In response to the Supplemental Recommendation by ICCAT Concerning the Conservation of Sharks Caught in Association with Fisheries Managed by ICCAT [Rec. 06-10], an updated assessment of the stocks of blue shark (Prionace glauca) and shortfin mako (Isurus oxyrinchus) was conducted in 2008. Ecological risk assessments (ERA) were also conducted for nine additional priority species of pelagic elasmobranchs, for which available data are very limited (Isurus paucus, Alopias superciliosus, Alopias vulpinus, Carcharhinus longimanus, C. falciformis, Lamna nasus, Sphyrna lewini, Sphyrna zygaena, and Pteroplatytrygon violacea).

The quantity and quality of the data available (e.g., historical catches and CPUE information) to conduct stock assessments have increased with respect to those available in the first (2004) shark assessments (Anon. 2005c) conducted by ICCAT. However, they are still quite uninformative and do not provide a consistent signal to inform the assessment. Unless these and other issues can be resolved, the assessments of stock status for all pelagic shark species will continue to be very uncertain and our ability to detect stock depletion to levels below the Convention Objective level will remain considerably low.

A summary of the Committee’s findings based on the 2008 (Anon. 2009c) assessment results is presented below. Although pelagic sharks are captured in the Atlantic Ocean with a wide variety of fishing gears, the largest volume of most of the species of major concern to ICCAT are captured by pelagic longline fisheries.

The Committee assessed in 2009 porbeagle sharks assuming the existence of four separate stocks: Northwest, Northeast (including the Mediterranean, for which only limited information is available), Southwest and Southeast. The assessment results for the southern porbeagle stocks also presented high levels of uncertainty due to data limitations.

Increased research and data collection are required to enable the Committee to improve the advice it can offer.

A great variety of shark species are found within the ICCAT Convention area, from coastal to oceanic species. Biological strategies of these sharks are very diverse and are adapted to the needs within their respective ecosystems where they occupy a very high position in the trophic chain as active predators. Therefore, generalization as regards to the biology of these very diverse species results in inevitable inaccuracies, as would occur for teleosts. To date, ICCAT has prioritized the biological study and assessment of the major sharks of the epipelagic system as these species are more susceptible of being caught as by-catch by oceanic fleets targeting tuna and tuna-like species. Among these shark species there are some of special prevalence and with an extensive geographical distribution within the oceanic-epipelagic ecosystem, such as the blue shark and shortfin mako shark, and others with less or even limited prevalence, such as porbeagle, hammerhead sharks, thresher sharks, white sharks, etc.

Porbeagle shows a distribution that is restricted to cold-temperate waters, preferably close to the continental shelf of both hemispheres where this species rarely overlaps with the fishing activity directed at tunas and tuna-like species. This species have an ovoviviparous reproductive strategy, which increases the probability of survival of their young, with litters of only a few individuals in the case of porbeagle. Their growth rates differ between sexes and among species. Females often reach first maturity at a large size. A characteristic of this species is usually its tendency to segregate temporally and spatially by size-sex, according to its respective processes of feeding, mating-reproduction, gestation and birth. Numerous aspects of the biology of this species are still poorly understood or completely unknown, particularly for some regions, which contributes to increased uncertainty in quantitative and qualitative assessments.

Porbeagle shows a distribution that is restricted to cold-temperate waters, preferably close to the continental shelf of both hemispheres.

The Committee assessed in 2009 porbeagle sharks assuming the existence of four separate stocks: Northwest, Northeast (including the Mediterranean, for which only limited information is available), Southwest and Southeast. The assessment results for the southern porbeagle stocks also presented high levels of uncertainty due to data limitations.

Fishery Indicators


Earlier reviews of the shark database resulted in recommendations to improve data reporting on shark catches. Though global statistics on shark catches included in the database have improved, they are still insufficient to permit the Committee to provide quantitative advice on stock status with sufficient precision to guide fishery management toward optimal harvest levels. Reported catches for porbeagle are provided in Table1. Given that catch reports to ICCAT are incomplete, the Committee attempted to develop a more accurate estimate of shark mortality and capture related to the Atlantic tuna fleets on the basis of the expected proportions among tunas and sharks and in the landings of these fleets (Figure 1) as well as using shark fin trade data. These information sets were used to reconstruct plausible estimates of historic catches used in blue shark and shortfin mako assessments in 2008.

During the porbeagle assessment in 2009 (SCRS/2009/014), standardized CPUE data were presented for three of the four stocks (NE, NW and SW; Figure 2). These series when referring to fisheries targeting porbeagle could fail to reflect the global abundance of the stock and where they refer to sharks caught as by-catch they could be highly variable.

With regard to the species for which ERAs were conducted, the Committee understands that, in spite of existing uncertainties, results make it possible to identify those species that are more susceptible and vulnerable (based only on productivity) to prioritize research and management measures (Table 2). These ERAs are conditional on the biological variables used to estimate productivity as well as the susceptibility values for the different fleets and thus may change in the future as new information becomes available.

Species	Productivity (r)	Productivity rank
BTH (Alopias superciliosus)	0.010	1
SMA (Isurus oxyrinchus)	0.014	2
LMA (Isurus paucus)	0.014	3
POR (Lamna nasus)	0.053	4
FAL (Carcharhinus falciformis)	0.076	6
OCS (Carcharhinus longimanus)	0.087	7
SPL (Sphyrna lewini)	0.090	8
SPZ (Sphyrna zygaena)	0.124	9
ALV (Alopias vulpinus)	0.141	10
PST (Pteroplatytrygon violacea)	0.169	11
BSH (Prionace glauca)	0.301	12
CRO (Pseudocarcharias kamoharai)	-	-

Figure 1. Left plate: Estimated potential catch of porbeagle by non-reporting longline fleets using catch ratios for the southwest stock. Very limited observations across the time-series result in a high but unquantified uncertainty in the estimates. Right plate: Comparison of estimates for non-reporting longline fleets with reported catch levels held in the Task I data set for the southwest stock area.

Figure 2. CPUE series for the porbeagle southwest stock .

Precautionary management measures should be considered for stocks where there is the greatest biological vulnerability and conservation concern, and for which there are very few data. Management measures should ideally be species-specific whenever possible.

For species of high concern (in terms of overfishing), which are expected to have high survivorship in fishing gears after release, particularly the bigeye thresher, the Committee recommends that the Commission prohibit retention and landings of the species to avoid fishing mortality. For other species which can be easily misidentified, such prohibitions could complicate compliance monitoring and therefore, other measures might be more appropriate. For example, minimum landing lengths or maximum landing lengths would afford protection to juveniles or the breeding stock, respectively, although other technical measures such as gear modifications, time-area restrictions, or other approaches, could be alternative means to protecting different life stages, provided they are tested for effectiveness through research projects before they are implemented.

Both porbeagle stocks in the northwest and northeast Atlantic are estimated to be overfished, with the northeastern stock being more depleted. The main source of fishing mortality on these stocks is from non-ICCAT, directed porbeagle fisheries that are being managed by most of the relevant Contracting Parties through quotas and other measures.

The Committee recommends that countries initiate research projects to investigate means to minimize by-catch and discard mortality of sharks, with a particular view to recommending to the Commission complementary measures to minimize porbeagle by-catch in fisheries for tuna and tuna-like species.

For porbeagle sharks, the Committee recommends that the Commission work with countries catching porbeagle, particularly those with targeted fisheries, and relevant RFMOs to ensure recovery of North Atlantic porbeagle stocks and prevent overexploitation of South Atlantic stocks. In particular, porbeagle fishing mortality should be kept to levels in line with scientific advice and with catches not exceeding current level. New targeted porbeagle fisheries should be prevented, porbeagles retrieved alive should be released alive, and all catches should be reported.

Management measures and data collection should be harmonized among all relevant RFMOs, and ICCAT should facilitate appropriate communication.

SOUTHWEST ATLANTIC PORBEAGLE SUMMARY
Current Yield (2008)		164.6 t2
Relative Biomass: B2008/BMSY		0.36-0.782
Relative Fishing Mortality: FMSY F2008/FMSY		0.025-0.0333 0.31-10.784
Management measures in effect		None

¹ Estimated catch allocated to theSoutheast stock area.
² Range obtained from BSP (low and high) and CFASP models. Value from CFASP model (SSB/SSBMSY) was 0.48 (0.20).
³ Range obtained from BSP (low) and CFASP (high) models.
⁴ Range obtained from BSP (low and high) and CFASP models. Value from CFASP model was 1.72 (0.51).