74-15 Save the Bay, Eat a Ray: A Purported Trophic Cascade Mediated by Declines in Large Shark Populations and the Consequences of Applying Simplistic Models to Complex Ecosystems
Top-down versus bottom-up control of marine ecosystems has been debated for decades. Mechanisms driving mortality and energy flow are often system dependent and difficult to distinguish, especially in large systems with complex trophic webs. Large sharks have long been described as “apex predators” that assert top-down influences on ecosystems, though little empirical evidence for this existed. As populations of large sharks declined in recent decades, numerous researchers have sought “evidence” for cascading effects of these predator reductions on lower trophic levels. For example, it was reported that predation release due to declines in large coastal sharks in the northwest Atlantic Ocean led to dramatic increases in populations of smaller elasmobranchs, including cownose rays (Rhinoptera bonasus). Increased predation by cownose rays was in turn reported to have caused declines in populations of commercially important bivalve mollusks, even causing fishery collapses for some taxa. This purported trophic cascade has been cited frequently in the scientific literature, in the media, and has been used successfully to justify market development and harvesting of cownose rays in spite of the facts that cownose rays have among the lowest lifetime fecundity of any marine vertebrate and congeners have been driven to endangered status by relatively small fisheries. Recently, the harvest and consumption of cownose rays has been touted as environmentally-friendly. For example, the “Save the Bay, Eat a Ray” media campaign suggests that by consuming cownose rays, citizens will be aiding recovery of the degraded Chesapeake Bay. We will challenge the validity of this trophic cascade by presenting alternate hypotheses for the observed changes in relative abundances of large sharks and meso-predators and challenge the magnitudes of these population changes. For example, abundance trends from stock wide surveys do not infer similar population declines for large sharks. Data on shark diets also suggest the potential for top-down regulation may have been exaggerated and in some cases the reported trophic linkages may not exist. The marine food web along the East Coast is extremely complex and we suggest the use of simplistic three-level trophic models may lead to inappropriate conclusions concerning the mechanisms behind observed changes in relative abundances. We will also demonstrate that harvesting cownose rays will not have the desired effect of limiting mortality on commercial bivalves and will propose alternatives. Finally, we will call for objective interpretation and the consideration of alternative explanations from future research analyzing relative abundance data for elasmobranch fishes.