Salish Sea Marine Survival Project

49 Figure 2. Microtroll-captured juvenile Chinook Salmon (211 mm) with small spoon in its mouth. Photo by Will Duguid. Figure 4. Microtrolling locations in Sansum Narrows and Maple Bay, locations located ~3 km apart. Figure provided by Will Duguid. Microtrolling has been used for a number of purposes as part of the SSMSP BC Conservation Foundation (BCCF) and University of Victoria researchers used microtrolling to apply PIT tags to juvenile Chinook Salmon to investigate mortality periods of Cowichan River Chinook. Effort was concen- trated in the Southern Gulf Islands from August to October 2014-2017. Juvenile Chinook were tagged and tissue samples (fin clips or scales) were taken for genetic stock identification to determine which fish would be expected to return to the Cowichan River. University of Victoria researchers employed a standard- ized (gear, depth, deployment time) microtrolling protocol to investigate fine scale variation in distribution, diet and growth of juvenile Chinook Salmon in the Southern Gulf Islands. Deployment of hooks coupled with temperature loggers at standardized depths allowed detailed depth distribution and thermal strat- ification data to be collected. Fish diets were sampled non-lethally by gastric lavage (stomach flushing) and growth rate was analyzed by measuring the width of circuli (growth rings) on scales. Microtrolling was used by Kintama Research Services, University of Victoria and DFO to economically capture juvenile Chinook Salmon for an acoustic tagging study in the Southern Gulf Islands in 2017. SUMMARY OF RESULTS TO DATE Microtrolling has proven to be an effective method for capturing juvenile Chinook Salmon in good condition for tagging. Since 2014, over 4,000 juvenile Chinook Salmon have been PIT tagged by 2-3 person crews operating out of small (5-7 m) boats. This tagging effort has been a key component of the ongoing Cowichan Chinook Salmon PIT tagging program which is providing new insights on hatchery and wild Chinook Salmon survival Microtrolling has also allowed diet, distribution and growth of juvenile Chinook Salmon in late summer of their first year at sea to be analyzed at a spatial and temporal scale not possible before. Summer growth of juvenile fish in Cowichan Bay is remarkable (Figure 3). Researchers have found that size, diet and growth rate of juvenile Chinook Salmon can vary between closely adjacent locations (<4 km apart) (Figures 4 & 5). Larger, faster growing Chinook Salmon appear to spend more time in areas where juvenile Pacific Herring are available. In fact, sampling the same region throughout late summer and fall has revealed that only larger juvenile salmon appear able to consume juvenile Pacific Herring, while smaller salmon rely on crab larvae and other plank- tonic organisms. Co-occurrence of larger, faster growing juvenile Chinook Salmon with juvenile Pacific Herring also has potential to bring them into contact with predators which are targeting herring schools. Figure 3. Average size per day by origin for PIT tagged fish (2014-2019) in Cowichan Bay. (Caveats: early season fish only big enough to tag (>60 mm) are shown, no tagging done in July and microtrolling may exclude smaller fish especially in August). Fish in August and September are caught by mictrolling. Figure credit: Kevin Pellett, DFO. 19-Apr 9-May 29-May 18-Jun 8-Jul 28-Jul 17-Aug 6-Sep 26-Sep 16-Oct 5-Nov Wild Hatchery 7 per. Mov. Avg. (Wild) 7 per. Mov. Avg. (Hatchery) 50 70 90 110 130 150 170 190 210 230 250 Average Fork Length (mm)

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