The influence of seasonal migrations on fishery selectivity

Based on previous work, dome-shaped fishery selectivity patterns are expected in place of asymptotic patterns when one-way fish movements among areas are considered. It is less clear if this occurs when the “round-trip” seasonal movements are considered. A simulation of a long-distance migrating fish stock (Atlantic menhaden) was used to study the influence of life history and fishery processes on selectivity, under an “areas as fleet” stock assessment context. When age-constant two-way migration was assumed to occur at a low rate, a domed selectivity pattern in the area experiencing the highest fishing mortality was produced, consistent with previous work. However, as the two-way migration rate increased, the domed selectivity pattern diminished and eventually disappeared. When age-varying migration was introduced, with a higher movement probability for older fish, domed selectivity prevailed in the source (i.e. spawning) area. If movement away from the spawning area occurs at younger ages than are selected by the fishing gear, the extent of the dome in this area is reduced. When movement away from the spawning area occurs at ages that are already available to the fishing gear, the dome in the spawning area is exaggerated. The area in which domed selectivity occurred was primarily determined by whether the probability of movement increased or decreased with age. In contrast to previous work that considered one-way or diffusive movement, the temporal or spatial distribution of recruitment and overall fishing mortality did not have a significant influence on selectivity. Building simulations that reflect the life history of the stock can guide assessment efforts by placing priors and constraints on model fits to selectivity patterns and be used to explore trade-offs between model complexity and the ability to produce reasonable management advice. Their development is encouraged as a standard feature in the assessment of migratory fish stocks.