Research Document - 2015/050
Habitat-Based Model and Stock-Recruit Productivity Estimates for Coho Salmon in Georgia Strait Mainland, Georgia Strait Vancouver Island and Lower Fraser Management Units
By C.A.J. Noble, J. Korman and R. Bocking
Abstract
Identifying biological reference points or benchmarks for management of Coho Salmon is a critical component of the Wild Salmon Policy, and key to sustainable fishery management; yet data and budget restrictions limit the use of traditional stock recruit methods to identify benchmarks. Here, we combine a habitat-based model and Bayesian stock-recruit and stock-smolt analysis to estimate average CU smolt production and the number of spawners required to achieve this, as well as stock productivity parameters and three potential benchmarks (Umsy, Smsy and Sgen) for wild (non-enhanced) Coho Salmon populations. Stock recruit analyses were conducted using both Beverton-Holt and Logistic Hockey Stick models and spawner-to-smolt and spawner-to-recruit data sets. Stream length accessible to Coho Salmon was determined from terrain resource inventory maps (TRIM) using GIS and maps at 1:20,000 scale. Stream order, gradient and known barriers were used to define the accessible length of stream. The number of smolts per kilometer was derived using a log-linear predictive regression of smolt yield given stream length for 22 streams within the CUs of interest. 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 (EVI-GS); 395,603 and 11,968 (GSM); 751,868 and 22,784 (HS-BI); 1,484,479 and 46,005 (LFR); 910,977 and 27,605 (LILL); and 608,082 and 18,427 (BB). Estimated average smolt production and 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 (GS-VI). Results of the Habitat Model are dependent on the amount of habitat available, particularly as it applies to stream order, and to the number of smolts produced per spawner. The Logistic Hockey Stick stock-recruit model estimates that at an assumed future marine survival rate of 2.5%, harvest rates of approximately 35-40% will produce MSY for EVI-GS and GSM CUs; however, at 1.0% survival, harvest rates to produce MSY drop to 1-4% for EVI-GS and GSM CUs, a level more in line with current management practices. While we model, and provide, estimates of Sgen and Smsy, we abstain from recommending these benchmarks due to implementation challenges relating to the fact that escapement is not monitored completely to determine if the benchmark was met and because it requires a reliable pre-season forecast of abundance to determine how much catch to take to end up at Sgen or Smsy. The results of the stock-recruit analysis are highly dependent on marine survival estimates. Data deficiencies prevented stock recruit analyses to be completed on all other CUs, which resulted in no stock recruit analysis conducted on the GSM and LFR MUs.
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