Saturday, April 21, 2012

Shark Populations: Rate of Decline, Rebound Potential and Conservation Strategies!

Fiji's Sharks - still in relatively good shape, especially in the SRMR!

Great post by David!
It ties in with this and describes various ways in which researchers try to document the rate of decline of Shark stocks. The takeaway message: all methods are prone to errors - but regardless of the method, all results indicate that in most areas that have been surveyed, Shark stocks have undergone sometimes precipitous decreases.
This as always with the caveat that the numbers vary considerably depending on species and locations, and that there are many areas that have not been properly surveyed. The biggest threat to Sharks is clearly overfishing but they are obviously equally subjected to all other anthropogenic threats including, and here I differ with David, the depletion of their prey. Yes due to the complexity of food webs, it has not been specifically documented - but to me, it never the less remains a highly plausible hypothesis.

Enter this new paper by Christine Ward-Paige et al.
Yes that would be she of the Great Fiji Shark Count - and obviously, the paper is just great!


Many elasmobranchs have experienced strong population declines, which have been largely attributed to the direct and indirect effects of exploitation.
Recently, however, live elasmobranchs are being increasingly valued for their role in marine ecosystems, dive tourism and intrinsic worth.
Thus, management plans have been implemented to slow and ultimately reverse negative trends, including shark-specific (e.g. anti-finning laws) to ecosystem-based (e.g. no-take marine reserves) strategies. Yet it is unclear how successful these measures are, or will be, given the degree of
depletion and slow recovery potential of most elasmobranchs. Here, current understanding of elasmobranch population recoveries is reviewed. The potential and realized extent of population increases, including rates of increase, timelines and drivers are evaluated. Across 40 increasing populations, only 25% were attributed to decreased anthropogenic mortality, while the majority was attributed to predation release. It is also shown that even low exploitation rates (2-6% per year) can halt or reverse positive population trends in six populations currently managed under recovery plans.
Management measures that help restore elasmobranch populations include enforcement or near-zero fishing mortality, protection of critical habitats, monitoring and education. These measures are highlighted in a case study from the south-eastern U.S.A., where some evidence of recovery is seen in Pristis pectinata, Galeocerdo cuvier and Sphyrna lewini populations.
It is concluded that recovery of elasmobranchs is certainly possible but requires time and a combination of strong and dedicated management actions to be successful.

The take-away message?

A. Recovery is possible.

As per S. Smith et al., it is (obviously) easiest for smaller, short lived coastal Sharks whereas it is hardest for larger long lived coastal species, with pelagic species in the mid range. Whereas some of the smaller Sharks and Rays have principally profited from the demise of their predators (= so-called predator release), some larger species have experienced modest increases that are attributed to successful management and conservation strategies. These are the graphs for the Tiger Galeocerdo cuvier, the Smalltooth Sawfish Pristis pectinata and the Scalloped Hammerhead Sphyrna lewini from the South-Eastern USA - click for detail.

B. Even the slightest fishing pressure will greatly delay or even reverse the recovery.

Here are models for the population growth rates for the Grey Nurse Carcharias taurus, Great White Carcharodon carcharias, Basking Shark Cetorhinus maximus, Smalltooth Sawfish Pristis pectinata and the Whale Shark Rhincodon typus assuming different, incidentally very low mortality rates - click for detail.


For example, the recovery strategy for C. maximus has deemed that 10–17 mortalities per year across the entire population of 321–523 individualsto be acceptable (McFarlane et al., 2009).
This equates to an anthropogenic mortality rate of A ≈ 0·03, which would cause the population to decline according to present results (A > 0·023 caused a decline); however, McFarlane et al. (2009) used the maximum rv values (0·032–0·040) from the range of known life-history characteristics, while that used here (rv = 0·025, Table II) is more conservative. Even with an anthropogenic mortality rate of A = 0·02, the population would take c. 139 years to double, and even longer to recover to pre-exploitation levels.

Similarly, C. taurus in New South Wales, Australia, has failed to increase despite being legally protected from fishing since 1984 (Otway et al., 2004).
Because this population consists of only 300 individuals (Otway et al., 2004), an anthropogenic mortality rate of A < 0·05 or less than 15 individuals per year is required to allow for the population to increase, fewer than the estimated 14–20 per year that are killed by fishing and beach netting (Dulvy & Forrest, 2010).
Can you see why conservationists are so angry?

C. Management and Conservation strategies must be multi-faceted, this because single strategies are often inadequate.
This will be the topic of a forthcoming post.


Species-specific Conservation

Species-specific international and national instruments exist as a last resort to conserve individual species by identifying and listing those species at risk of extinction and implementing strategies to secure their long-term survival (Camhi et al., 2009).
The International Union for Conservation of Nature (IUCN), which provides species-specific conservation status at the global scale, has identified 67 elasmobranch species as critically endangered or endangered (Simpfendorfer et al., 2011). Other international instruments that aim to conserve threatened species, such as the Convention on International Trade in Endangered Species (CITES; php) and the CMS have recently listed a number of elasmobranch species, including C. carcharias, R. typus and C. maximus on both instruments, and L. nasus, spiny dogfish Squalus acanthias L. 1758, longfin mako Isurus paucus Guitart 1966 and shortfin mako Isurus oxyrinchus Rafinesque 1810 on CMS (CMS, 2005). All Pristis spp. are listed under CITES Appendix I or II. Some elasmobranchs are also included on national instruments, including the C. taurus, C. carcharias and R. typus on the Australian EPBC, C. maximus on the Canadian SARA and P. pectinata on the U.S. ESA.

Despite progress in providing legal protection to some species at risk of extinction, there are limitations that can prevent the success of these strategies.
These include: the challenge of establishing the extent of population decline and thus a proper assessment of the risk status (Marcus et al., 1999; Ferretti et al., 2008, 2010; Ward-Paige et al., 2010; Nance et al., 2011), hesitation to list species (Camhi et al., 2009, Lack & Sant, 2011), enforcement complexities such as distinguishing prohibited species from look-a-like species (Shivji et al., 2005) and monitoring remote areas (Graham et al., 2010), limited use of non-lethal monitoring techniques that inform about protected species (Domeier & Nasby-Lucas, 2007; Rowat et al., 2009; Bansemer & Bennett, 2010; Ward-Paige et al., 2010; Ward-Paige & Lotze, 2011), and a lack of information about life history, critical habitat and population dynamics at low abundance (Simpfendorfer, 2000, Kinney & Simpfendorfer, 2009; de la Parra Venegas et al., 2011).
Due to elasmobranchs’ low rate of recovery, the success of species-specific conservation initiatives might take decades to be fully revealed.

Based on the first decade(s) of legal protection for elasmobranchs the success of these initiatives is not encouraging.
For example, C. taurus in southern Australia have been legally protected from fishing since 1984, but incidental hooking rates remain high (Bansemer & Bennett, 2010) and populations continue to decline (Otway et al., 2004).
Rhincodon typus in Australia also continue to decline in both abundance and size (Bradshaw et al., 2007) despite being protected by CMS, CITES and EPBC.

While the success of these species-specific instruments for elasmobranchs remains to be seen, they almost certainly require long-term commitments, e.g. beyond species’ generation times, and should be combined with other conservation strategies such as no-take areas, habitat restoration and by-catch mitigation.

Here are Christine's recommendations.

Successful recovery of elasmobranch populations requires a long-term commitment with strong, dedicated management. The best strategy for elasmobranch recovery might be a multi-faceted conservation approach that includes
  • (1) science-based management and near-zero fishing mortality,
  • (2) clear and enforced anti-finning prohibitions and novel approaches (e.g. fin possession bans) to curb finning in unmanaged fisheries,
  • (3) enforced MPAs or shark sanctuaries that cover a range of habitat types used by elasmobranchs in different life stages,
  • (4) strong conservation and restoration initiatives for critical habitats and aggregation sites such as nurseries, breeding grounds and migration routes,
  • (5) legally binding legislation with a more rapid response that gives species the protection they need before population abundances decline to dangerously low levels and
  • (6) raised public and political awareness to reduce demand and increase support for conservation initiatives.

Since 2000, knowledge of elasmobranch biology and their population status have drastically improved, but there is still only localized evidence of rebuilding populations.
Proper management needs appropriate recovery targets, good life-history information, accurate population assessments and precise taxonomic descriptions. Development and implementation of cost-efficient, long-term and broad-scale monitoring of different conservation strategies, e.g. shark sanctuaries, is needed. Because a large portion of the shark trade is illegal, unregulated and unreported, it cannot be the sole responsibility of conservation and management agencies, but also that of fishermen and the general public to raise awareness, promote good practices and curb demand for shark products.

Could not agree more - as I said, great stuff!

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