Science Advisory Report 2015/057
Evaluation of Marine Recreational Coho Mark-Selective Fisheries
Summary
- Declines in the abundance of southern British Columbia Coho salmon populations, in particular Interior Fraser River Coho (IFR) salmon, through the 1990’s, resulted in harvest restrictions for Coho in all sectors beginning in 1998. Mark-selective fisheries (MSF) were implemented under a domestic operational target exploitation rate ceiling for IFR Coho Salmon. Pre-season planning of mixed stock fisheries, where IFR Coho were prevalent, was limited to an exploitation rate for IFR Coho of 3%, through 2013 and, in 2014, up to 16% in Canadian domestic fisheries.
- Theoretical evaluation and simulation modeling indicate that MSFs can reduce exploitation rates on unmarked fish relative to a fishery that is not mark-selective. Mortality rates for unmarked fish depend not on capture mortality, but mortality experienced from hooking and releasing, or from interaction with fishing gear that doesn’t result in capture. A MSF will have a lower exploitation rate on unmarked fish than a non-MSF, as long as the mortality rate from release or gear interactions for unmarked fish is lower than the proportion of marked fish in the fished population.
- One of the primary concerns of using MSFs is the potential to complicate stock assessment, especially the use of coded wire tags (CWT’s) to estimate stock specific marine survival and fishery specific exploitation rates. The magnitude of this effect will scale with the number and magnitude of mark-selective fisheries.
- A Double Index Tag (DIT) approach was identified by the ad hoc Selective Fisheries Evaluation Committee as the primary assessment method to evaluate the effects of MSFs on survival of marked and unmarked fish. The DIT approach involves releasing two uniquely coded wire tagged groups of fish reared under exactly the same conditions, of which one group is adipose fin clipped.
- Coho DIT experiments from eight streams have marked more than five million Coho and recovered over 85,000 estimated tags at escapement. These data, analyzed with a risk ratio approach, suggest that although there may be an average 12% survival benefit (unmarked to marked risk ratio of 1.12 with an approximate 95% C.I. of 0.75 to 1.60) confidence intervals bridge a value of 1.0 indicating the possibility of no difference.
- A Bayesian hierarchical analysis, however, indicated that overall, there is a 70% probability that there is a positive survival benefit of being unmarked.
- A simple model integrating estimates of marked Coho exploitation rates from hatchery indicator stocks and average survival benefits of mark-selective fisheries suggest that exploitation rates on unmarked Coho are low; approximately 70% less, on average, than the exploitation rates estimated for marked Coho.
- Catch and assessment data are generally not of fine enough resolution to conduct MSF analyses at the stock level, limiting their application. An increased level of support for MSF, including the convention of an oversight committee familiar with current limitations, is recommended.
This Science Advisory Report is from the March 3-5, 2015 regional peer review on the Evaluation of Marine Recreational Coho Mark-selective Fisheries in British Columbia, including an evaluation of the Canadian marine fishery exploitation model for Interior Fraser Coho. Additional publications from this meeting will be posted on the Fisheries and Oceans Canada (DFO) Science Advisory Schedule as they become available.
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