|Shortfin mako - North Atlantic|
|Marine Resource Fact Sheet|
|Shortfin mako - North Atlantic|
|FAO Names: en - Shortfin mako, fr - Taupe bleue, es - Marrajo dientuso, ru - Акула-мако|
|Considered a single stock: Yes Considered a single stock: Yes|
Spatial Scale: Sub-Regional Management unit: Yes
Reference year: 2010
Biological State and Trend
During 2012, a meeting was held to conduct a shortfin mako stock assessment, in response to ICCAT’s Recommendation Rec. 10-06. An Ecological Risk Assessment was also conducted for 16 shark species (20 stocks), which is detailed in document SCRS/2012/167.
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, and great white sharks. Shortfin mako is a large pelagic shark that show a wide geographic distribution from tropical to temperate waters worldwide. Shortfin mako has an ovoviviparous reproductive system, which increases the probability of survival of their young. The shortfin mako has an average litter size of around 12 individuals. Although high uncertainty regarding its biology remains, available life history traits (slow growth, late maturity and small litter size) indicate that it is vulnerable to overfishing. A behavioural characteristic of this species is its tendency to segregate temporally and spatially by size-sex, during feeding, mating-reproduction, gestation and birth processes. Tagging studies have suggested large-scale migratory behaviour and periodic vertical movement, but the lack of information on some components of the populations precludes a complete understanding of its distribution/migration pattern by ontogenetic stages and in some cases identifying their pupping/mating grounds. 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.
Jurisdictional distribution: Highly migratory
Shortfin mako shark shows a wide geographical distribution from tropical to temperate waters worldwide.
Water Area Overview
Spatial Scale: Sub-Regional
Considered a single stock: Yes
Considered a single stock: Yes
The Committee assessed shortfin mako shark in 2012 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 this species.
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 and estimated catches for shortfin mako is provided in Table1
and Figure 1. During the 2012 shortfin mako stock assessment, different standardized CPUE series were presented for the north stock. For this stock the series were conflicting and did not coincide with the catch tendencies (Figure 2). The group noted that the increase in the CPUE series could be due to an increase in abundance, an increase in catchability, in the fishing strategy or in data reporting for this species.
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 (Figure3). 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.
|Figure 1. Shortfin mako (SMA) catches reported to ICCAT (Task-I) and estimated by the Committee. (2011 landings are considered provisional). |
|Figure 2. Indices of abundance for North Atlantic shortfin mako shark, along with total catches input into the BSP model. |
|Figure 3. Vulnerability ranks for 20 stocks of pelagic sharks calculated with three methods: Euclidean distance (v1), multiplicative (v2), and arithmetic mean (v3). A lower rank indicates higher risk. Stocks listed in decreasing risk order according to the sum of the three indices. Red highlight indicates risks scores 1-5; yellow, 6-10; blue, 11-15; and green, 16-20. Productivity values ranked from lowest to highest. |
Overall Assessment Results
The results of the stock assessments and the 2012 ERA carried out for elasmobranchs [within the ICCAT Convention Area] are summarised below. To date, these assessments have focused only on Atlantic stocks, and not on shark stocks in the Mediterranean Sea stocks. Nevertheless, it should be noted that two Mediterranean-specific measures relevant to sharks species of interest were adopted during 2012. First, ten elasmobranch species were strictly protected under Annex II of the Barcelona Convention (under the Protocol Concerning Specially Protected Areas and Biological Diversity in the Mediterranean). These species include shortfin mako (Isurus oxyrinchus), porbeagle (Lamna nasus), smooth hammerhead (Sphyrna zygaena), scalloped hammerhead (Sphyrna lewini), great hammerhead (Sphyrna mokarran), and tope (Galeorhinus galeus). Under Annex II protection, these shark species can no longer be captured or sold, and plans for their recovery should be developed.
The ERA conducted by the committee was a quantitative assessment consisting of a risk analysis to evaluate the biological productivity of these stocks and a susceptibility analysis to assess their propensity to capture and mortality in pelagic longline fisheries. Three metrics were used to calculate vulnerability (Euclidean distance, a multiplicative index, and the arithmetic mean of the productivity and susceptibility ranks). The five stocks with the lowest productivity were the bigeye thresher (Alopias superciliosus), sandbar (Carcharhinus plumbeus), longfin mako (Isurus paucus), night (Carcharhinus signatus), and South Atlantic silky shark (Carcharhinus falciformis). The highest susceptibility values corresponded to shortfin mako (Isurus oxyrinchus), North and South Atlantic blue sharks (Prionace glauca), porbeagle (Lamna nasus), and bigeye thresher. Based on the results, the bigeye thresher, longfin and shortfin makos, porbeagle, and night sharks were the most vulnerable stocks. In contrast, North and South Atlantic scalloped hammerheads (Sphyrna lewini), smooth hammerhead (Sphyrna zygaena), and North and South Atlantic pelagic stingray (Pteroplatytrygon violacea) had the lowest vulnerabilities. More detailed analyses of productivity and susceptibility of some species, as well as improvements in the method used to estimate the overall longline effort (EFFDIS) will be conducted in 2013. The Committee observed that the data regarding night shark distribution was considered to be incomplete and therefore the results with regard to this species should be considered preliminary and requiring revision before publication.
Regarding stock assessment, the 2012 assessment of the status of North Atlantic stock of shortfin mako shark was conducted with updated time series of relative abundance indices and annual catches. Coverage of Task I catch data and number of CPUE series increased since the last stock assessment conducted in 2008, with Task I data now being available for most major longline fleets. The available CPUE series showed increasing or flat trends for the finals years of each series (since the 2008 stock assessment) for the North stock, hence the indications of potential overfishing shown in the previous stock assessment have diminished and the current level of catches may be considered sustainable.
For the North Atlantic stock, results of the two stock assessment model runs used indicated almost unanimously that stock abundance in 2011 was above BMSY and F was below FMSY (Figure 4). Thus, these results indicated that the North Atlantic stock is healthy and the probability of overfishing is low. However, they also showed inconsistencies between estimated biomass trajectories and input CPUE trends, which resulted in wide confidence intervals in the estimated biomass and fishing mortality trajectories and other parameters. The high uncertainty in past catch estimates and deficiency of some important biological parameters are still obstacles for obtaining reliable estimates of current status of the stocks.
|Figure 4. For North Atlantic shortfin mako sharks, median biomass relative to BMSY and median fishing mortality rate relative to FMSY, with 80% credibility intervals, from BSP model. |
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.
The SCRS welcomed the conservation and management measures adopted by the Commission in the past three years regarding the species ranked as the most vulnerable in the 2008 and 2010 Ecological Risk Assessments and for which almost no data have been submitted (bigeye thresher, oceanic whitetip, hammerhead sharks and silky shark).
Considering the need to improve stock assessments of pelagic shark species impacted by ICCAT fisheries, the committee recommends that the CPCs provide the corresponding statistics of all ICCAT and non-ICCAT fisheries capturing these species, including recreational and artisanal fisheries. The committee considers that a basic premise for correctly evaluating the status of any stock is to have a solid basis to estimate total removals.
The Committee recommends that joint work with the ICES Working Group on Elasmobranch Fishes should be continued. In addition, stocks of mutual interest and areas of overlap, particularly species occurring in the Mediterranean Sea, should be discussed.
The Committee recommends that the commission adopt measures that allow scientific observers to collect biological samples (vertebrae, tissue, reproductive tracts, and stomachs) from species whose retention is prohibited by current regulations and which are already dead at haul-back. For all of these species, biological knowledge is severely lacking therefore the committee strongly recommended that these samples be collected.
The Committee reiterates that the CPCs explore methods to estimate catches of sharks in purse seine and artisanal fisheries. Management measures should be applied to these sectors where catches of shark species are determined to be significant. Methods for mitigating shark by-catch by these fisheries also need to be investigated and applied.
Taking into consideration the continued high vulnerability ranking in the ERA, results from the modeling approaches used in the assessment, the associated uncertainty, and the relatively low productivity of shortfin mako sharks, the committee recommends, as a precautionary approach, that the fishing mortality of shortfin mako sharks should not be increased until more reliable stock assessment results are available for both the northern and southern stocks.
NORTH ATLANTIC SHORTFIN MAKO SUMMARY
Provisional Yield (2011)
Relative Fishing Mortality:
Overfished 2010 (Y/N)
Overfishing 2010 (Y/N)
Management Measures in Effect:
[Rec. 04-10]. [Rec. 07-06]. [Rec. 10-06]
1 Task I catch.
2 Range obtained from BSP.
Source of information
“Report of the 2012 Meeting of the Standing Committee on Research and Statistics.” International Commission for the Conservation of Atlantic Tunas (ICCAT). 2013 ICCAT Report for biennial period, 2012-13 PART I (2012) - Vol. 2 ICCAT http://www.iccat.int/Documents/BienRep/REP_EN_12-13_I_2.pdf