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.

Globally, bycatch in tropical/subtropical shrimp trawl and longline fisheries is threatening many marine species. Here we examine the joint effects of increased mortality caused by shrimp trawling bycatch, and reduced predation caused by losses of large sharks because of longline fishing. Research surveys in the Gulf of Mexico demonstrated precipitous declines in shallow water coastal elasmobranchs where shrimping effort was highest (bonnethead 96%, Bancroft's numbfish (lesser electric ray) 98%, smooth butterfly ray > 99%) and consistent increases in deeper water elasmobranchs (Atlantic angel shark, smooth dogfish). These increases are the first empirical support for predation release caused by the loss of large sharks, which have been theorized to structure tropical/subtropical marine ecosystems. Bycatch of elasmobranchs in shrimp trawls is a critical conservation concern which is not solved by present mitigation measures; similar loss of elasmobranchs is expected to be occurring in tropical/subtropical regions worldwide where ever intensive shrimp trawling occurs.

Introduction 142Abundance-area relationships 142Habitat specific -total population growth relationship 144Commonality in spatial dynamic models 145Does density-dependent habitat selection exist in marine fish? 148Effect of spatial scale 149 Conclusion 150Acknowledgements 150References 151Abstract Current methods and theory used in the study of the spatial dynamics of marine fish are problematic. Positive relationships between population abundance and occupied area are typically interpreted as evidence of density-dependent habitat selection. However, both abundance and area may co-vary with an un-parameterized variable, such as a density-independent effect. In addition, if density-dependent habitat selection is present, population growth rates in optimal habitats would be expected to be lower than in marginal habitats. This same pattern can also evolve from a largescale, spatially autocorrelated change in a density-independent factor. The theory underlying density-dependent habitat selection, the ideal free distribution, can be tautological when no a priori information of how habitat suitability changes with density is known. In this case, an ideal free distribution can be defined for any pattern of habitat-specific population growth rates. However, these problems are not insurmountable and solutions may be found by considering spatial variation in proxies of fitness and explicitly allowing for the relative importance of habitat selection (density dependent) and environmental (density independent) effects to vary with spatial scale.

No abstract

No abstract