Colin P. Millar

Can Conservation Stocking Enhance Juvenile Emigrant Production in Wild Atlantic Salmon

AbstractConservation stocking is frequently used by fishery managers to stabilize or increase production of depleted fish stocks. However, in the case of salmonids the benefits are increasingly questioned and generally poorly quantified. We investigated the effects of ova stocking on freshwater emigrant production in a declining Scottish population of “spring” Atlantic Salmon Salmo salar. The stocking program was designed to minimize risks, maximize expected benefits, and bypass population bottlenecks between the ova and fry stages. Long-term data (33 cohorts over 42 years, including 8 years of stocking) on numbers of ova and juvenile emigrant production were used to investigate whether the ova–emigrant stock–recruitment relationship differed between conditions of natural spawning and stocking. We considered Ricker and Beverton–Holt models that included terms for the effects of stocking, intercohort competition, and changes in trap efficiency on emigrant production. The “best model” was considered to be t...

Fishing for MSY: using “pretty good yield” ranges without impairing recruitment

28 Pretty Good Yield (PGY) is a sustainable fish yield corresponding to obtaining no less than a specified large 29 percentage of the Maximum Sustainable Yield (MSY). We investigated 19 European fish stocks to test the 30 hypothesis that 95% PGY yield range is inherently precautionary with respect to impairing recruitment. An FMSY 31 range was calculated for each stock as the range of fishing mortalities (F) that lead to an average catch of at 32 least 95% of MSY in the long term simulations. Further, a precautionary reference point for each stock (FP.05) 33 was defined as the F resulting in a 5% probability of the spawning stock biomass falling below an agreed 34 biomass limit below which recruitment is impaired (Blim) in long‐term simulations. For the majority of the stocks 35 analysed, the upper bound of the FMSY range exceeded the estimated FP.05. However, larger fish species had 36 higher precautionary limits to fishing mortality, and species with larger asymptotic length were less likely to 37