Wednesday, December 02, 2009

What everybody needs to know

I stand corrected.

I was intuitively of the opinion that oceanic Sharks are most likely doomed for extinction, a hypothesis that reverberates in some of my posts like this one.
I was obviously influenced by the fact that I live in the South Pacific where fishing for coastal Sharks, compared to the killing of Sharks by the Tuna fleets, appears to be a relatively minor threat. But CJA Bradshaw is of course right when he says that globally, the threats to coastal species are higher as they don't only have to contend with fishing pressure, but with habitat degradation and climate change on top of that, likely most of it anthropogenic (the latter being a comment by me, not him).

The relevant paper he has co-authored is called Susceptibility of Sharks, Rays and Chimaeras to Global Extinction and I invite anybody who wants to talk about Shark conservation with any degree of authority to download it here. Yes you will have to pay for it - but it's a must-read and must-have and the authors need to be compensated for their hard work.

Please, take the time to read it in its entirety.
But for those in a hurry, here's a synopsis on Bradshaw's own excellent conservation blog. The authors have also published the following FOC abstract (highlights in italic are mine).

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 compared 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 unregulated (IUU) fishing.
The lack of specific management and reporting mechanisms 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 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 experienced the same magnitude of deterministic decline as Red-Listed teleosts, their larger size and lower fecundity (not included in the analysis) predispose chondrichthyans to a higher risk of extinction overall.

Removal of these large predators 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 synergistic 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 geographic ranges, and how environmental stressors modify these, can extinction 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 management worldwide.

I find this paper so important that I've taken the liberty of posting its conclusions below.
They tie in beautifully with some of this blog's principal threads, among which the requirement to push for sustainable fisheries. What I however miss is the call for applying the precautionary principle (much called for and rarely heeded, especially in Fisheries management) until the necessary data have been collected and evaluated - but then again, this is a paper dealing with facts and not a conservation manifesto.

Required reading and kudos to the authors for having written an excellent, informative, exhaustive and in so may ways, seminal paper for Shark conservation!

7. Concluding Remarks

We are still in the fortunate situation that
there are no recorded cases of chondrichthyan extinction in modern times.
However, we have identified that the largest, most range-restricted and heavily harvested species might be easily pushed below their MVP sizes, which could be much larger than those estimated under stable environmental conditions.
Fishing, at all scales, represents one of the largest mortality sources for many chondrichthyan species, but there are some examples of small local fisheries that have operated without clear declines in population size of targeted species. However, mixed-species fisheries that harvest poorly measured, but presumably large quantities of chondrichthyans are of particular concern, as is IUU fishing.

The lack of specific management and reporting mechanisms for the latter types means that many species might already be reduced to densities where extinction risk is unacceptably high.
It is almost universally recognised now that so-called ‘sustainable’ fisheries will have to be the norm if they are to survive economically, and that they will have to demonstrate negligible or minimal impacts to ecosystems through careful management and stewardship (Hilborn, 2007). IUU fishing can affect shark populations and community structure, and this might be a far greater challenge to control. Recreational fishing and beach meshing can also contribute to local declines. Climate change and habitat degradation will further erode certain populations to the point where extinction risk rises appreciably.

The idea that chondrichthyans have life history characteristics that might predispose them to extinction in a rapidly changing world (e.g. relatively low reproductive potential, growth and capacity for population recovery; Pratt and Casey, 1990) is generally upheld by our results.
Furthermore, because chondrichthyans tend to occupy the highest trophic levels, it is arguable that degradation of marine communities might limit the prey quality and quantity available to chondrichthyan predators, further exacerbating population reductions. We found no strong evidence, from admittedly simple models with few parameters, that chondrichthyans are intrinsically more susceptible to extinction than other marine fishes in relation to their evolved niches and life history characteristics. However, chondrichthyans tend to be larger than many other marine fish taxa, and large body size generally correlates with slower growth and lower reproductive capacity. As such, it is the rapid pace of environmental change and harvesting that have the greatest potential to impede certain species from maintaining large population sizes. Any species can withstand moderate to even extreme exploitation if it does not outpace intrinsic replacement rates and adaptation potential (Brook et al., 2008).

We were unable to examine all plausible correlates of threat risk due to data paucity.

Many studies have examined age at maturity and growth rates in terms of vulnerability to extinction, with late-maturing and slow-growing species apparently at greater risk (Reynolds et al., 2005). Therefore, a better compilation of data incorporating these and other possible correlates could reveal further subtleties in the drivers of threat risk in this taxon and other marine fishes. Another caveat is that predictors of threat risk indicate a species’ sensitivity to the largely systematic (deterministic) drivers of population decline (declining population paradigm) (Cardillo, 2003; Sodhi et al., 2008a), whereas actual extinction appears to correlate poorly with ecological and life history traits given that the final coup de
grĂ¢ce tends to result from largely stochastic processes that act independently of a species’ evolutionary history (Brook et al., 2006, 2008; Sodhi et al., 2008b; Traill et al., 2007)

There are many examples of how large predators influence communities and ecosystems via top-down (and in some cases, bottom-up) control of species occupying lower trophic levels.
Thus, the removal of large predators can elicit trophic cascades and destabilise the relative abundance of smaller prey and non-prey species. However, these effects are still poorly understood, especially for large, complex trophic webs where interactions are largely unquantified. Specifically, chondrichthyans can alter prey diversity and size distributions, and their mere presence can affect the foraging behaviour of prey that alters ecosystem functions such as nutrient recycling and structural habitat complexity.

Severe predator depletions can lead to permanent shifts in marine communities and alternate equilibria.

Management of shark populations must therefore take into account the rate at which drivers of decline affect specific species.
Only through detailed collection of data describing demographic rates, habitat affinities, trophic linkages and geographic ranges, and how environmental stressors modify these, can extinction risk be estimated and reduced. The estimation of MVP sizes is an essential component of this endeavour and should, in our view, eventually accompany the current approaches used to manage sharks worldwide.

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