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Susceptibility of Sharks, Rays and Chimaeras to Global Extinction

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Susceptibility of Sharks, Rays and Chimaeras to Global Extinction CHAPTER FOUR Susceptibility of Sharks, Rays and Chimaeras to Global Extinction Iain C. Field,*,†,1 Mark G. Meekan,†,2 Rik C. Buckworth,‡ and Corey J. A. Bradshaw§,} Contents 1. Introduction 277 1.1. Aims 280 2. Chondrichthyan Life History 281 2.1. Niche breadth 281 2.2. Age and growth 282 2.3. Reproduction and survival 283 3. Past and Present Threats 284 3.1. Fishing 284 3.2. Beach meshing 305 3.3. Habitat loss 306 3.4. Pollution and non-indigenous species 306 4. Chondrichthyan Extinction Risk 308 4.1. Drivers of threat risk in chondrichthyans and teleosts 309 4.2. Global distribution of threatened chondrichthyan taxa 310 4.3. Ecological, life history and human-relationship attributes 313 4.4. Threat risk analysis 317 4.5. Modelling results 320 4.6. Relative threat risk of chondrichthyans and teleosts 326 5. Implications of Chondrichthyan Species Loss on Ecosystem Structure, Function and Stability 328 5.1. Ecosystem roles of predators 328 * School for Environmental Research, Institute of Advanced Studies, Charles Darwin University, Darwin, Northern Territory 0909, Australia { Australian Institute of Marine Science, Casuarina MC, Northern Territory 0811, Australia { Fisheries, Northern Territory Department of Primary Industries, Fisheries and Mines, Darwin, Northern Territory 0801, Australia } The Environment Institute and School of Earth and Environmental Sciences, University of Adelaide, Adelaide, South Australia 5005, Australia } South Australian Research and Development Institute, Henley Beach, South Australia 5022, Australia 1 Present address: Graduate School of the Environment, Macquarie University, Sydney, New South Wales 2109, Australia 2 Present address: Australian Institute of Marine Science, University of Western Australia Ocean Sciences Institute (MO96), Crawley, Western Australia 6009, Australia Advances in Marine Biology, Volume 56 # 2009 Elsevier Ltd. ISSN 0065-2881, DOI: 10.1016/S0065-2881(09)56004-X All rights reserved. 275 276 Iain C. Field et al. 5.2. Predator loss in the marine realm 331 5.3. Ecosystem roles of chondrichthyans 333 6. Synthesis and Knowledge Gaps 335 6.1. Role of fisheries in future chondrichthyan extinctions 335 6.2. Climate change 337 6.3. Extinction synergies 339 6.4. Research needs 340 7. Concluding Remarks 341 Acknowledgements 343 References 343 Abstract Marine biodiversity worldwide is under increasing threat, primarily as a result of over-harvesting, pollution and climate change. Chondrichthyan fishes (sharks, rays and chimaeras) have a perceived higher intrinsic risk of extinction com- pared to other fish. Direct fishing mortality has driven many declines, even though some smaller fisheries persist without associated declines. Mixed- species fisheries are of particular concern, as is illegal, unreported and unreg- ulated (IUU) fishing. The lack of specific management and reporting mechan- isms for the latter means that many chondrichthyans might already be susceptible to extinction from stochastic processes entirely unrelated to fishing pressure itself. Chondrichthyans might also suffer relatively more than other marine taxa from the effects of fishing and habitat loss and degradation given coastal habitat use for specific life stages. The effects of invasive species and pollution are as yet too poorly understood to predict their long-term role in affecting chondrichthyan population sizes. The spatial distribution of threatened chondrichthyan species under World Conservation Union (IUCN) Red List criteria are clustered mainly in (1) south-eastern South America; (2) western Europe and the Mediterranean; (3) western Africa; (4) South China Sea and Southeast Asia and (5) south-eastern Australia. To determine which ecological and life history traits predispose chondrichthyans to being IUCN Red-Listed, and to examine the role of particular human activities in exacerbating threat risk, we correlated extant marine species’ Red List categorisation with available ecological (habitat type, temperature preference), life history (body length, range size) and human-relationship (whether commercially or game- fished, considered dangerous to humans) variables. Threat risk correlations were constructed using generalised linear mixed-effect models to account for phylogenetic relatedness. We also contrasted results for chondrichthyans to marine teleosts to test explicitly whether the former group is intrinsically more susceptible to extinction than fishes in general. Around 52% of chondrichthyans have been Red-Listed compared to only 8% of all marine teleosts; however, listed teleosts were in general placed more frequently into the higher-risk categories relative to chondrichthyans. IUCN threat risk in both taxa was Susceptibility of Sharks, Rays and Chimaeras to Global Extinction 277 positively correlated with body size and negatively correlated albeit weakly, with geographic range size. Even after accounting for the positive influence of size, Red-Listed teleosts were still more likely than chondrichthyans to be classified as threatened. We suggest that while sharks might not have necessarily experi- enced the same magnitude of deterministic decline as Red-Listed teleosts, their larger size and lower fecundity (not included in the analysis) predispose chon- drichthyans to a higher risk of extinction overall. Removal of these large pre- dators can elicit trophic cascades and destabilise the relative abundance of smaller species. Predator depletions can lead to permanent shifts in marine communities and alternate equilibrium states. Climate change might influence the phenology and physiology of some species, with the most probable response being changes in the timing of migrations and shifts in distribution. The syner- gistic effects among harvesting, habitat changes and climate-induced forcings are greatest for coastal chondrichthyans with specific habitat requirements and these are currently the most likely candidates for extinction. Management of shark populations must take into account the rate at which drivers of decline affect specific species. Only through the detailed collection of data describing demographic rates, habitat affinities, trophic linkages and geo- graphic ranges, and how environmental stressors modify these, can extinc- tion risk be more precisely estimated and reduced. The estimation of minimum viable population sizes, below which rapid extinction is more likely due to stochastic processes, is an important component of this endeavour and should accompany many of the current approaches used in shark manage- ment worldwide. 1. Introduction Humans have depended on marine resources since prehistory (Walker and Deniro, 1986), with the commonly held belief until even recent times that it was beyond human capability to cause the extinction of marine species. This is summarised by two of the foremost thinkers of the eighteenth and nineteenth centuries, Jean Baptiste de Lamarck and Thomas Huxley, who reflected a widespread belief that the high fecundity and wide distributions of marine fishes made the seas an inexhaustible source of food and wealth, and that people could use but a small fraction of the total resources available using fishing methods employed at the time (Garibaldi and Caddy, 2004; Sims and Southward, 2006). Even only a decade ago, a survey of marine scientists revealed that nearly one-third believe marine extinctions are currently not a serious problem (Roberts and Hawkins, 1999). In the past decade, it has become clear that marine biodiversity world- wide is under increasing threat, primarily as a result of over-harvesting, 278 Iain C. Field et al. pollution and the direct and indirect impacts of climate change (Gardner et al., 2003; Harley et al., 2006; Harvell et al., 2002, 2004; Hutchings and Reynolds, 2004; Jackson et al., 2001; Jones et al., 2004; Lotze et al., 2006; Pauly et al., 2002; Roberts, 2002). At present, around 40% of the world’s human population lives within 100 km of the coast (Martinez et al., 2007) and this proportion is increasing. With the median global human population predicted to increase to over 9 billion by 2050 (McMichael, 2001) and more people choosing to live along the coastal fringes, marine habitats are likely to suffer increasing degradation and over-exploitation (Worm et al., 2006). As a corollary, anthropogenic stresses and climatic changes have reduced the resilience of ecosystems in many locations around the globe by slowly degrading habitats and directly harvesting species, causing many ecosystems to switch unexpectedly into alternate states (Folke et al., 2004; Hughes et al., 2003; Nystrom et al., 2000; Scheffer et al., 2001; Worm et al., 2006). Stressors can operate singly or synergistically at multiple scales (Brook et al., 2008), resulting at times in large shifts in species composition. Familiar examples include regime or phase shifts on coral reefs (Aronson et al., 2004; Bellwood et al., 2004; Hawkins and Roberts, 2004; McManus and Polsenberg, 2004), in kelp forests following declines in canopy-forming species (Steneck et al., 2002, 2004), and the abandonment of many coastal and oceanic fisheries (Dulvy et al., 2004b, 2006; Jennings and Kaiser, 1998; Pauly et al., 2002; Roberts, 2002, 2003; Worm et al., 2006). Indeed, despite having sometimes wide geographic distributions and unique regional his- tories, many marine systems have experienced long periods of slow degra- dation followed by rapid acceleration in collapse of the biological communities they support (Lotze et al., 2006). This has been largely attrib- uted to
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