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Study Looks At Injuries To Coho In Purse Seine Nets That Determine Survival/Mortality After Capture
Posted on Friday, November 02, 2018 (PST)

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 waters.”


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.


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 injuries.


“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 osmolality.”


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.


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