Cascading effects of the loss of apex predatory sharks from a coastal ocean.

Department of Biology, Dalhousie University, 1355 Oxford Street, Halifax, NS B3H 4J1, Canada.
Science (Impact Factor: 31.48). 03/2007; 315(5820):1846-50. DOI: 10.1126/science.1138657
Source: PubMed

ABSTRACT Impacts of chronic overfishing are evident in population depletions worldwide, yet indirect ecosystem effects induced by predator removal from oceanic food webs remain unpredictable. As abundances of all 11 great sharks that consume other elasmobranchs (rays, skates, and small sharks) fell over the past 35 years, 12 of 14 of these prey species increased in coastal northwest Atlantic ecosystems. Effects of this community restructuring have cascaded downward from the cownose ray, whose enhanced predation on its bay scallop prey was sufficient to terminate a century-long scallop fishery. Analogous top-down effects may be a predictable consequence of eliminating entire functional groups of predators.


Available from: Julia K Baum, May 29, 2015
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    Marine Historical Ecology in Conservation: Applying the Past to Manage for the Future, Edited by John N. (Jack) Kittinger, Loren McClenachan, Kerun B. Gedan, Louise K. Blight, 01/2014: chapter Using Disparate Datasets to Reconstruct Historical Baselines of Animal Populations: pages 63-85; University of California Press.
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    ABSTRACT: Shark fishery is not a target fishery in Bangladesh water rather regarded as by-catch. A total of 36 species of sharks, five species of skates and 30 species of rays were so far recorded from Bangladesh waters but no chimaerids. Fish Act has no restriction on harvesting of sharks, while Forestry Act restricts it in and around the Sundarbans. Catch records reflect that catches are declining and bulk of the catches are composed of small sized individulas. Study on the biology of Elasmobranch fishes of Bangladesh is very scanty and this is probably because of the difficulty in getting adequate statistics and samples. Review of stock assessments and present stock status of sharks is essential with a National Plan of Action (NPOA) for a regional fishery management plan which would help in introducing and promoting collaborative fisheries management approaches in the BoB region. Fishery Act should be updated with strong rules and proper monitoring, and control and surveillance (MCS) should be implemented for restricting indiscriminate exploitation of the Elasmobranchs.
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    ABSTRACT: Elasmobranchs are among the oldest and most successful predators in the ocean, yet one of the most vulnerable to the direct and indirect effects of fishing. Many populations are rapidly declining around the world, and an increasing number is listed as threatened or endangered. The broader ecosystem consequences of these declines, and whether other marine predators can replace sharks, are open questions. In this thesis, I used a diverse set of data and modeling techniques to analyze long-term changes in elasmobranch populations in the Mediterranean Sea, and the consequences of shark declines on marine ecosystems. Because of its long history of fishing, the Mediterranean offers a unique perspective on the response of marine communities to exploitation over long time scales. Here, I reconstructed the history of elasmobranch exploitation over the past 200 years in pelagic, coastal and demersal communities. Results were combined meta-analytically to derive a general pattern of change for the entire region. Overall, I detected multiple cases of regional species extirpations, a strong correlation between historical intensity of exploitation and the stage of community degradation, and some cases of compensatory species increases. My results suggest that compared to other marine ecosystems worldwide, the Mediterranean Sea might be in an advanced stage of overexploitation. To gain more general conclusions about the patterns and consequences of shark declines in the ocean, I reviewed and reanalyzed documented changes in exploited elasmobranch communities around the world, and synthesized the effects of sharks on their prey and wider communities. This work revealed that sharks are abundant and diverse in little exploited or unexploited marine ecosystems but vulnerable to even light levels of fishing. The decline in large sharks has reduced natural mortality in a range of their prey, contributing to changes in abundance, distribution, and behaviour of marine megafauna that have few other predators. In some cases, this has resulted in cascading changes in prey populations and food-web structure. Overall, my thesis greatly enhanced our knowledge about the critical state of elasmobranchs in the Mediterranean Sea and the consequences of the declines of these important marine predators on marine ecosystems.
    12/2010, Degree: PhD, Supervisor: Ransom A. Myers, Heike K. Lotze, Andrew A. Rosenberg