Every other year, large numbers of eastern Kamchatka pink
salmon appear in the eastern Pacific Ocean, impacting the amount of large
phytoplankton and copepods available for salmon to eat in the southern Bering
Sea and around the Aleutian Islands where they rear, according to a recent
Pink salmon abundance rises in odd years and falls in even
years. The abundance of the large copepods in areas where the salmon are
located drops as pink populations rise. Copepod abundance rises again when
pinks are less present.
The study shows that this biennial pattern of copepod
abundance (prey eaten by salmon of all types) is likely caused by the predation
pressure on the prey by eastern Kamchatka pink salmon.
While typically finding that plankton abundances from
continuous plankton recorder data over a 15-year period (2000 – 2014) vary in
relation to physical processes in this region around the Aleutian Islands and
southern Bering Sea, the study found alternating annual abundances of large
copepods and opposing alternating abundances of diatoms which could only be
explained by the odd-even year run patterns of pink salmon in what's referred
to as a “trophic cascade,” said researcher Sonia Dawn Batten, a research
scientist and biological oceanographer with Marine Biological Association of
the UK in Nanaimo, British Columbia.
“High numbers of the predator (pink salmon) cause lower
numbers of the prey (copepods) which then can't feed on the phytoplankton,
allowing high numbers (phytoplankton) in those years,” she said. “We saw the
reverse pattern in the lower run years. Since pink salmon are only one of the
many species that feed on copepods in this region, this has implications for
other species in years with lots of pink salmon.”
She added that she believes the number of pink salmon in
the region studied can exceed the carrying capacity. “While I don’t know much about
the other salmon species distributions in this area, there are also impacts (of
the large pink salmon presence) on sea birds, a conclusion reported by other
researchers (Alan Springer et al).
Managers should also understand that there are limits to how
much fish production can be supported in regions of the North Pacific, she said
of the releases of pink salmon from hatcheries.
“Simply releasing more fish from hatcheries won't
necessarily increase the numbers returning and may have knock on effects for
other species (some of which are economically important, some which are
important in the ecosystem),” Batten said.
However, most of the pink salmon in this region are of
natural origin, likely for eastern Kamchatka.
This study shows the value of monitoring, Batten said.
“It’s often not well supported but it needs to be if we are going to have the
data that scientists require to understand these relationships and predict
future scenarios. Managers need that science input.”
“Pink Salmon induce a trophic cascade in plankton
populations in the southern Bering Sea and around the Aleutian Islands,” was
published online June 8, 2018 in Fisheries Oceanography (https://onlinelibrary.wiley.com/doi/full/10.1111/fog.12276).
Batten’s co-authors are Gregory Ruggerone, fisheries
scientist at Natural Resources Consultants, Inc. and Ivonne Ortiz, fisheries
scientist at the Joint Institute for the Study of the Atmosphere and Ocean,
University of Washington.
Ruggerone had previously published a study on the high
number of pink and chum salmon that may be depleting the numbers of other
salmon in the North Pacific Ocean, such as chinook.
“Numbers and Biomass of Natural-and Hatchery-Origin Pink Salmon,
Chum Salmon, and Sockeye Salmon in the North Pacific Ocean, 1925–2015” was
published online April 4, 2018 in Marine and Coastal Fisheries: Dynamics,
Management and Ecosystem Science (https://onlinelibrary.wiley.com/doi/full/10.1002/mcf2.10023). Ruggerone’s co-author is James Irvine, Fisheries and
Oceans Canada, Pacific Biological Station in Nanaimo. British Columbia.
(See CBB, April 13, 2018, “Carrying Capacity: High Numbers
Of Pink, Chum Salmon In North Pacific May Be Hurting Chinook,” http://www.cbbulletin.com/440518.aspx.)
The impact of pink salmon abundance on copepod abundance is
not apparent every year in every area of the North Pacific, according to the
“We find regional differences in the expression of these
effects with alternating odd/even year patterns being strongest in the central
Southern Bering Sea and eastern Aleutians and reduced, or absent, in the
western Aleutians,” the study says.
The abundance of eastern Kamchatka pink salmon were eight
times higher in the odd years of their two-year cycle (about 122 million fish)
as opposed to 15 million salmon on off years 2000-2012.
The high abundance unexpectedly declined by 73 percent in
2013 compared with previous odd years when just 33 million fish were present in
the southern Bering Sea and Aleutian Island areas, but plankton populations
were higher, the study says.
“These findings emphasize the importance of variability in
predator abundance and its effect across the ecosystem, which in this case was
greater than physical oceanographic variability,” the study says.
Although 2013 was a poor year for pink salmon in the
western Pacific, across the ocean to America’s West Coast (eastern Pacific),
pink salmon returned to Alaska and Prince William Sound in large numbers –
about 265 million. Those stocks were largely distributed east of the southern
Bering Sea and Aleutian Islands.
In 2015, the next odd year for Kamchatka pink salmon, the
pinks returned in very high numbers, so the decline in 2013 did not persist,
the study says.
“The results described here show that top-down effects need
to be considered where large numbers of predators occur,” the study concludes.
“The influence of Pink Salmon on the summer plankton around the Aleutian
Islands and in the southern Bering Sea was a much stronger signal than
environmental variability caused by the cold/warm stanzas experienced by the
Bering Sea during the same time period.”