The largest and oldest chinook salmon –
“kings” – have mostly disappeared along the West Coast.
That's the main finding of a new University of
Washington-led study published Feb. 27 in the journal Fish and Fisheries http://onlinelibrary.wiley.com/doi/10.1111/faf.12272/full.
The researchers analyzed nearly 40 years of
data from hatchery and wild chinook populations from California to Alaska,
looking broadly at patterns that emerged over the course of four decades and
across thousands of miles of coastline. In general, chinook salmon populations
from Alaska showed the biggest reductions in age and size, with Washington
salmon a close second.
"Chinook are known for being the largest
Pacific salmon and they are highly valued because they are so large," said
lead author Jan Ohlberger, a research scientist in the UW's School of Aquatic
and Fishery Sciences. "The largest fish are disappearing, and that affects
subsistence and recreational fisheries that target these individuals."
Chinook salmon are born in freshwater rivers
and streams, then migrate to the ocean where they spend most of their lives
feeding and growing to their spectacular body size. Each population's lifestyle
in the ocean varies, mainly depending on where they can find food. California chinook
salmon tend to stay in the marine waters off the coast, while Oregon and
Washington fish often migrate thousands of miles northward along the west coast
to the Gulf of Alaska where they feed. Western Alaska populations tend to
travel to the Bering Sea.
After one to five years in the ocean, the fish
return to their home streams, where they spawn and then die.
Despite these differences in life history,
most populations analyzed saw a clear reduction in the average size of the
returning fish over the last four decades -- up to 10 percent shorter in
length, in the most extreme cases.
These broad similarities point to a cause that
transcends regional fishing practices, ecosystems, or animal behaviors, the
"This suggests that there is something
about the larger ocean environment that is driving these patterns,"
Ohlberger said. "I think fishing is part of the story, but it's definitely
not sufficient to explain all of the patterns we see. Many populations are
exploited at lower rates than they were 20 to 30 years ago."
The study’s abstract says while “it remains to
be explored whether these trends are caused by changes in climate, fishing
practices or species interactions such as predation, our qualitative review of
the potential causes of demographic change suggests that selective removal of
large fish has likely contributed to the apparent widespread declines in
average body sizes.”
It used to be common to find chinook salmon 40
inches or more in length, particularly in the Columbia River or Alaska's Kenai
Peninsula and Copper River regions. The reductions in size could have a
long-term impact on the abundance of chinook salmon, because smaller females
carry fewer eggs, so over time the number of fish that hatch and survive to
adulthood may decrease.
There are likely many reasons for the changes
in size and age, and the researchers say there is no "smoking gun."
Their analysis, however, points to fishing pressure and marine mammal predation
as two of the bigger drivers.
Commercial and sport fishing have for years
targeted larger chinook. But fishing pressure has relaxed in the last 30 years
due to regulations to promote sustainable fishing rates, while the reductions
in chinook size have been most rapid over the past 15 years. Resident killer
whales, which are known chinook salmon specialists, as well as other marine
mammals that feed on salmon are probably contributing to the overall changes,
the researchers found.
"We know that resident killer whales have
a very strong preference for eating the largest fish, and this selectivity is
far greater than fisheries ever were," said senior author Daniel
Schindler, a UW professor of aquatic and fishery sciences.
While southern resident killer whales that
inhabit Puget Sound are in apparent decline, populations of northern resident
killer whales, and those that reside in the Gulf of Alaska and along the
Aleutian Islands, appear to be growing at extremely fast rates. The paper's
authors propose that these burgeoning northern populations are possibly a
critical, but poorly understood, cause of the observed declines in chinook
Scientists are still trying to understand the
impacts of orcas and other marine mammals on chinook salmon, and the ways in
which their relationships may have ebbed and flowed in the past. It may not be
possible, for example, for marine mammals and Chinook salmon populations to be
robust at the same time, given their predator-prey relationship.
"When you have predators and prey
interacting in a real ecosystem, everything can't flourish all the time,"
Schindler said. "These observations challenge our thinking about what we
expect the structure and composition of our ecosystems to be."
Co-authors are Eric Ward of NOAA's Northwest
Fisheries Science Center and Bert Lewis of the Alaska Department of Fish and