Comparing Dynamic and Static Time-Area Closures for Bycatch Mitigation: A Management Strategy Evaluation of a Swordfish Fishery

Abstract

Time-area closures are a valuable tool for mitigating fisheries bycatch. There is increasing recognition that dynamic closures, which have boundaries that vary across space and time, can be more effective than static closures at protecting mobile species in dynamic environments. We created a management strategy evaluation to compare static and dynamic closures in a simulated fishery based on the California drift gillnet swordfish fishery, with closures aimed at reducing bycatch of leatherback turtles. We tested eight operating models that varied swordfish and leatherback distributions, and within each evaluated the performance of three static and five dynamic closure strategies. We repeated this under 20 and 50% simulated observer coverage to alter the data available for closure creation. We found that static closures can be effective for reducing bycatch of species with more geographically associated distributions, but to avoid redistributing bycatch the static areas closed should be based on potential (not just observed) bycatch. Only dynamic closures were effective at reducing bycatch for more dynamic leatherback distributions, and they generally reduced bycatch risk more than they reduced target catch. Dynamic closures were less likely to redistribute fishing into rarely fished areas, by leaving open pockets of lower risk habitat, but these closures were often fragmented which would create practical challenges for fishers and managers and require a mobile fleet. Given our simulation’s catch rates, 20% observer coverage was sufficient to create useful closures and increasing coverage to 50% added only minor improvement in closure performance. Even strict static or dynamic closures reduced leatherback bycatch by only 30–50% per season, because the simulated leatherback distributions were broad and open areas contained considerable bycatch risk. Perfect knowledge of the leatherback distribution provided an additional 5–15% bycatch reduction over a dynamic closure with realistic predictive accuracy. This moderate level of bycatch reduction highlights the limitations of redistributing fishing effort to reduce bycatch of broadly distributed and rarely encountered species, and indicates that, for these species, spatial management may work best when used with other bycatch mitigation approaches. We recommend future research explores methods for considering model uncertainty in the spatial and temporal resolution of dynamic closures.

Trade-offs between bycatch and target catches in static versus dynamic fishery closures

“…A major concern for the efficacy of spatial and temporal closures is the extent to which, rather than reducing total fishing effort, they simply redistribute effort away from some areas/times while concentrating it in others ( 17 – 20 ). If fishing effort is displaced rather than removed, spatial and temporal closures can create trade-offs between reduced fishing mortality on bycatch and target species inside no-take protected areas or during protected times, and potentially increase fishing mortality of nontarget species in waters surrounding closures ( 19 – 21 ).…”

“…However, the optimal habitats of mobile species can shift within and across years due to environmental variability and climate change, thereby modifying interactions between fisheries and bycatch species in space and time. Dynamic ocean management is an emerging tool in which management measures (e.g., closures) can change across space and time in response to environmental variability and ocean uses ( 28 ), helping to balance bycatch and economic opportunity for fisheries ( 18 , 29 , 30 ). Ideally, dynamic management tools can integrate near-real-time environmental data to track multiple species’ habitats to optimize bycatch to target catch ratios ( 31 , 32 ).…”

“…Some studies have considered the general advantages of dynamic ocean management ( 29 , 31 , 33 ) and provided assessments of the role of spatial closures in bycatch reduction in specific case studies ( 18 ). Along with bycatch reduction alone, a key consideration is how policies can be designed to minimize bycatch while maintaining the economic, nutritional, and cultural benefits obtained by sustainably managing target species ( 34 – 36 ).…”

Management Strategy Evaluation: Allowing the Light on the Hill to Illuminate More Than One Species

“…The swordfish MSE was created to evaluate spatial management strategies used for bycatch mitigation (Smith et al, 2021). The drift gillnet fishery (DGN) was used as a case study, given the availability of spatially explicit catch data from an extensive observer program, the existence of a large static spatial closure aimed at reducing bycatch of leatherback turtles, Dermochelys coriacea (the Pacific Leatherback Conservation Area, PLCA), and the development of a multi-species dynamic bycatch risk tool for this fishery ('EcoCast'; Hazen et al, 2018).…”

“…The ten radar plot performance metrics are: 'TotSF' total swordfish catch per fishing season; 'SFset' mean number of swordfish caught per set; 'TotLB' total number of leatherback turtles caught per season; 'LBset' mean number of turtles caught per set; 'LB/SF' the number of turtles caught per swordfish caught; 'TotBS' the total number of blue sharks caught per season; 'Profit' the mean profit per fishing trip from swordfish revenue minus fuel and crew costs; 'Dist' the mean distance traveled per fishing trip; 'Sets' the number of successful fishing sets (i.e., effort); 'Area' the amount of area open to fishing. Figure adapted fromSmith et al (2021).…”