Temporal Trends in Largemouth Bass Mortality, with Fishery Implications

—We reviewed estimates of annual exploitation (u) and total mortality (Z) for populations of largemouth bass Micropterus salmoides and used a simulation model to explore how temporal changes in u have influenced those populations. The review produced 32 estimates of u and 30 of Z spanning 51 years. Fishing mortality was roughly parabolic through time, with a mean of 0.35 for 1976–1989 and a mean of 0.18 for 1990–2003. Thus, average fishing mortality rates have declined by about one-half since about 1990. Total mortality declined with the decline in u, suggesting that changes in u caused lower overall total mortality rates. The evidence further suggests that the decline in u was caused by the voluntary release of fish by anglers rather than by changes in overall fishing effort. The simulation model showed that the decline in exploitation increased adult largemouth bass abundance but reduced the ability of size and bag regulations to improve population metrics owing to low rates of directed harvest. Discard mortality (i.e., the mortality of fish caught and released) would not negate the benefits of lower exploitation unless the mortality of fish caught and released was 0.3 or higher. Changes in angler behavior have substantially reduced fishing mortality for largemouth bass fisheries, which should be considered when developing management plans for this species and others with high rates of voluntary release. Recreational fisheries are often considered less likely to suffer from recruitment overfishing than commercial fisheries (e.g., Walters and Maguire 1996; Cook et al. 1997; Jackson et al. 2001), but the impacts of recreational fishing can be severe. Post et al. (2002) showed that freshwater recreational fisheries can suffer from growth overfishing across broad geographic scales. Despite greater use of bag and size limits to lower exploitation over the past three decades (Radomski 2003), increasing angler effort in recreational fisheries still results in growth overfishing (Beard and Kampa 1999; Cox and Walters 2002). Identifying trends in fishing mortality for recreational fisheries is difficult owing to a lack of data to assess broad temporal or spatial scales (Post et al. 2002; Beard et al. 2003; Radomski 2003; Cooke and Cowx 2004). Angler behavior also influences the potential for overfishing in recreational fisheries. Beard et al. (2003) found that angler effort declined as bag limits became more restrictive for walleye Sander vitreus in northern Wisconsin lakes. In contrast, voluntary release of fish that are legal to harvest has increased for many recreational fisheries (Quinn 1996). We found no studies evaluating whether observed shifts in angler behavior (e.g., Quinn 1996) can result in long-term trends in fish population metrics such as fishing or total mortality, and there is a need to evaluate broad spatial and temporal trends in mortality sources. Largemouth bass Micropterus salmoides support some of the most popular recreational fisheries in North America, and some analyses have evaluated fishery responses to size limits across broad geographic scales. Beamesderfer and North (1995) used growth and natural mortality rates across largemouth bass populations to simulate potential fishery yield responses to harvest. Wilde (1997) conducted a meta-analysis of largemouth bass population responses to harvest restrictions. He concluded that minimum length limits did not influence fish abundance and size, whereas protective slot limits had some ability to increase both variables. His results were surprising because if fishing mortality were high, we would expect that protective size limits would influence fish abundance and size. Detecting effects of size limits can be difficult owing to * Corresponding author: [email protected] Received November 9, 2006; accepted April 27, 2007 Published online March 13, 2008 418 North American Journal of Fisheries Management 28:418–427, 2008 Copyright by the American Fisheries Society 2008 DOI: 10.1577/M06-264.1 [Article]