger age-at-maturity) consistently explains a greater proportion of overall size changes than do changing growth rates (smaller size-at-age),” they wrote. “Salmon are getting smaller primarily because they are returning to reproduce at a younger age than they did in the past. Climate change and competition with highly abundant wild and hatchery-produced salmon appear to be widespread drivers of size declines.”

Although the shrinkage to date has been small enough to go unnoticed by most Alaskans, the researchers warned that “serious consequences for ecosystems and people could result from salmon size declines.

“Smaller salmon transport less marine-derived nutrients and produce fewer offspring. Smaller salmon could threaten food security in rural salmon-dependent communities, where diminished access to calorie-rich salmon directly influences well-being and human health.

“From an economic perspective, smaller salmon translate to lost commercial fisheries profit due to reduced flesh recovery rates (proportionally more skin, viscera, and bones but less muscle), increased processing cost, and lower prices.”

More fish could, however, offset the costs of smaller fish in some cases, they admitted. Alaska last year set a record for value with the harvest of 206.9 million salmon netting commercial fishermen a record $657.6 million. Sockeyes, primarily from Bristol Bay, accounted for about 62 percent of that value but only 27 percent of the harvest, according to Fish and Game.

While loses due to the shrinking size of fish in some areas can be made up by higher volumes in those areas where larger numbers of fish are returning, “the opportunity for mitigation will be limited for species like Chinook salmon that have generally experienced declines in abundance concurrent with size declines or for ecosystem services for which abundance cannot replace size,” the researchers said.

“For example, recreational anglers highly value catching large fish, which influences decisions on fishing trip destinations. In addition, abundant species like sockeye and pink salmon cannot replace many ecosystem services provided by Chinook salmon because Chinook salmon generally have much greater migration distances, fat content, and cultural importance.”

Still, the researchers admitted, “for salmon in Alaska, the extent to which body size is changing across species and regions, the causes of size changes, and the consequences for nature and people are poorly known.

And the drivers of changes may vary considerably by species and regions.

“Some factors emerged as particularly important for individual species,” the researchers said. “For sockeye salmon, North Pacific pink salmon abundance had a particularly strong negative association with body size. For chum salmon, a strong negative association with the North Pacific Gyre Oscillation (NPGO) contrasted with a similarly strong positive association for coho salmon. No single factor was a particularly important predictor of body size in Chinook salmon; instead many factors had moderate contributions to body size change.”

The results, they said, show “that salmon might be responding to one or more physical or biological drivers that were not included among the environmental” variables and even among the responses to the variables studied the researchers found significant variations.

Warmer summers appear to increase sockeye production while decreasing coho numbers. The Pacific Decadal Oscillation, which brings a shift in Gulf of Alaska water temperatures, benefits chum, coho and Chinook salmon, sockeyes suffer.

Warmer waters nearshore in summer help Chinook and cohos, but hurt chums and sockeyes. The picture is complicated, but the study did largely rule out climate change as the primary factor in shrinkage at this time.

“Relationships between salmon body size and temperature did not fit the predictions of the metabolic theory of ecology,” they added. “Rather, the variable influence of climate drivers suggests that the impact of climate on salmon body size is species-specific and to a lesser extent location-specific, perhaps occurring through climate-mediated changes in food availability or quality.”

While climate-driven shrinkage might be hard to get a handle on – if it exists at all – the scientists said there is a clear connection “between sockeye salmon body size and the North Pacific-wide abundance of pink salmon. This result corroborates previous studies documenting negative influences of Asian pink salmon abundance on Alaskan sockeye salmon, which share similar prey communities and distributions during their final years at sea.”

The state of Alaska now manages both hatchery and wild pink salmon runs for maximum sustained yield (MSY). The Alaska Constitution stipulates that “replenishable resources belonging to the State shall be utilized, developed, and maintained on the sustained yield principle, subject to preferences among beneficial uses.”

That has been read by some to mean all salmon runs should be managed for MSY. No one to date has suggested what the state Board of Fisheries should do in situations where maximizing the yield in one fishery might actually reduce the yield in another.

The Board was asked to reduce hatchery pink production in Prince William Sound in 2018 in the interest of Cook Inlet sockeyes, but refused to do so. Lobbied heavily by commercial fishing interests, the Board decided it couldn’t limit hatchery production without strong evidence proving hatchery operations are harming wild fish.

Bill Templin, the state’s chief fishery biologist, told the Board that such evidence doesn’t exist. No one can really predict what will happen once fish disappear into the big black box of the Pacific, and thus no one can prove adding ever more hatchery pink salmon to the ocean will harm wild fish, he said.

Scientists know there is a limit to the carrying capacity of the marine environment, he admitted, but they don’t know what the limit. Hatchery advocates believe the limit will make itself evident when hatchery returns start to falter.

By and large, that has yet to happen in Alaska. But if the lastest study and some before it are to be believed, competition between hatchery pinks and wild salmon can have consequences for the latter even before declines in hatchery production appear.

The operative word there is “could.” As Templin observed, correlation is not causation, and causation in the complicated ecosystem hidden beneath the waters of the North Pacific is damn hard to prove.