Science Advisory Report  2015/045

Habitat-based escapement benchmarks for Coho Salmon in Georgia Strait Mainland, Georgia Strait Vancouver Island, and Lower Fraser River Management Units

Summary

• In order to fulfill Pacific Salmon Treaty (PST) obligations and inform the implementation of Strategy 1 of the Wild Salmon Policy (WSP), biologically-based benchmarks to assess the status of PST Management Units (MUs) and WSP Conservation Units (CUs) must be derived for Southern BC Coho Salmon.
• An analysis, funded by the Southern Endowment Fund of the PST, to provide science-based recommendations for escapement and exploitation rate benchmarks for the Strait of Georgia Mainland, Strait of Georgia Vancouver Island and Lower Fraser River Coho salmon MUs (and their component CUs) was undertaken by LGL Ltd of Sidney, BC and Ecometric Research of Vancouver, BC
• The analysis utilizes a habitat-based Coho Salmon carrying capacity model, originally developed for the Nass River watershed, to generate estimates of average smolt production and spawners required to achieve average smolt production for each CU.  The habitat-based model was also used to provide a priori information to Bayesian stock-recruit and stock-smolt analyses, which were then used to generate estimates of stock productivity parameters and three common biological benchmarks (Umsy, Smsy and Sgen).
• From the habitat-based model, average estimated smolt production and the number of spawners required to produce the average number of smolts for each CU were calculated respectively as 1,603,226 and 49,422 (East Coast Vancouver Island-Georgia Strait, ECVI-GS); 395,603 and 11,968 (Georgia Strait Mainland, GSM); 751,868 and 22,784 (Howe Sound-Burrard Inlet, HS-BI); 1,484,479 and 46,005 (Lower Fraser River, LFR); 910,977 and 27,605 (Lillooet, LILL); and 608,082 and 18,427 (Boundary Bay, BB).  Similarly, estimated average smolt production and required spawners for each MU were calculated respectively as 1,147,471 and 34,752 (GSM); 3,003,538 and 92,037 (LFR); and 1,603,226 and 49,422 (Georgia Strait-Vancouver Island, GS-VI).
• Estimates of average exploitation rates to produce maximum sustainable yield (Umsy) obtained from Logistic Hockey Stick spawner smolt-recruit models (assuming future marine survival rate of 2.5% and prCV of 0.6) are roughly 41% for the GSM CU and 35% for the ECVI-GS CU, respectively.  There is a high degree of uncertainty in these results (indicated by wide credible intervals associated with the reported averages).  
• Results of the spawner smolt-recruit analysis are highly dependent on marine survival estimates (results were provided at three marine survival levels for comparison: 1%, 2.5% and 5%).  If assumed future marine survival is reduced from 2.5% to 1%, Umsy drops to 4% and 1% for GSM and ECVI-GS CUs, respectively (from 41% and 35%).
• Potential impact on the results from a number of assumptions and uncertainties in the models were evaluated via sensitivity analyses.  Results of the habitat model are highly dependent on the definition of accessible habitat, particularly as it applies to stream order, and to the estimated number of smolts produced per spawner. 
• There are varying degrees of uncertainty associated with many of the inputs to both models presented, leading to high uncertainty in the model outputs.  Caution should be exercised when interpreting or applying these results.
• Insufficient data for some CUs prevented stock recruit analyses to be completed on four of six CUs ( HS-BI, LFR, LILL, and BB), which resulted in no stock recruit analysis results for the GSM and LFR MUs.  For CUs lacking sufficient data to calculate benchmarks, analyses using a high-quality regional data set, similar to the approach used in Korman and Tompkins (2014), would provide a method to generate results for these CUs.

This Science Advisory Report is from the November 4-5, 2014 and April 14, 2015 regional peer review on Determination of Escapement and Exploitation Rate Benchmarks for the Three Georgia Strait Southern BC Coho Management Units.  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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