Effects of cryptic mortality and the hidden costs of using length limits in fishery management

Abstract

Introduction 197Methods 197 Model formulation 198Parameter value specification 199 Results 200Discussion 203Abstract Fishery collapses cause substantial economic and ecological harm, but common management actions often fail to prevent overfishing. Minimum length limits are perhaps the most common fishing regulation used in both commercial and recreational fisheries, but their conservation benefits can be influenced by discard mortality of fish caught and released below the legal length. We constructed a computer model to evaluate how discard mortality could influence the conservation utility of minimum length regulations. We evaluated policy performance across two disparate fish life-history types: short-lived high-productivity (SLHP) and long-lived low-productivity (LLLP) species. For the life-history types, fishing mortality rates and minimum length limits that we examined, length limits alone generally failed to achieve sustainability when discard mortality rate exceeded about 0.2 for SLHP species and 0.05 for LLLP species. At these levels of discard mortality, reductions in overall fishing mortality (e.g. lower fishing effort) were required to prevent recruitment overfishing if fishing mortality was high. Similarly, relatively low discard mortality rates (>0.05) rendered maximum yield unobtainable and caused a substantial shift in the shape of the yield response surfaces. An analysis of fishery efficiency showed that length limits caused the simulated fisheries to be much less efficient, potentially exposing the target species and ecosystem to increased negative effects of the fishing process. Our findings suggest that for overexploited fisheries with moderate-to-high discard mortality rates, reductions in fishing mortality will be required to meet management goals. Resource managers should carefully consider impacts of cryptic mortality sources (e.g. discard mortality) on fishery sustainability, especially in recreational fisheries where release rates are high and effort is increasing in many areas of the world.

Direct observation of fishing effort, catch, and discard rates of charter boats targeting reef fishes in the northern Gulf of Mexico

“…Catch rates for species other than red snapper were generally low, and overfished species (i.e., gag, gray triggerfish, and greater amberjack) were discarded because of either minimum length limits or closed seasons. High discard mortality of physoclistous fishes caught in deep waters (depths >40 m), combined with intense gulf-wide recreational fishing effort (Coleman et al, 2004;Hanson and Sauls, 2011), likely amplifies the impact of discarding practices during closed-season trips given the greater depths fished compared to openseason trips (Wilson and Burns, 1996;Coggins et al, 2007;Rummer, 2007;Campbell et al, 2014).…”

Ecosystem-based fisheries management requires a change to the selective fishing philosophy

“…They suggest that the widespread potential for rapid and large effects on size-or life historymediated ecological dynamics might imperil populations, industries, and ecosystems (31,37). Although regulations on minimum size limits have often been imposed for good reasons, their negative impacts on genetic, population, and ecosystem diversity are being increasingly recognized (39,40). Evidence also shows that size selection increases the variability of fish population abundance (22,41,42) and that fishery-induced evolution increases the stock rebuilding and recovery periods when large fish are selectively removed (43).…”

Depth interactions and reproductive ecology of sympatric Sillaginidae: Sillago robusta and S. flindersi

“…In high-volume multispecies trawl fisheries, it is often preferable to manage and regulate the sizes of fish retained by prescribing the selectivity of the fishing gears rather than enforce-ment of specific legal lengths, which can be logistically problematic for fisheries operators as well as compliance officers. Nevertheless, prior to introducing and mandating any specific fishing gears (or retained legal lengths for these sillaginid species) in any particular fishery, assessments of rates of survival of non-retained individuals in such fishing gears (Broadhurst et al 2006, Coggins et al 2007, as well as broader market and economic impacts need to be considered. Moreover, the potential effects on population reproductive output and resource sustainability of alternate management arrangements that protect larger (more fecund) fish, either by harvesting particular slot sizes (Gwinn et al 2013) or provision of refuge (no-take) areas (Roberts et al 2005) needs investigating.…”