A Kenai River pink salmon, the northern pike of the sea?/Craig Medred photo

A group of Canadian and West Coast U.S. scientists think they have an answer as to why returns of sockeye salmon to the Kenai, Copper and other Western North American rivers have been creeping downward since the 1980s while Bristol Bay has boomed.

Credit, or blame, a warmer North Pacific Ocean that has greatly benefited salmon in streams draining into the Bering Sea, and an abundance of pink salmon that has hurt salmon in streams draining into the Gulf of Alaska.

“…We present evidence that the magnitude and direction of climate and competition effects vary over large spatial scales,” writes Brendan Connors of the Institute of Ocean Sciences with Fisheries and Oceans Canada in British Columbia. “In the south, a warm
ocean and abundant salmon competitors combined to strongly reduce sockeye productivity, whereas in the north, a warm ocean substantially increased productivity and offset the negative effects of competition at sea. From 2005 to 2015, the approximately 82 million adult pink salmon (Oncorhynchus gorbuscha) produced annually from hatcheries were estimated to have reduced the productivity of southern sockeye salmon by 15 percent on average.”

The study scheduled for publication in the peer-reviewed Canadian Journal of Fisheries and Aquatic Sciences suggests that overabundant, at-sea pink salmon are reducing other salmon species via competition for food much as the increasing abundance of northern pike reduces salmon via predation in Cook Inlet streams.

Connors’ co-authors include Michael Malick from the National Marine Fisheries Service’s Northwest Fisheries Science Center in Seattle, and scientists Greg Ruggerone and James Irvine, who are credited with writing the definitive paper on a North Pacific salmon abundance in 2018. 

Their study pegged those salmon populations as the highest in recorded history but found the population dominated by pinks, the smallest and fastest-growing of the Pacific salmon. 

Ever since a debate has been raging within scientific circles about the ecosystem-wide impacts of pink salmon. Alaska is a huge producer of pinks and a world leader in farmed pink salmon, which are hatched and raised by human hands before their release into the ocean.

When they return, they are netted and marketed as “wild caught.”

Alaska Department of Fish and Game’s chief fisheries scientist, Bill Templin, has pushed back strongly against the theory of problematic conflicts between pink salmon and sockeye, coho, and Chinook (king) salmon.

“Correlation is not causation” he reminded the policy-setting Alaska Board of Fisheries in October 2018 when it was asked to consider reigning in the hatchery production of pinks. The Board subsequently refused to cap or rollback production.

A year later, the state’s salmon farmers and their critics did come to an agreement that the almost 2 billion hatchery salmon dumped in the Pacific each year do warrant at least an annual review.

Little fish are big business in the 49th state, especially in Prince William Sound just to the east of the state’s largest city. The commercial-fishermen-run Prince William Sound Aquaculture (PWSAC) – one of two big, commercial hatchery operations in the region – is an almost $200-million economic engine in a region with almost no other business opportunities, according to one study.. 

Good hatcheries, bad hatcheries?

Three years ago, scientists looking for signs of longterm damage from the 11 million gallons of Alaska crude oil the tanker Exxon Valdez spilled in the Sound in 1989 stumbled on indications the hatcheries there were driving declines in salmon returns to the fabled Copper River.

“All sockeye salmon stocks examined exhibited a downward trend in productivity with increasing PWS hatchery pink salmon returns,” they reported in a peer-revied study.  “While there was considerable variation in sockeye salmon productivity across the low- and mid-range of hatchery returns (0–30 million), productivity was particularly impacted at higher levels of hatchery returns.

“We do not know if possible deleterious interactions between hatchery pink salmon and wild sockeye salmon in this study are from predation or competition, or whether they occur in nearshore or offshore areas,” the study says. “Pink salmon feeding may cause a general depletion of prey availability that could impact sockeye salmon without tight spatial overlap of these two species. In this regard, the apparent impact to sockeye productivity may reflect a general increase in pink salmon abundance across the northeast Pacific rather than increased abundance of hatchery pink salmon to PWS (Prince William Sound) in particular.”

Despite the study. Sound commercial fishermen have stood steadfastly behind the hatcheries. Last year, those hatcheries produced about 31 million fish worth $56 million to commercial fishermen, “or 52 percent of the total ex-vessel value for common property commercial salmon fisheries in the region,” according to the Alaska Department of Fish and Game.

“Pink salmon contributed most to the value of the common property commercial harvest ($33 million), followed by sockeye ($11 million) and chum ($10 million) salmon.
The 31 million hatchery-produced salmon harvested in the Prince William Sound commercial common property fishery accounted for 61 percent of the total common property commercial catch in the region.”

The new, oceanwide study in preparation for NRC Research Press claims those profits in the Sound come at the cost of wild fish from the Pacific Northwest to Kodiak Island. To the west of the big iland in the Gulf of Alaska, however, the study says competition between pinks and other species diminishes and ocean warming begins to enter the picture.

Big winner?

Bristol Bay sockeye salmon “survival benefits from greater early marine growth offset the adverse effects of pink salmon on sockeye salmon during late marine life,” the study concludes. “Abundances of both pink and sockeye salmon in the North Pacific doubled after the 1977 ocean regime shift, and greater productivity of Bristol Bay sockeye salmon is associated with greater early marine growth.

“Sockeye salmon originating from the Bering Sea interact with relatively few pink salmon during early marine life and numerous pink salmon during subsequent years when they are distributed farther west, leading to reduced growth, survival, and abundance of sockeye salmon.”

There is now a broad scientific consensus that Bristol Bay salmon have benefitted from a warmer ocean. The Bay witnessed a record harvest of 62.3 million sockeye in 2018 followed by a record value harvest last year when the harvest of 56.5 million salmon was worth $306.5 million at the dock.

The 2019 harvest was the fourth largest in Bay history, worth about two and a half times the long-term average value, and marked the fifth straight year the catch topped 50 million.

Elsewhere the picture is not so bright.

As one moves east to west and north to south in the Gulf, the authors of the new report wrote, sockeye increasingly suffer at the expense of warmer water and competition from pinks.

“The combined effects of a warming ocean and increasing salmon competitor abundance (and their interaction) across the North Pacific shifted from negative to positive across the sockeye range from south to north,” the study says. “At the southern end of their range our analysis predicts a 30 percent reduction in recruits produced per spawner for every increase of 1.5 °C in sea-surface temperature (SST) and 119 million salmon competitors.”

The Kenai and Copper rivers – far and away Alaska’s most popular sockeye salmon systems – are midway between the winning fish in Bristol Bay and the biggest losers in the Pacific Northwest.

The author concede the issue of competition between various salmon species at sea is highly debated, but argue that the hatchery contribution to the number of pinks at sea can no longer be ignored.

“…The abundance of hatchery pink salmon during 2005–2015 (82 million adults per year, or 17 percent of all pink salmon) exceeded the abundance of wild chum salmon and was equal to the abundance of wild sockeye salmon over the same time period,” they note. “In addition, there are strong geographic differences in hatchery production. For example, Alaskan hatchery production of pink salmon represented 18 percent to 49 percent of total annual pink salmon produced in Alaska from 2005 to 2015.

“Our analyses allow us to quantify what the potential consequences of hatchery production may be for sockeye productivity across their range. Using the parameter estimates…we estimate that total hatchery production of pink salmon has reduced sockeye productivity at the southern end of their range by 15 percent on average, over the past decade.

The gradient goes from a 30 percent reduction in the Pacific Northwest to a low of 5 percent in the least affected areas of Alaska. The data, they argue, could be read to indicate “that hatchery production has contributed to the depressed productivity of sockeye salmon in British Columbia, some of which have recently been assessed as at risk of extinction.

“In contrast, above-average sea-surface temperature conditions in the Gulf of Alaska and Bering Sea regions over the past decade are estimated to have largely offset the negative effects of hatchery production on sockeye productivity. In the Gulf of Alaska, hatchery pink salmon production is estimated to have reduced sockeye productivity by 5 percent, on average, over the past decade, while in the Bering Sea the positive influence of above-average SST has led to an increase in productivity of 5 percent, on average, compared with an increase of 10 percent, on average, if no hatchery production had occurred.”

The study has the potential to add to a political-legal firestorm in the Pacific Northwest where environmental groups are already in court arguing a decline in wild Chinook (king) salmon is threatening an endangered pod of killer whales.

The Wild Fish Conservancy, a Washington-state based advocacy group, has charged that Southern resident killer whales are starving to death because of a lack of Chinook – their preferred prey. Chinook are another species in a decline possibly linked to hatchery additions of pinks in the North Pacific.

“Increasing abundances of salmon across the North Pacific, and in particular pink salmon, have been linked to a trophic cascade in epipelagic waters, leading to fewer zooplankton, reduced growth, survival and delayed maturation of salmon, reduced reproductive success of seabirds, and perhaps reduced foraging efficiency of southern resident killer whales,” the new study says.

“Nonetheless, some jurisdictions (e.g., Alaska and Russia) continue to allow increasing hatchery production of pink and chum salmon with minimal consideration of adverse effects on distant salmon populations. Our findings highlight the importance of international cooperation to consider and potentially constrain the number of hatchery salmon released into the ocean to help Pacific salmon adapt to a warming and increasingly uncertain future.”

To date, Alaska has been the big beneficiary of warming. Alaska salmon harvests are now at levels never imagined,but some other West Coast fishermen are starting to wonder if their fisheries have suffered to pay for the boom.