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Blue shark - North Atlantic, 2007
Marine Resource  Fact Sheet
Stock status report 2008
Blue shark - North Atlantic, 2007
Fact Sheet Citation  
Owned byInternational Commission for the Conservation of Atlantic Tunas (ICCAT) – More
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Species:
FAO Names: en - Blue shark, fr - Peau bleue, es - Tiburón azul, ru - Акула синяя
Geographic extent of Blue shark - North Atlantic
Main Descriptors
Considered a single stock: Yes        Spatial Scale: Sub-Regional
Management unit: Yes
 
 
History
 

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 blue and shortfin mako sharks in 2008 assuming the existence of three separate stocks: North, South and Mediterranean. However, the data available to the Committee for the Mediterranean were not considered sufficient to conduct quantitative assessments for these species. The assessment results presented high levels of uncertainty due to data limitations. Similarly.

Increased research and data collection are required to enable the Committee to improve the advice it can offer.
Habitat and Biology
Climatic zone: Tropical; Temperate.   Horizontal distribution: Oceanic.   Vertical distribution: Pelagic.  


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.

Blue shark shows a wide geographical distribution, most often between 50ºN and 50ºS latitude. This species have an ovoviviparous reproductive strategy, which increases the probability of survival of their young, with abundant litters of about 40 pups in the case of blue shark. 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.
Geographical Distribution
Jurisdictional distribution: Highly migratory


Blue shark shows a wide geographical distribution, most often between 50ºN and 50ºS latitude.
Water Area Overview
Spatial Scale: Sub-Regional

Geo References
Resource Structure
Considered a single stock: Yes


The Committee assessed blue and shortfin mako sharks in 2008 assuming the existence of three separate stocks: North, South and Mediterranean. However, the data available to the Committee for the Mediterranean were not considered sufficient to conduct quantitative assessments for these species. The assessment results presented high levels of uncertainty due to data limitations. Similarly.
Exploitation
 

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 blue shark, shortfin mako and 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.

A number of standardized CPUE data series for blue shark were presented in 2008 as relative indices of abundance. The Committee placed emphasis on using the series that pertained to fisheries that operate in oceanic waters over wide areas. Figure 2 presents the central tendency of the available series for the two stocks of this species.

Considering the quantitative and qualitative limitations of the information available to the Committee, the results presented in 2008, as those of the 2004 assessment (Anon. 2005c), are not conclusive.

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. Blue shark catches reported to ICCAT and estimated by the Committee.
Figure 2. Average trends in the CPUE series used in the assessments of blue shark (BSH). The averages were calculated by weighting the available series either by their relative catch or by the relative spatial coverage of the respective fisheries.
Assessment
 
Overall Assessment Results

Ecological risk assessments for eleven priority species of sharks (including blue shark and shortfin mako) caught in ICCAT fisheries demonstrated that most Atlantic pelagic sharks have exceptionally limited biological productivity and, as such, can be overfished even at very low levels of fishing mortality. Specifically, the analyses indicated that bigeye threshers, longfin makos, and shortfin makos have the highest vulnerability (and lowest biological productivity) of the shark species examined (with bigeye thresher being substantially less productive than the other species). All species considered in the ERA, particularly smooth hammerhead, longfin mako, bigeye thresher and crocodile sharks, are in need of improved biological data to evaluate their biological productivity more accurately and thus specific research projects should be supported to that end. Table 2 provides a productivity ranking of the species considered. ERAs should be updated with improved information on the productivity and susceptibility of these species.

For both North and South Atlantic blue shark stocks, although the results are highly uncertain, biomass is believed to be above the biomass that would support MSY and current harvest levels below FMSY. Results from all models used in the 2008 assessment (Anon. 2009c) were conditional on the assumptions made (e.g., estimates of historical catches and effort, the relationship between catch rates and abundance, the initial state of the stock in the 1950s, and various life-history parameters), and a full evaluation of the sensitivity of results to these assumptions was not possible during the assessment. Nonetheless, as for the 2004 stock assessment (Anon. 2005c), the weight of available evidence does not support hypotheses that fishing has yet resulted in depletion to levels below the Convention objective (Figure 3).


Figure 3. Phase plots summarizing base scenario outputs for the current stock status of blue shark (BSH). BSP=Bayesian Surplus Production model; CFASPM=catch-free, age-structured production model. The shaded box represents the area at which the biomass at MSY is estimated to be reached. Any points inside or to the left of the box indicate the stock is overfished (with respect to biomass). Any points above the horizontal line indicate overfishing (with respect to F) is occurring.
Management
Management unit: Yes


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.

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.

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


NORTH ATLANTIC BLUE SHARK SUMMARY

2007 Yield

 

61,845 t1

     

Current Yield (2008)

 

30,545 t2

     

Relative Biomass:
B2007/BMSY
B2007/B0

 


1.87-2.743
0.67-0.934

 

 

 

Relative Fishing Mortality:
FMSY
F2007/FMSY

 


0.155
0.13-0.176

1 Estimated catch used in the 2008 assessments.
2 Task I catch.
3 Range obtained from the Bayesian Surplus Production (BSP) (low) and the Catch-Free Age Structured Production (CFASP) (high) models. Value from CFASP is SSB/SSMSY.
4 Range obtained from BSP (high), CFASP and Age-Structured Production Model (ASPM) (low) models.
5From BSP and CFASP models (same value). CV is from CFASP model.
6 Range obtained from BSP (high) and CFASP (low) models.

Source of information
 
“Report of the 2009 Meeting of the Standing Committee on Research and Statistics.” International Commission for the Conservation of Atlantic Tunas (ICCAT). 2010 ICCAT Report for biennial period, 2008-09 PART II (2009) - Vol. 2 ICCAT Click to openhttp://www.iccat.int/Documents/BienRep/REP_EN_08-09_II_2.pdf
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