(This column is adapted from a longer piece published in the North Pacific Anadromous Fish Commission Newsletter.)
By Gregory T. Ruggerone (Natural Resources Consultants, Inc.), James R. Irvine (Fisheries and Oceans Canada, Pacific Biological Station), and Brendan Connors (Fisheries and Oceans Canada, Institute of Ocean Sciences)
At the Third NPAFC-IYS Virtual Workshop on Linkages between Pacific Salmon Production and Environmental Changes that took place in May 2021, we hypothesized that an overabundance of salmon, combined with effects of recent marine heat waves, may have been responsible for unexpectedly low returns of all five species of Pacific salmon across the North Pacific in 2020.
We were subsequently invited to provide this updated summary of our report for the NPAFC Newsletter so that our findings could reach a broader audience. We hope that our article, which includes updated and preliminary estimates of commercial catch for 2021, will stimulate further discussions about the interactions of salmon at sea, especially in light of the dynamic and shifting ocean environment in which they live.
Since everyone wants more salmon, the question “Are there too many salmon in the North Pacific Ocean?” might seem odd. However, it is worth considering the potential effects of healthy and abundant salmon populations that often migrate thousands of kilometers at sea where they intermingle and compete with distant depleted populations for prey.
It is not surprising that fishery managers are primarily concerned with maintaining those populations that return to regions they manage with little consideration for how these populations might adversely affect other salmon. Likewise, hatchery managers release large numbers of juvenile salmon to maximize harvests in nearby salmon fisheries, often with little consideration for, or understanding of, potential competition effects on other distant salmon populations that compete for the same common pool of resources at sea.
The numbers of Pacific salmon surviving to adulthood increased following the 1977 ocean regime shift, peaking in 2018 when approximately 950 million pink, chum, and sockeye salmon returned from the ocean. This increase was likely the result of favorable ocean conditions combined with the release of large numbers of hatchery-origin juvenile salmon. Releases of hatchery salmon into the North Pacific reached approximately 5.5 billion juvenile salmon in 2019, a sharp increase since the 1960s when approximately 0.6 billion hatchery salmon were released each year. Approximately 40% of the total salmon biomass in the Pacific during 1990 to 2015 was made up of hatchery salmon, especially chum and pink salmon. Clearly hatchery salmon are now key components of the epipelagic North Pacific Ocean. Then in 2020, the harvest of Pacific salmon unexpectedly and precipitously declined. We wanted to know why.
In 2019, salmon abundance remained exceptionally high (~854 million salmon). Together the 2018/2019 period was the highest two-year period of salmon abundance on record since 1925, nearly 20% greater than the previous two-year high in 2009/2010, and more than 3.2 times higher than average abundance during relatively low salmon production years from 1960 to 1975.
Pink salmon dominate the abundance of Pacific salmon returning from the North Pacific, reaching approximately 700 million maturing fish in 2018 and nearly 640 million fish in 2019. The exceptional return in 2018 was highly unusual because pink salmon abundance is typically highest in odd-numbered years.
Approximately 88% and 68% of the total pink salmon abundance were from Asia in 2018 and 2019, respectively. In contrast, peak abundance of pink salmon in North America occurred in 2013 and 2015 (more than 300 million fish per year). Overall, pink salmon represented approximately 74% of total salmon abundance in 2018/2019.
Most pink salmon are of natural origin, but abundance of hatchery pink salmon during 2005 to 2015 was greater than abundance of wild chum salmon and approximately equal to abundance of wild sockeye salmon. Total chum and sockeye salmon represented only 14% and 12%, respectively, of total salmon abundance in 2018/2019. These values exclude Chinook and coho salmon, whose combined reported commercial catch was 1.5% of total salmon catch from the North Pacific during 2018/2019 and approximately 5% of total salmon catch, on average, during 1925 to 2020.
The high abundance of Pacific salmon in recent decades came to an abrupt end in 2020. Commercial catch statistics for all salmon species indicate Pacific salmon harvests, which provide an index of abundance, declined more in 2020 than in any other year on record since 1930. Commercial salmon catch declined by approximately 187 million fish compared with average catch during the previous 10 years. Although the COVID-19 pandemic likely reduced commercial catch to some extent in some regions, most fishery reports and preliminary escapement estimates indicate that low abundance rather than harvest reductions from the COVID-19 pandemic were primarily responsible for the unusually low catch in 2020.
The following details illustrate the issue of commercial catch declines in 2020 for each species. Harvests of each species of Pacific salmon declined 35%, on average, in 2020 when compared with the previous 10-year average. Harvests of Chinook salmon in 2020 were the lowest on record since 1925, declining 54% compared with the previous ten years. Chum salmon harvests in 2020 declined 42%, followed by pink (-40%), coho (-27%), and sockeye salmon (-10%).
Sockeye salmon harvests declined relatively little because returns to Bristol Bay in the southeastern Bering Sea remained robust and offset the exceptionally low harvests of sockeye salmon in the Gulf of Alaska, British Columbia, Washington, and the Columbia River. The decline in sockeye harvests in regions beyond Bristol Bay and the Alaska Peninsula, which intercepts many Bristol Bay sockeye salmon, was 44%. Pink salmon returning to the Norton Sound region in northern Alaska in 2020 continued with relatively high returns that began in 2016, providing evidence for the beneficial effects of the warming ocean on some salmon species in the extreme north.
We hypothesized that a tipping point was reached in the North Pacific Ocean, leading to the substantial decline of all five species of Pacific salmon in 2020. We suggested that the tipping point was caused by the combined effects of unusually frequent marine heatwaves since and exceptionally abundant pink salmon during 2018 and 2019. This hypothesis is partially supported by recent research on the combined effects of sea surface temperature (SST) and pink salmon abundance on productivity (return per spawner) of 47 sockeye salmon populations (brood years 1976–2009) ranging from the Fraser River in British Columbia to Bristol Bay, Alaska.
This research found that a 1.5°C increase in SST was associated with a 23% increase in sockeye productivity in the Bering Sea, a 9% productivity increase in the Gulf of Alaska, but with a 12% decline in productivity in the southern region (British Columbia and Southeast Alaska). Frequent heatwaves likely contributed to the growing abundance of pink salmon in the north while also contributing to a northward shift in the adverse effects of high SST on production of other salmon species.
The research also found that a 119 million increase in pink salmon abundance was historically associated with a 9% decline in sockeye productivity in the Bering Sea and the Gulf of Alaska, and a 21% decline in British Columbia. This finding is consistent with a trophic cascade caused by abundant pink salmon and other studies indicating adverse effects of pink salmon on the growth, age-at-maturation, survival, and abundance of sockeye salmon, Chinook salmon, coho salmon, chum salmon, marine fishes, seabirds, and potentially southern resident killer whales. Additional evidence of adverse interactions between pink salmon and other species is shown by the biennial patterns in marine species that are consistent with the biennial pattern in pink salmon; a pattern that cannot be explained by physical oceanography alone.
The adverse effect of numerous pink salmon on vital rates of other salmon species has the potential to be far-reaching because salmon migrate long distances. For example, 11–38% of Chinook salmon sampled on the southeastern Bering Sea shelf during 2005–2010 originated from the west coast of the contiguous United States. Furthermore, commercial catch of Chinook salmon in Alaska, Russia, and British Columbia (which includes many southward migrating Chinook salmon), and the average weight of Chinook salmon in Alaska are negatively correlated with pink salmon abundance during the three years in which Chinook salmon overlap with pink salmon at sea. The diets of Chinook and pink salmon can significantly overlap, especially when pink salmon are in their second year at sea, as both species consume small fishes, squid, and zooplankton.
The tipping point hypothesis stems from the record-setting back-to-back-year abundances of pink salmon and unique ocean conditions leading up to the salmon decline in 2020. Both pink salmon abundance and SST were relatively high leading up to the salmon decline in 2020.
Furthermore, pink salmon abundance in 2018 did not decline as in past even-numbered years. This hypothesis is consistent with research that has uncovered non-stationary effects of ocean temperatures on salmon catch in the Gulf of Alaska, including negative effects of recent heatwaves on catch and the interactive effect of climate and competition among wild and hatchery pink salmon.
Preliminary 2021 commercial harvest data indicate a tremendous surge in pink salmon from the low numbers observed in 2020, despite the ongoing COVID-19 pandemic. Commercial harvests of pink salmon in Alaska and Russia rebounded and led to the largest harvest of pink salmon on record since 1925 (~515 million pink salmon, all regions combined).
Sockeye salmon abundance in Bristol Bay, Alaska, set a record high in 2021 — 66 million fish (catch and escapement). apparently in response to favorable early marine growth in the Bering Sea, relatively few pink salmon returning from the North Pacific in 2020, and few pink salmon in the Bristol Bay region. Pink salmon and Bristol Bay sockeye salmon are the primary reason for the resurgence of total salmon in 2021.
In contrast, overall commercial harvests of Chinook, chum, and coho salmon, as well as non-Bristol Bay sockeye salmon, remained low throughout Asia and North America during 2021. Relative to harvests during 2010 to 2019, chum salmon harvests declined the most (-38%), followed by Chinook (-33%), coho (-25%) and sockeye salmon beyond the Bristol Bay and the Alaska Peninsula management area (-27%).
In British Columbia, harvests of all five species appear to have been very low in 2021, with preliminary estimates of total commercial harvest being less than 10% of the average harvest during 2010–2019.
The jury is still out on the validity of our tipping point hypothesis in which the combined effects of high back-to-back pink salmon abundance (2018 and 2019) and frequent marine heatwaves led to large reductions in the abundance of all species in 2020.
The record high harvest of pink salmon in 2021 represented approximately 81% of all salmon harvests, and approximately 87% of all harvests if the large harvest of Bristol Bay sockeye salmon are excluded. In contrast, harvests of other salmon species in most regions of the North Pacific remained very low relative to 2010 to 2019.
Given the sudden and widespread decline in salmon abundance in 2020, we suspect that factors during late marine life were important to the widespread decline in addition to factors during early marine life and freshwater residence. The exceptional abundance of pink salmon in 2021 raises the concern for rapid recovery of salmon in many regions, but it is difficult to predict whether high pink salmon abundance will exacerbate poor feeding conditions for other salmon species in the near future or partially offset the benefit of favorable ocean conditions if conditions improve.
Regardless, with such high abundances of pink salmon returning from the North Pacific as it warms and their effect on the growth and survival of other salmon species, we ask: are there too many salmon in the ocean and if so, should hatcheries continue to release up to 5.5 billion salmon each year of which nearly 1.5 billion are pink salmon?
For the full report with graphs and research citations see the NPAFC newsletter.
Ruggerone, G.T., J.R. Irvine, and B. Connors. 2021. Did recent marine heatwaves and record high pink salmon abundance lead to a tipping point that caused record declines in North Pacific salmon abundance and harvest in 2020?
— CBB, June 30, 2021, RESEARCH: TIPPING POINT REACHED IN NORTH PACIFIC, LEADING TO SUBSTANTIAL DECLINE IN ALL FIVE PACIFIC SALMON SPECIES IN 2020 https://cbbulletin.com/research-tipping-point-reached-in-north-pacific-leading-to-substantial-decline-in-all-five-pacific-salmon-species-in-2020/
— CBB, April 13, 2018, CARRYING CAPACITY: HIGH NUMBERS OF PINK, CHUM SALMON IN NORTH PACIFIC MAY BE HURTING CHINOOK https://www.www.www.cbbulletin.com/carrying-capacity-high-numbers-of-pink-chum-salmon-in-north-pacific-may-be-hurting-chinook/
— CBB, April 3, 2015, HOW MANY SALMON CAN NORTH PACIFIC SUPPORT? STUDY LOOKS AT COMPETITION BETWEEN SOCKEYE, PINKS https://www.www.www.cbbulletin.com/how-many-salmon-can-north-pacific-support-study-looks-at-competition-between-sockeye-pinks/
— CBB, Oct. 8, 2010, HUGE HATCHERY RELEASES FROM JAPAN, ALASKA FILL NORTH PACIFIC WITH PINK, CHUM, SOCKEYE; TOO MUCH? https://www.www.www.cbbulletin.com/huge-hatchery-releases-from-japan-alaska-fill-north-pacific-with-pink-chum-sockeye-too-much/