A Common Sense Approach to Ecosystem-Based Fisheries Management

1. A Common Sense Approach to Ecosystem-Based Fisheries Management Rainer Froese, Henning Winker, and Amanda Stern-Pirlot IfM-GEOMAR Düsternbrooker Weg 20 24105 Kiel, Germany rfroese@ifm-geomar.de In this study we show that substantial gains towards the goals of ecosystem-based fisheries management can be achieved by applying ecological common sense to current single-species management, and that this can be done without drastic changes in current catch levels. We used data on the North Sea stock of the Atlantic Cod and compared current impact of fishing on first-time spawners and large adults with an alternative fishing scenario where the same catch is taken only from specimens larger than 88 cm length, i.e., after the size where the biomass of cohorts reaches a maximum (Lopt). Under the current fishing regime only about 20% of first time spawners, 3% of specimens with optimum size (Lopt), and 1% of large spawners survive compared to a no-fishing scenario. In contrast, under the alternative fishing scenario with the same catch 100% of first-time spawners, 100% of specimens at Lopt, and 88% of large spawners survive and are able to fulfil their respective roles in the ecosystem. We also present one example where the alternative fishing scenario has already been successfully implemented. We show here that substantial gains towards the goals of ecosystem based fisheries management can be achieved by applying ecological common sense to current single species management. This can be even done without drastic changes in the current catch levels. North Sea Cod (Gadus morhua) L∞: 119.3 K: 0.24 T0: -0.01 M: 0.25 a: 0.014 b: 3.004 • Methods • We used data on North Sea Cod (Gadus morhua) as published by ICES (2005) and took the means of three cohorts entering the fishery at age two in the years 1992-1994. • The length where the biomass of an unfished cohort reaches its maximum can be obtained from Lopt = Linf (3 / (3 + M/K)) (Beverton 1992). • Biomass for three different fishing scenarios were calculated: (1) current, (2) Lopt, i.e. fishing starts abruptly at Lopt and generates the current catch, and (3) unfished. • For Alaska Pollock (Theragra chalcogramma) we used data by the AFSC (Dorn et al. 2005) and followed the same approach as for the North Sea Cod to calculate scenarios (1) and (3). Fig. 1. Cohort biomass over length, with no exploitation (blue line), exploitation only around Lopt (green line), and actual current exploitation (red line) for the North Sea Atlantic Cod, Gadus morhua. Results and Discussion While attaining the same yield, fishing according to the Lopt regime would increase survival of Atlantic Cod first-time spawners from 20% to 100%, and megaspawners from 1% to 88% (Fig. 1). An upper size limit is not crucial in protecting large fish, because the natural decline in biomass after Lopt is quite steep and thus even without an upper size limit most specimens in the catch will be of Lopt size and enough large ones will survive. This is visible in the current catch regime for Alaska Pollock (Fig. 2), which approaches the Lopt scenario described here: individuals are caught starting around Lopt without an upper size limit. This huge and profitable fishery has a Marine Stewardship Council certification for sustainability. Gulf of Alaska Pollock (Theragra chalcogramma) L∞: 62.58 K: 0.3 T0: -0.05 M: 0.3 a: 0.006 b: 3.019 Fig. 2. Cohort biomass over length, with no exploitation (blue line) and actual exploitation (red line) for the Gulf of Alaska Pollack (Theragra chalcogramma). • Ecosystem benefits of an Lopt fishing regime: • Juveniles are protected, therefore they can remain an important food source for a wide variety of their natural predators. • 2. Most individuals are allowed to grow into large adults which play an important role as top predators shaping the ecosystem. • 3. Bycatch of non-target species is substantially reduced because of the use of larger mesh sizes targeting larger individuals. • 4. Genetic diversity is maintained through reducing impact on all size classes, therefore protecting the species in the face of environmental perturbations. This has positive implications for biodiversity and ecosystem functionality.