Understanding the Mechanisums that Regulate Coho Salmon Abundance inthe Strait of Georgia, British Columbia, Canada

There was an important commercial and recreational fishery for coho salmon in the Strait of Georgia, British Columbia, Canada in the 1970s and 1980s.The recreational fishery was one of the largest in Canada with average annual catches of 508,000 fish from 1980to 1990(Beamish et al. 1999)).However, in the late 1980s, it was evident that abundances were declining (DFO 1990(DFO , 1992;;Irvine et al. 1992).The declines continued into the 1990s with the closure of the commercial fishery in 1995 and the recreational catches diminishing to very low levels after 1994 (Beamish et al. 2008).The declines in abundance were related to the loss of freshwater habitat and to an exploitation rate that was too high (DFO 1990(DFO , 1992)).However, the synchronous declines in abundance for coho salmon from the Strait of Georgia, Puget Sound and off the coasts of Washington and Oregon (Beamish et al. 2000) indicated that the major reason for the declines resulted from a declining marine survival that was related to a change in the ocean ecosystem.Importantly, the abundance declines in all these areas occurred despite additions of smolts from hatcheries that annually averaged about 100 million fish (Beamish et al. 1997), clearly showing that the declines were mainly a result of changing ocean conditions.Coho salmon smolts enter the marine environment in about May and remain and rear within the Strait of Georgia until October or November (Chittenden et al. 2009;Beamish et al. 2010).Historically, coho salmon originating from rivers flowing into the Strait of Georgia were caught within the strait and off the southwest portion of Vancouver Island.Prior to 1994, the proportion captured within the strait or outside of the strait varied among years.When catches were greater outside the Strait of Georgia, the year was classified as an outside year and an inside year when the proportion was greater within the strait (Beamish et al.1999).From the mid-1970s through to the early 1990s, there was no trend in the inside or outside catches.Beginning in 1995, virtually all juvenile coho salmon left the Strait of Georgia late in their first ocean year and did not return until their spawning migration later in their second ocean year (Beamish et al. 1999).Thus, the collapse of the recreational fishery resulted from this behavioural change during their first marine winter as well as a decline in marine survival from about 10% in the late 1970s to about 1% in the 1990s (Beamish et al. 2010).Our discussions with First Nations Elders on Vancouver Island indicate that historically the presence of coho salmon in the Strait of Georgia in all months was typical and that these fish were so numerous they could be readily caught throughout the year.Thus, the migration out of the Strait of Georgia beginning in 1995 represented a behavioural change that was unprecedentedIn this study, we show that abundances of coho salmon occurred within the Strait of Georgia in the winter of 2017-2018 for the first time since 1994.These fish remained in the strait until they returned to their natal rivers.We speculate that the change in migratory behaviour was related to improved growth in the early marine period in the Strait of Georgia. MethodsSurveys examining the distribution and abundance of juvenile salmon in the Strait of Georgia have been conducted in the early summer (June/July) and early fall (September) each year since 1998.The standard track line extending from Cape Mudge in the northern Strait of Georgia to the Canada/US border in the south is surveyed (Fig. 1).The surveys use a modified mid-water trawl with a mouth opening of approximately 14m deep by 30m wide and with small mesh cod-end designed to retain juvenile salmon.Fishing occurs at 15m depth intervals from the surface to 60m head-rope depth and sets are typically 30 minutes.Detailed information on the survey protocol is provided in Beamish et al. (2000) and Sweeting et al (2003).Random or haphazard samples of juvenile Pacific salmon (30-100 per set) are selected and measured for fork-length (mm).Catch-per-unit-effort (CPUE) is calculated by set and by survey and is standardized to catch per hour.Stock composition of the coho salmon collected in the Strait of Georgia has been determined using DNA analysis since 2008.DNA analysis is conducted using a small piece of fin tissue collected during sampling and stored in 95% ethanol.The analysis is conducted at the Molecular Genetics Laboratory at the Pacific Biological Station in Nanaimo, BC following the procedures described in Beacham et al. (2012).

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