Linking the dynamics of harvest effort to recruitment dynamics in a multistock, spatially structured fishery

Abstract

A freshwater sport fishery that targets hundreds of geographically isolated stocks is simulated by combining a model of angler behavior with a model of rainbow trout (Oncorhynchus mykiss) population dynamics. Ideal free distribution (IFD) theory, which suggests that angling quality will be similar on all lakes, is used to drive angler effort distribution. Model parameters are based on creel survey data from 53 lakes and empirical relationships between growth, survival, and density derived from whole-lake density manipulations on nine lakes over a period of 10 years. We compared angling quality, population density, fish size, and yield under unfished conditions, harvest rates that maximize sustained yields (MSY), and an IFD equilibrium driven by angler behavior. The IFD equilibrium rarely maximized yields. Stocks with high MSY angling quality are overexploited at the IFD equilibrium because anglers move to take advantage of exceptional angling opportunities. These stocks would often be viewed as more resistant to harvest pressure because they have higher stock productivities and habitat capacities. However, in our model, they are systematically overharvested because their high fish density attracts excessive angling pressure. Conversely, stocks with low MSY angling quality are underexploited because anglers move to take advantage of better angling quality on other lakes.

Mechanisms for climate-induced mortality of fish populations in whole-lake experiments

“…For instance, survival of young trout in the presence of larger cannibalistic adult trout increases as a steep nonlinear function of body size in our lakes (21). By late-August, fish have grown sufficiently to have reached the upper asymptotic size-dependent survival (21), and at that time age-0 trout use deep littoral and pelagic habitats that are normally highly risky (7,13). Survival rates of young trout reach a high asymptote at a length of Ϸ80 mm (21), which corresponds with observed trout size by late August (7).…”

“…These values were averaged and summed, respectively, over the period from stocking until mid-August to correspond with our behavioral measures and to correspond with the period over which age-0 trout mortality is highest. For instance, survival of young trout in the presence of larger cannibalistic adult trout increases as a steep nonlinear function of body size in our lakes (21). By late-August, fish have grown sufficiently to have reached the upper asymptotic size-dependent survival (21), and at that time age-0 trout use deep littoral and pelagic habitats that are normally highly risky (7,13).…”

“…By late-August, fish have grown sufficiently to have reached the upper asymptotic size-dependent survival (21), and at that time age-0 trout use deep littoral and pelagic habitats that are normally highly risky (7,13). Survival rates of young trout reach a high asymptote at a length of Ϸ80 mm (21), which corresponds with observed trout size by late August (7). Average air temperatures over time were obtained from the nearest weather station located at the nearby town of Merritt, BC.…”

Predicting walleye recruitment as a tool for prioritizing management actions

“…Although most recruitment analyses have considered variability over time within a single population, management of inland recreational fisheries requires that spatial variability in recruitment also be considered. Many recreational fisheries consist of a landscape of relatively isolated stocks linked by mobile anglers, and these fisheries are managed using a small set of regulations to limit harvest on a large number of systems Carpenter and Brock 2004;Parkinson et al 2004). Failure to account for heterogeneity among systems -including recruitment heterogeneity -may contribute to collapse of important recreational fisheries Parkinson et al 2004).…”

“…Many recreational fisheries consist of a landscape of relatively isolated stocks linked by mobile anglers, and these fisheries are managed using a small set of regulations to limit harvest on a large number of systems Carpenter and Brock 2004;Parkinson et al 2004). Failure to account for heterogeneity among systems -including recruitment heterogeneity -may contribute to collapse of important recreational fisheries Parkinson et al 2004). Recruitment variability can also obscure population trends (e.g., Peterman and Bradford 1987) and responses to management actions (e.g., Allen and Pine 2000).…”