Alaska's salmon are getting smaller, affecting people and ecosystems


Date:
August 19, 2020
Source:
University of California - Santa Cruz
Summary:
The size of salmon returning to rivers in Alaska has declined
dramatically over the past 60 years because they are spending
fewer years at sea, scientists report. Salmon are critically
important to both people and ecosystems in Alaska, supporting
commercial and subsistence fisheries and transporting nutrients
from the ocean to inland areas. Smaller salmon provide less food
for people who depend on them, less value for commercial fishers,
and less fertilizer for terrestrial ecosystems.



FULL STORY ==========================================================================
The size of salmon returning to rivers in Alaska has declined dramatically
over the past 60 years because they are spending fewer years at sea,
according to a new study led by researchers at the University of
California, Santa Cruz, and the University of Alaska Fairbanks.


========================================================================== Salmon are critically important to both people and ecosystems in Alaska, supporting commercial and subsistence fisheries and transporting
nutrients from the ocean to inland areas, fertilizing the ecosystems
in and around the rivers where they spawn. Smaller salmon provide less
food for people who depend on them, less value for commercial fishers,
and less fertilizer for terrestrial ecosystems.

For years, people in Alaska have been noticing that wild salmon were
getting smaller, but the reasons have been unclear. In the new study,
published August 19 in Nature Communications, researchers compiled and
analyzed data collected over six decades (1957 to 2018) from 12.5 million
fish by the Alaska Department of Fish and Game. This unprecedented
dataset enabled them to see patterns of body size changes for four
species of salmon -- Chinook, chum, coho, and sockeye -- across all
regions of Alaska.

The results showed that the decreases in body size are primarily due
to salmon returning to their spawning grounds at younger ages than they
have in the past.

Alaskan salmon can spend up to seven years at sea, although this varies
by species. During this time they feed and grow to maturity, migrating
great distances in the North Pacific Ocean before returning to fresh
water to spawn.

"There are two ways they could be getting smaller -- they could be growing
less and be the same age but smaller, or they could be younger -- and
we saw a strong and consistent pattern that the salmon are returning
to the rivers younger than they did historically," said corresponding
author Eric Palkovacs, professor of ecology and evolutionary biology
and associate director of the Fisheries Collaborative Program in the
Institute of Marine Sciences at UC Santa Cruz.

The researchers identified a range of factors that appear to be driving
this shift, some acting across all regions and others affecting only
certain species or populations.



========================================================================== "There's not a single smoking gun," said first author Krista Oke, a postdoctoral scientist initially at UC Santa Cruz and now at University of Alaska Fairbanks. "Small contributions from a lot of factors are adding up
to drive these changes." Two factors -- climate change and competition
with growing numbers of wild and hatchery salmon in the ocean -- have
clearly contributed to size declines across all species and regions,
Palkovacs said. In contrast, the effect of commercial fishing appears
to be important only for some salmon populations.

Similarly, the results were mixed for another proposed driver of size
declines, the recovering populations of marine mammals that prey on
salmon.

"We know that climate drives changes in ocean productivity, and we
see a consistent signal of climate factors associated with decreasing
salmon size," Palkovacs said. "Another consistent association is with the abundance of salmon in the ocean, especially pink salmon. Their abundance
in the North Pacific is at historic highs due in part to hatchery
production in Alaska and Asia, and they compete with other salmon for
food." The observation that salmon are returning to freshwater streams
at younger ages implies that the ocean is becoming a riskier place for
them to be, he said. By staying in the ocean longer and growing larger,
salmon can have greater success in spawning and lay more eggs, but each additional year increases the risk of not returning to reproduce at all.

"Natural selection has always pushed in both directions, but the balance between the two is changing, pushing harder against the older, larger
salmon," Palkovacs said. "It seems that the ocean is becoming a riskier
place to be." According to Oke, understanding exactly what is going
on in the ocean to drive this shift is a difficult challenge that will
require further study. "That's the next hard step I hope we can get to
soon," she said. "It could be that they're having to spend more time
feeding, which is putting them in risky places. Lots of things could
be happening to increase the overall risk of mortality in the ocean,
but we weren't able to pin that down."


==========================================================================
The consequences for people and ecosystems, however, are more
clear. Smaller salmon means fewer meals per fish for subsistence
fishers, lower profits for commercial fishers, fewer eggs laid to sustain salmon populations, and fewer nutrients to support the productivity and biodiversity of freshwater and riparian ecosystems.

"Smaller fish is a real problem for people who depend on salmon for
their food and well being," Oke said. "For commercial fishers, smaller
fish tend to fetch lower prices, and below a certain size they can't
be made into high-value products and might have to be canned." On the ecosystem side, the nutrients delivered by salmon runs provide critical
support for bears, insects, birds, trees, and juvenile salmon themselves.

Palkovacs noted that an extensive body of research has tracked the
movement of marine nitrogen from salmon into the terrestrial ecosystems
around the streams where they spawn.

"Salmon go up into these small streams, and whether they are caught by predators or die after spawning, their nutrients are transferred into
the forests and freshwater ecosystems," he said. "It's a classic salmon ecosystem service, and the amount of nutrients they deliver depends
on their body size." The study had its origins in a working group
organized by the National Center for Ecological Analysis and Synthesis
(NCEAS) at UC Santa Barbara through its State of Alaska's Salmon and
People project. With funding from the Gordon & Betty Moore Foundation,
the researchers were able to work with the Alaska Department of Fish and
Game to compile data the agency had been collecting for decades, but which
was dispersed among different field offices in various smaller databases.

"At NCEAS, we had two data scientists who compiled all the data into
one massive database on Alaskan salmon that is now publicly available," Palkovacs said. "It took a lot of time and energy, but that's what enabled
us to do this comprehensive analysis." Oke added that getting the data
in the first place was no small task either.

"When you think about the fact that we used data from 12.5 million
salmon, that's how many times someone from ADF&G measured a salmon. It's
an exceptional amount of work to make a dataset like this possible,"
she said.

In addition to Oke and Palkovacs, the coauthors of the paper include corresponding author Peter Westley at University of Alaska Fairbanks,
as well as researchers at Alaska Pacific University in Anchorage, UC
Davis, UC Berkeley, NCEAS, McGill University in Montreal, Washington
Department of Fish and Wildlife, Virginia Polytechnic Institute, Alaska Department of Fish and Game, Simon Fraser University, and Tanana Chiefs Conference, Fairbanks.


========================================================================== Story Source: Materials provided by
University_of_California_-_Santa_Cruz. Original written by Tim
Stephens. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. K. B. Oke, C. J. Cunningham, P. A. H. Westley, M. L. Baskett, S. M.

Carlson, J. Clark, A. P. Hendry, V. A. Karatayev, N. W. Kendall, J.

Kibele, H. K. Kindsvater, K. M. Kobayashi, B. Lewis, S. Munch, J. D.

Reynolds, G. K. Vick, E. P. Palkovacs. Recent declines in salmon
body size impact ecosystems and fisheries. Nature Communications,
2020; 11 (1) DOI: 10.1038/s41467-020-17726-z ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/08/200819085003.htm

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