Most coho salmon caught in commercial purse
seines in the Strait of Juan de Fuca recover within 48 hours unless they have
mostly visible dermal injuries. Those fish failed to recover within the 84 hour
holding period, according to a recent study.
The coho were caught in purse seine nets off
the West Coast of Vancouver Island in the Strait. Because the species is of
conservation concern in Western Canada and an interior Fraser River coho is
listed in Canada as a threatened species, when caught the fish must be returned
to the water.
The difference between those that survived and
the fish that didn’t? Coho with dermal injuries showed disruptions to blood ion
levels that failed to recover during the holding period. That suggests that
failure to maintain ion balances is a possible mechanism by which
capture-induced injury leads to mortality.
“We conducted holding studies to assess how
severity of injury and reflex impairment influences the time course of
physiological recovery in coho salmon following capture by purse seine,” said
researcher Katrina Cook, a PhD candidate in the Department of Forest Sciences
and Conservation at the University of British Columbia. “Although
reflex-impairments and blood lactate levels suggested fish were exhausted upon
capture, they were able to recover by 48 hours.”
An assessment of the presence of involuntary
responsiveness to stimuli, according to the study, reflex impairment is the
most common way to measure a captured fish’s vitality. As such, reflex
impairment is a reliable predictor of post-release mortality, especially when
coupled with visible dermal injuries, the study says.
Although the study focused on coho salmon in
British Columbia, the findings are applicable to any fishery in which
non-target fish may suffer from capture-induced dermal injuries, Cook said.
“Of note is that the study was conducted in
marine waters during their approach to freshwater, and thus during an
iono-regulatory transition period,” she said. “It is during this time that
dermal injuries may be most impactful. Therefore, although injuries may cause
mortality in any fishery in any location, the particular mechanism of mortality
we identified is likely most relevant to salmonids captured in coastal approach
For Columbia River coho, that could be as the
fish enter the Columbia River estuary.
She added that coho are thought to be more
sensitive to dermal injuries than other species of salmonids. “A failure to
regulate ion homeostasis given dermal injuries has not been identified in any
other salmonid species,” she said.
“Dermal injuries caused by purse seine capture
result in lasting physiological disturbances in coho salmon,” was published
online in Comparative Biochemistry and Physiology, Part A: Molecular &
Integrative Physiology. https://www.sciencedirect.com/science/article/pii/S1095643318301855?via%3Dihub.
Cook’s co-authors are Scott Hinch, professor
in the Department of Forest Sciences and Conservation at UBC; Matt Drenner,
also at UBC and senior biologist at Cramer Fish Sciences; Graham Raby,
post-doctoral research fellow at Great Lakes Institute for Environmental
Research, Carleton University in Ottawa; David Patterson, research biologist
with Fisheries and Oceans Canada, Cooperative Resource Management Institute, Simon
Fraser University; and Steven Cooke, professor, Department of Biology and
Institute of Environmental Sciences, Carleton University.
If the goal of release practices is to make
sure that fish returned to the water survive, then the primary management
implication is that any means by which capture-induced injuries can be reduced
would improve the survival of these fish, according to Cook.
“In the fishery at study, this could mean
using wet sorting tables and having release chutes affixed to sorting tables
such that fish are released as quickly as possible while minimizing handling, a
practice that is currently not a condition of license,” she said.
Seining nets that cause less dermal injury are
more appropriate for fisheries employing release practices, she added.
“Research has found that Pacific salmon are
less susceptible to dermal injuries as they mature,” Cook said. “Therefore, timing
fisheries such that non-target fish are more mature, thus less vulnerable to
dermal injuries, may also improve their survival.”
The study found that 19 percent of fish
experienced scale loss, 23 percent of fish had net marks and 14 percent had
other dermal injuries. Smaller coho in the study experienced more severe dermal
“Of all blood parameters measured, lactate and
the ion homeostasis score showed the largest differences among vitality
scores,” the study says. “During anaerobic exhaustive exercise, lactate
accumulates in white muscle and a portion leaks into the blood stream. The
osmotic pressure created by intracellular metabolic acidosis causes water to
move into white muscle cells, increasing plasma ion concentrations and
Still plasma lactate concentrations recovered
among all groups within 48 hours, but the significant interactions between
injury classifications simply reflect the greater recovery required among more
exhausted fish, the study says. What was apparent is that ion concentrations
recovered within 48 hours in reflex impaired fish, but the ion levels did not
recover in the injured fish. For those fish, it took up to 84 hours or longer.