Unraveling the ecological importance of elasmobranchs

Ongoing and rapid declines in populations of many large-bodied sharks throughout the world are now widely recognized as a critical conservation challenge (e.g., Musick, Branstetter, and Colvocoresses 1993; Baum et al. 2003; Baum and Myers 2004; Clarke et al. 2006; Robbins et al. 2006; Myers et al. 2007; Heithaus et al. 2007a; Ferretti et al. 2008). Despite conservation efforts, shark bycatch and a growing demand for shark fins and meat have increased exploitation rates in recent years and many large sharks are listed as Endangered, Vulnerable, or Threatened on the IUCN Red List (but see Okey, Wright, and Brubaker 2007 for a possible exception). Populations of many other elasmobranchs, including rays, skates, and smaller-bodied sharks are responding directly to fishing pressure and indirectly to the removal of large sharks and other marine top predators. Many species that are captured in targeted fisheries or as bycatch are declining, while those exempt from significant fishing mortality may be increasing in locations where their predators or competitors have declined (Shephard and Myers 2005; Okey, Wright, and Brubaker 2007). Further, ongoing climate change is predicted to modify distributions of elasmobranchs and other apex predators (Cairns, Gaston, and Huettmann 2008). Understanding and predicting the broader ecological consequences of these changes in elasmobranch populations is important for marine conservation. To date, however, few studies have provided detailed insights into the ecological role of these fishes.