Shooting double-crested cormorants and
suffocating their eggs with corn oil to reduce their predation on juvenile
Snake River steelhead likely has no impact on the number of adult steelhead
returning from the ocean to spawn, a US. Fish and Wildlife scientist has
concluded in a preliminary analysis.
Snake River steelhead are the species believed
to be most affected by the fish-eating birds whose numbers have increased by
remained stable since 2004.
In fact, culling double-crested cormorants
probably won’t improve the survival of other fish species, either, according to
a preliminary analysis by Steven Haeseker, a fish biologist and biometrician
with the agency’s Columbia River Fisheries office in Vancouver.
A blog item on the Northwest Power and
Conservation Council’s website www.nwcouncil.org
describes Haeseker’s presentation at a recent
meeting of the Council’s Independent Scientific Advisory Board.
For the second year in a row, the U.S. Army Corps
of Engineers is shooting cormorants and dousing their eggs on a tiny island
built from dredged material near the mouth of the Columbia River. The intent is
to reduce predation on salmon and steelhead smolts migrating through the
estuary on their way to the ocean. Reducing predation by double-crested
cormorants is among the actions in a 2014 federal plan to protect ESA-listed
salmon and steelhead (recently rejected by a federal judge). The plan calls for
reducing the double-crested cormorant population in the estuary from about
13,000 nesting pairs to 5,300-5,900.
This year the killing began April 7,
authorized by a “depredation permit” issued to the Corps by the U.S. Fish and
Wildlife Service on March 18. The birds hunt for fish in the area of East Sand
Island, where they also build nests. The island is just south of the town of
Chinook, Washington. The permit also allows the Corps to kill birds and oil
nests on any other dredge-remain islands in the lower river. The limits in the
permit are 3,114 double-crested cormorants and 5,000 nests in 2016. Last year
more than 1,700 birds were killed and more than 5,000 nests were oiled.
But Haeseker’s analysis suggests it might not
He summarized his preliminary results, in
part: “Cormorant consumption rates were not a significant factor for steelhead
[smolt-adult-return rates] after accounting for the other freshwater and ocean
Using data from tagged Snake River steelhead,
Haeseker concluded that predation by double-crested cormorants was balanced by
decreased mortality from other sources and that there was no impact on the
number of adult fish returning from the ocean based on the number of outgoing
smolts several years earlier, said the Council report.
Thus, says the Council report, predation by
cormorants appeared to have no effect on the number of returning adult fish.
In comparison, however, the same data
suggested that mortality of juvenile Snake River steelhead by passage through
the hydrosystem did reduce the number of returning adult fish, Haeseker said.
“It's important to note that the Fish and
Wildlife Service considers Haeseker's research preliminary and that it is
currently being updated with additional data,” said the Council blog item.
The Service noted that while the analysis
focused on Snake River steelhead, cormorant predation extends to a variety of
juvenile Pacific salmon in the lower Columbia, primarily sub-yearling fall chinook. As such, the study methods are being examined
to see if they can be applied to other salmon stocks subject to cormorant
predation. In addition, the study needs to be peer-reviewed, according to the
Service. These actions are underway, but are not yet complete. Once the study is complete, a new draft will be
submitted to a peer-reviewed journal.
In April 2015, the Audubon Society of Portland
and four other groups sought an injunction in federal court to prevent the
killing, but a federal judge denied the request. The lawsuit continued this
year but had not been decided by the time the killing began.