¹Range obtained from 8 Bayesian production and 1 SS3 model runs. Value from SS3 is SSF/SSFMSY. Low value is lowest value from 4 production model (JABBA) runs and high value is from the SS3 base run.

²Range obtained from 2 Bayesian production (BSP2JAGS) and 2 catch-only (CMSY) model runs. Low value is lowest value from the CMSY model runs and high value is highest value from the BSP2JAGS model runs.

The most recent assessment for shortfin mako was conducted in 2017 through a process that included a data preparatory meeting in March 2017 (Anon., 2017a) and an assessment meeting in June 2017 (Anon., 2017b). The last year of fishery data used in the assessment was 2016.

Shortfin mako are large pelagic sharks that show a wide geographic distribution; from tropical to temperate waters worldwide. Shortfin mako have an aplacental viviparity reproduction strategy with an oophagy behavior, which limits their fecundity but increases the probability of survival of their young. The shortfin mako has an average litter size of around 12 pups. Although high uncertainty regarding their biology remains, available life history traits (slow growth, late maturity and small litter size) indicate that they are vulnerable to overfishing. A behavioral characteristic of these species is their tendency to segregate temporally and spatially by size and/or sex, during feeding, mating-reproduction, gestation and birth processes.

Shortfin mako are large pelagic sharks that show a wide geographic distribution; from tropical to temperate waters worldwide. Tagging studies have suggested that they exhibit large-scale migratory behaviour and periodic vertical movement, but the lack of information on some components of the populations precludes a complete understanding of their distribution/migration pattern by ontogenetic stage and in some cases identifying their pupping/mating grounds. Numerous aspects of the biology of these species are still poorly understood or completely unknown, particularly for some regions, which contributes to increased uncertainty in quantitative and qualitative assessments.

Based on the biological information available for assessment purposes, it is currently assumed that there are Northern and Southern shortfin mako stocks.

Figure 1. Time series of reported (Task I) and estimated shortfin mako shark (SMA) catches, between 1971 and 2015, for the North and South Atlantic stocks.

Earlier reviews of the shark database resulted in recommendations to improve data reporting on shark catches. Reported and estimated catches for shortfin mako have been considered sufficiently complete for the purpose of quantitative stock assessment and are provided in Figure 1 and in Table 1.

The CPUE series available for the 2017 shortfin mako stock assessments showed decreasing trends since approximately 2010 for the North Atlantic stock and generally increasing trends since approximately 2008 for the South Atlantic stock (Figure 2).

Figure 2. Indices of abundance for North Atlantic shortfin mako shark used in the 2017 stock assessment.

The 2017 assessment of the status of North and South Atlantic stocks of shortfin mako shark was conducted with updated time series of relative abundance and annual Task I catches (Figure 1), life history, and with the inclusion of length composition data. An alternative series of catch data based on ratios of shark catches to catches of the main target species was also estimated and used in the assessments. It was the Committee’s view that the 2017 stock assessment represents a significant improvement in our understanding of current stock status, for North Atlantic shortfin mako in particular.

For the North Atlantic stock, results of nine stock assessment model runs were selected to provide stock status and management advice. Although all results indicated that stock abundance in 2015 was below BMSY, results of the production models (BSP2JAGS and JABBA) were more pessimistic (B/BMSY deterministic estimates ranged from 0.57 to 0.85) and those of the age-structured model (SS3), which indicated that stock abundance was near MSY (SSF/SSFMSY = 0.95 where SSF is spawning stock fecundity), were less pessimistic. F was overwhelmingly above FMSY (Figure 3), with a combined 90% probability from all the models of being in an overfished state and experiencing overfishing (Figure 3).

Figure 3. Stock status (2015) of North Atlantic shortfin mako based on Bayesian production models (4 BSP2JAGS and 4 JABBA runs) and 1 length-based, age-structured model (SS3). The clouds of points are the bootstrap estimates for all model runs showing uncertainty around the median point estimate for each of nine model formulations (BSP2JAGS: solid pink circles; JABBA: solid cyan circles; SS3: solid green circle). The marginal density plots shown are the frequency distributions of the bootstrap estimates for each model with respect to relative biomass (top) and relative fishing mortality (right). The red lines are the benchmark levels (ratios equal to 1).

Projection

For shortfin mako, projections were carried out with the BSP2JAGS production model for the North Atlantic. Projections indicated that current catch levels (3,600 t for the Task I catches and 4,750 t for the alternative catches estimated based on ratios, mean of 2011-2015) in the North Atlantic will cause continued population decline and that catches would need to be 1,000 t or lower to prevent further population declines (SHK-Figure 4). However, the Kobe II strategy matrices showed that for a constant annual catch of 1,000 t, the probability of being in the Kobe plot green zone would only be 25% by 2040 (Table 2). The Committee noted that the Kobe II strategy matrices may not reflect the full range of uncertainty in the outlook. Although in terms of current stock size the SS3 model is more optimistic than the aggregated biomass dynamic production models, the future outlook is probably more pessimistic because the fisheries are removing mostly juveniles and thus it can be anticipated that spawning stock will keep declining for years after fishing pressure has been reduced and before new recruits reach maturity. It should be noted that ICCAT fisheries are not removing mature females.

Figure 4. Median constant catch projections (0 – 4000 t) from BSP2-JAGS for the North Atlantic shortfin mako (2017 assessment). for 4 model runs: (a) C1 catch with a Schaefer model, (b) C2 catch with a Schaefer model, (c) C1 catch with a generalized production model, and (d) C2 catch with a generalized production model.

Table 2. Kobe II strategy matrix giving the probability that the fishing mortality will be below the fishing mortality rate at MSY (top), the probability that the biomass will exceed the level that will produce MSY (middle), and the two combined (bottom) based on production model (BSP2-JAGS) projection results for North Atlantic shortfin mako.

The Commission adopted Rec. 17-08, which aims to reduce the fishing mortality to end overfishing of the northern stock of shortfin mako. It does this by strengthening data collection (including collection of statistics on discards, biological parameters, weight of landing products) and establishing regulatory options (including promoting fish releases in a manner that increases survival, establishing minimum sizes) for ICCAT CPCs. In response to this recommendation several CPCs have adopted national regulations. Rec. 17-08 will be reviewed by the Commission in 2019.