Extinction, survival or recovery of large predatory fishes

Abstract

Large predatory fishes have long played an important role in marine ecosystems and fisheries. Overexploitation, however, is gradually diminishing this role. Recent estimates indicate that exploitation has depleted large predatory fish communities worldwide by at least 90% over the past 50-100 years. We demonstrate that these declines are general, independent of methodology, and even higher for sensitive species such as sharks. We also attempt to predict the future prospects of large predatory fishes. (i) An analysis of maximum reproductive rates predicts the collapse and extinction of sensitive species under current levels of fishing mortality. Sensitive species occur in marine habitats worldwide and have to be considered in most management situations. (ii) We show that to ensure the survival of sensitive species in the northwest Atlantic fishing mortality has to be reduced by 40-80%. (iii) We show that rapid recovery of community biomass and diversity usually occurs when fishing mortality is reduced. However, recovery is more variable for single species, often because of the influence of species interactions. We conclude that management of multi-species fisheries needs to be tailored to the most sensitive, rather than the more robust species. This requires reductions in fishing effort, reduction in bycatch mortality and protection of key areas to initiate recovery of severely depleted communities.

Recovery of marine animal populations and ecosystems

“…Here, crucial knowledge gaps exist in how changes in ecosystems or environmental conditions over time have altered such baselines and, hence, recovery prospects. There is also a lack of research on the recovery of currently 11 threatened (e.g. sharks) or highly exploited (e.g.…”

“…A global review of historical baselines for large marine animals found that only 40 out of 256 depleted populations (15.6%) experienced some recovery, 33 of them marine mammals [5]. A more detailed study on marine mammal recovery revealed that 35% of 88 distinct populations experienced significant population increases 11 over three generations (Anna M. Magera, MSc thesis, Dalhousie University, 2011). Interestingly, recovery was more common among pinnipeds (50%) than among cetaceans (16%) and, within cetaceans, was more common in coastal (32%) than offshore (6%) populations.…”

“…Today, many marine mammal, bird, reptile and fish populations are at low abundance, and several species are endangered or extinct on regional or global scales [5,[10][11][12]. However, despite long periods of intense human impacts, most marine species persist and some populations do show signs of recovery [3,8,9].…”

Measuring marine fish biodiversity: temporal changes in abundance, life history and demography

“…With some possible exceptions (Jackson et al 2001), rate of decline among the most severely affected fish has accelerated throughout the latter half of the twentieth century when reductions of more than 80%, relative to recorded (as opposed to true) historical levels, were not uncommon (Hutchings 2000;, particularly among large predators, e.g. Atlantic cod, Gadus morhua (Hutchings & Reynolds 2004;Myers & Worm 2005) and coastal and oceanic sharks (Baum et al 2003;Baum & Myers 2004).…”

“…Using macroecological theory, Jennings & Blanchard (2004) predicted that the current biomass of large fish (4-66 kg) in the intensively fished North Sea is more than 97% lower than that expected in the absence of fisheries exploitation. Myers & Worm (2005) have estimated that the abundance of many large, predatory fish in temperate marine waters is less than 10% of pre-exploitation levels. Among 21 populations of Atlantic cod, for example, they estimate that all have declined more than 70% relative to their estimated abundance in an unfished state, with 18 of the populations declining by more than 90% (Myers & Worm 2005).…”