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

Abstract

While there have been recent improvements in reducing bycatch in many fisheries, bycatch remains a threat for numerous species around the globe. Static spatial and temporal closures are used in many places as a tool to reduce bycatch. However, their effectiveness in achieving this goal is uncertain, particularly for highly mobile species. We evaluated evidence for the effects of temporal, static, and dynamic area closures on the bycatch and target catch of 15 fisheries around the world. Assuming perfect knowledge of where the catch and bycatch occurs and a closure of 30% of the fishing area, we found that dynamic area closures could reduce bycatch by an average of 57% without sacrificing catch of target species, compared to 16% reductions in bycatch achievable by static closures. The degree of bycatch reduction achievable for a certain quantity of target catch was related to the correlation in space and time between target and bycatch species. If the correlation was high, it was harder to find an area to reduce bycatch without sacrificing catch of target species. If the goal of spatial closures is to reduce bycatch, our results suggest that dynamic management provides substantially better outcomes than classic static marine area closures. The use of dynamic ocean management might be difficult to implement and enforce in many regions. Nevertheless, dynamic approaches will be increasingly valuable as climate change drives species and fisheries into new habitats or extended ranges, altering species-fishery interactions and underscoring the need for more responsive and flexible regulatory mechanisms.

Keywords: bycatch mitigation; fisheries management; marine protected areas; static and dynamic closures.

Fig. 1.

Location of the fishery case studies used in the analysis. LL: Longline; DGN: Drift Gillnet; TRW: Trawling; PS: Purse-seine.

Fig. 2.

Representation of 1) a static area closed around a centroid (no movement from year to year; first row) or dynamic (moves from year to year; second row); and 2) a mosaic area, static (third row) and dynamic (fourth row).

Fig. 3.

Relative changes for each type of closure for bycatch (Top), target catch when total effort remains the same (constant effort, Middle), and effort when total catch remains the same (constant catch, Bottom). For bycatch, relative changes for both constant effort and constant catch scenarios were combined for simplicity and because there were almost no differences between them (SI Appendix, Fig. S11). Points represent individual case studies; lines are a smooth curve with the band around them representing one SD. The column on the left represents when fishing efficiency remain constant, and the column on the right when fishing efficiency (target species CPUE) decreases. The primary x-axis shows the proportion of area closed from 0.1 or 10% to 0.5 or 50% of the fishing zone. For temporal closures, the number of months closed are represented on the secondary x-axis at the top (gray line only).

Fig. 4.

Relative changes for each type of closure when closing 30% of the total area to fishing for bycatch (Top), target catch when total effort remains the same (constant effort, Middle), and effort when total catch remains the same (constant catch, Bottom). For bycatch, both constant catch and constant effort scenarios were combined for simplicity and because there were almost no differences between them (SI Appendix, Fig. S12). The column on the left represents when fishing efficiency is constant, and the column on the right when fishing efficiency decreases. The box represents the quartiles (25, 50, or 75 percentiles) in which 50% (horizontal line in the box) is the median. The upper whisker is the maximum value of the data that is within 1.5 times the interquartile range over the 75th percentile. The lower whisker is the minimum value of the data that is within 1.5 times the interquartile range under the 25th percentile. Each case study is represented by the gray dots. The horizontal dashed line is the status quo.

Fig. 5.

Relative changes for each type of closure and each case study (rows) for total bycatch and catch of target species. These results are for the scenarios during which total effort remains the same (constant effort) and fishing efficiency decreases (Right) or remains constant (Left). The primary x-axis shows the proportion of area closed from 0.1, or 10%, to 0.5, or 50%, of the fishing zone. For temporal closures, the number of months closed are represented on the secondary x-axis at the top.

Fig. 6.

Relative changes for each type of closure and each case study (rows) for total bycatch and effort. These results are for the scenarios when total catch of target species remains the same (constant catch) and fishing efficiency decreases (Right) or remains constant (Left). The primary x-axis shows the proportion of area closed from 0.1, or 10%, to 0.5, or 50%, of the fishing zone. For temporal closures, the number of months closed are represented on the secondary x-axis at the top.

Fig. 7.

Relationship between predicted bycatch reduction (x-axis) and correlation between total bycatch and total target species (y-axis). Bycatch reduction indicates relative change compared to no closure (= 1). Each dot represents a different case study, and this plot shows, as an example, the results from a 30% closed area in a dynamic mosaic approach. The solid line represents a simple regression and the gray area the 95% CI. LL: Longline; DGN: Drift Gillnet; TRW: Trawling; PS: Purse-seine. US: United satte; EU: European Union; IATTC: Inter-American Tropical Tuna Commission.