Bull Trout Population Responses to Reductions in Angler Effort and Retention Limits

Abstract

We compared historical (1977–1980) and recent (1997–2001) abundance, catch-per-unit-effort (CPUE), and growth data to assess whether the implementation of restrictive sportfishing regulatory regimes in the 1990s led to changes in abundance and population structure of bull trout Salvelinus confluentus in two small Rocky Mountain lakes in Alberta, Canada. For remote Harrison Lake, we used changes in gill-net CPUE to infer a fivefold increase in bull trout abundance after closure of an access road and implementation of catch-and-release (CR) regulations. Bull trout growth rates decreased as their abundance increased. All large (fork length > 420 mm), old (age >12) bull trout were eliminated after regulatory changes were imposed. Reductions in prey abundance and size as bull trout abundance increased probably contributed to the demise of the large bull trout. For road-accessible Osprey Lake, no change in bull trout mark–recapture abundance estimates or growth rates were observed despite implementation of CR regulations and, later, closure of the lake to angling. We speculate that illegal angling and migration of bull trout between Osprey Lake and its tributary streams limited abundance responses at this site. A variety of site-specific factors, including ease of access, angler noncompliance with regulations, and local metapopulation structure of bull trout, led to variable bull trout population responses to the implementation of restrictive angling regulations. Active advertisement and enforcement of regulations may be required to achieve increased bull trout abundance at small, easily accessible waterbodies.