Hyperstability in an inland recreational fishery: Are catch-per-unit-effort data masking the magnitude of steelhead declines?

Abstract Objective Recreational fisheries are complex social–ecological systems, with interactions and feedbacks across local and regional scales and among individual fish, anglers, and managers. Understanding the link between true fish abundance and angler catch per unit effort (CPUE) is crucial to inform management decisions in these systems, especially in the absence of independent monitoring data. For instance, the relationship between fish abundance and CPUE could be linear or nonlinear, such as a hyperstable relationship, which occurs when CPUE remains high even as population abundance declines. We investigated the relationship between fisheries-independent abundance and CPUE in steelhead Oncorhynchus mykiss and evaluated the extent to which hyperstability may obscure underlying population declines. Methods We analyzed the relationship between steelhead abundance and CPUE in 14 streams across the province of British Columbia, Canada. To explore the impact of hyperstability on our capacity to detect changes in abundance, we simulated three scenarios of steelhead decline, comparing the rate of change in CPUE versus abundance. Results Our findings revealed sweeping patterns of hyperstability, indicating that when populations are depressed, CPUE does not decrease as rapidly as abundance. Additionally, we found that the magnitude of CPUE overestimation varied with the extent of population decline. For example, when the population declined by 50%, CPUE decreased by only 40%, representing a 28% overestimation of remaining abundance. This disparity increased in more extreme scenarios of decline. Conclusions Our findings underscore that catch data can mask fish population declines and highlight the need for improved fish population monitoring in recreational fisheries.