Research Document - 2001/154

Mortality rates of coho salmon caught by commercial salmon gillnets and the effectiveness of revival tanks and reduced soak time for decreasing coho mortality rates

By N.B. Hargreaves and C. Tovey


In 1998 a number of events changed the conservation, management and harvest opportunities for salmon in the Pacific Region of Canada. Due to growing concerns about the declining abundance of key coho salmon (Oncorhynchus kisutch) populations in both northern and southern B.C., an intensive review and public consultation process was initiated by Fisheries and Oceans Canada. On May 12, 1998 the Department's Coho Response Team issued its report titled "Selective Fisheries Approach For Management of B.C. Salmon Fisheries in 1998". This report provided a Selective Fisheries Management Framework and new options for management of salmon fisheries that included near-zero mortality for critical coho populations and requirements for fishing gear and methods to become more selective. Selective fishing was defined as the ability to avoid, or release alive and unharmed, non-target stocks or species of concern.

Since 1998 more than 100 selective fisheries experiments have been conducted in B.C. by the First Nations, recreational and commercial salmon fishing sectors with direct support from the Department's Pacific Fisheries Adjustment and Restructuring program (PFAR). The main purpose of these selective fishing experiments was to develop new fishing gear and methods aimed at reducing the coho catches, and reduce the mortality rates of any coho that are captured incidentally while harvesting other target species or stocks of salmon.

The purpose of this Research Document was to summarize and evaluate the results obtained from selected selective fishing experiments conducted since 1998. The main focus was on the mortality rates of coho salmon caught with commercial salmon gillnets. The three components of coho mortality that were examined included: 1) the immediate mortality that occurred when coho were captured; 2) the additional mortality that occurred during subsequent holding of coho in revival tanks aboard the fishing vessels; and 3) the short-term (up to 48 hour) delayed mortality that occurred after the coho were released from the fishing vessels.

The other objective of this paper was to evaluate the effect on coho mortality of reduced net soak time, and the use of on-board revival tanks for resuscitating coho caught in commercial salmon gillnets. Short soak times and revival tanks were introduced by the Department in 1998 as mandatory requirements in commercial salmon gillnet fisheries.

The results presented in this Research Document indicate that the incidental mortality rates of coho caught in commercial gillnets are highly variable. The immediate (catch) mortality rates ranged from 47.4% to 0.0% among the 11 different studies that were examined in detail. The additional mortality that occurred during holding of coho in onboard revival tanks was typically in the range of 5-10%, although 30% of the coho held in revival tanks died in one study. The additional short-term mortality of coho that were held in net pens for up to 48 hours after release from the fishing vessels ranged from 2.3% to 27.3%.

In the two studies that had the most comprehensive data, much of the variation in coho mortality rate at the time of capture was explained by a linear relationship between coho mortality rate and gillnet soak time (the duration that the gillnet remained in the water for each fishing operation). In the Alberni Inlet experiment in 1998, which involved only 7 commercial gillnet vessels, more than 95% of the observed variation in coho catch mortality rate was explained by the variation in soak time. In the Skeena River demonstration gillnet fishery in 2000, which involved more than 30 commercial gillnet vessels, about 70% of the variation in coho catch mortality rates was explained by the variation in soak time.

It was concluded that coho catch mortality rates increase rapidly with increasing soak time, and that short soak times of less than 60 minutes (defined as first cork in to last cork out) are likely required to achieve coho mortality rates that are substantially lower than the rate (60%) that the Department of Fisheries and Oceans (DFO) has assumed for most commercial gillnet fisheries since 1998.

It was concluded that revival tanks are generally beneficial for improving the condition of many coho prior to release. Appropriate use of revival tanks likely also improves the post-release survival rates of coho that are released from commercial gillnet vessels.

The capability of revival tanks to revive coho caught in commercial gillnets decreases with increasing soak time. The data currently available are insufficient to confirm which of the current two main alternative designs (the DFO "blue box", or the newer JF revival tank) is more effective for reviving coho, particularly in either demonstration scale or regular full-fleet gillnet fisheries. Additional direct experiments (e.g. side-by-side comparisons with adequate experimental controls) are still required to clearly confirm if the new JF revival tank design is superior. The current data do suggest that the JF revival tank design likely provides little advantage over the standard DFO "Blue Box" revival tank when soak times are very short (e.g. <30 minutes; first cork in to last cork out). However, the new JF design may significantly reduce coho mortality rates for moderately longer soak times (e.g. 30-100 minute soak times), compared to the standard DFO revival tank. Neither the new JF revival tank or standard DFO revival tank designs are likely to be effective for reviving coho caught in sets with longer soak times (e.g. >100 minutes).

The authors emphasized that the analyses and conclusions in this Research Document apply only to the short-term mortality of coho and do not address the additional important question of what effect capture and release from commercial gillnets may ultimately have on coho mortality beyond 48 hours after release from fishing vessels, or on the ultimate spawning success of these coho salmon. It was also noted that the actual impact commercial gillnet fisheries on salmon stocks of concern depends on both the mortality rates and the encounter rates. This Research Document considers factors that affect the mortality rate of incidentally caught coho, but does not evaluate or consider the factors that affect coho encounter rates.

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