Stock Status and Conservation Information
(From ISC25 Plenary Report)

Shortfin Mako Shark

Click here to see the ISC24 stock status and conservation information.

Stock Status

The current assessment provides the best scientific information available on North Pacific shortfin mako shark (SMA) stock status. Results from this assessment should be considered with respect to the management objectives of the WCPFC and the IATTC, the organizations responsible for management of pelagic sharks caught in international fisheries for tuna and tunalike species in the Pacific Ocean. Target and limit reference points have not been established for pelagic sharks in the Pacific Ocean. In this assessment, stock status is reported in relation to maximum sustainable yield (MSY).

A BSPM ensemble was used for this assessment; therefore, the reproductive capacity of this population was characterized using total depletion (D) rather than spawning abundance as in the previous assessment. Total depletion is the total number of SMA divided by the unfished total number (i.e., carrying capacity). Recent D (𝐷_2019-2022) was defined as the average depletion over the period 2019-2022. Exploitation rate (U) was used to describe the impact of fishing on this stock. The exploitation rate is the proportion of the SMA population that is removed by fishing. Recent U (𝑈_2018-2021) is defined as the average U over the period 2018-2021.

During the 1994-2022 period, the median D of the model ensemble in the initial year 𝐷₁₉₉₄ was estimated to be 0.19 (95% CI: credible intervals = 0.08-0.44), and steadily improved over time and 𝐷_2019-2022 was 0.60 (95% CI = 0.23-1.00). Although there are large uncertainties in the estimated population scale, the best available data for the stock assessment are four standardized abundance indices from the longline fisheries of Japan, Taiwan, and the US; and all four indices indicate a substantial (›100%) increase in the population during the assessment period. The population was likely heavily impacted prior to the start of the modeled period (1994), after which it has been steadily recovering. It is hypothesized that the fishing impact prior to the modeled period was likely due to the high-seas drift gillnet fisheries operating from the late 1970s until it was banned in 1993, though specific impacts from this fishery on SMA are uncertain as species specific catch data are not available for sharks. Consistent with the estimated trends in depletion, the exploitation rates were estimated to be gradually decreasing from 0.023 (95% CI = 0.004-0.09) in 1994 to the recent estimated exploitation rate (𝑈_2018-2021) of 0.018 (95% CI = 0.004-0.07). The decreasing trends in estimated exploitation rates were likely due to the increase in estimated population size being greater than increases in the observed catch.

The median of recent D (𝐷_2019-2022) relative to the estimated D at MSY (𝐷_MSY = 0.51, 95% CI = 0.40-0.70) was estimated to be 1.17 (95% CI = 0.46-1.92). The recent median exploitation rate (𝑈_2018-2021) relative to the estimated exploitation rate at MSY (𝑈_MSY= 0.05, 95% CI =0.03-0.09) was estimated to be 0.34 (95% CI = 0.07-1.20). Surplus production models are a simplification of age-structured population dynamics and can produce biased results if this simplification masks important components of the age-structured dynamics (e.g., index selectivities are dome shaped or there is a long time-lag to maturity). Simulations suggest that under circumstances representative of the observed SMA fishery and population characteristics (e.g., dome-shaped index selectivity, long lag to maturity, and increasing indices), the BSPM ensemble may produce biased results. Representative simulations suggested that the D_2019-2022 estimate has a positive bias of approximately 7.3 % (median). The trajectories of stock status from the model ensemble revealed that North Pacific SMA had experienced a high level of depletion prior to the start of the model and was likely overfished in the 1990s and 2000s, relative to MSY reference points.

Based on these findings, the following information on the status of the NPO SMA is provided by the ISC25 Plenary:

1. No biomass-based or fishing mortality-based limit or target reference points have been established for NPO SMA by the IATTC or WCPFC;
2. Recent median depletion (𝑫_{𝟐𝟎𝟏𝟗-𝟐𝟎𝟐𝟐}) is estimated from the model ensemble to be 0.60 (95% CI = 0.23-1.00). The recent median 𝑫_{𝟐𝟎𝟏𝟗-𝟐𝟎𝟐𝟐} was 1.17 times 𝑫_𝑴𝑺𝒀 (95% CI = 0.46-1.92) and the stock is likely (66% probability) not in an overfished condition relative to MSY-based reference points;
3. Recent harvest rate (𝑼_{𝟐𝟎𝟏𝟖-𝟐𝟎𝟐𝟏}) is estimated from the model ensemble to be 0.018 (95% CI = 0.004-0.07). 𝑼_{𝟐𝟎𝟏𝟖-𝟐𝟎𝟐𝟏} was 0.34 times (95% CI = 0.07-1.20) U_𝑴𝑺𝒀 and overfishing of the stock is likely not occurring (95% probability) relative to MSY-based reference points;
4. The model ensemble results showed that there is a 65% joint probability that the North Pacific SMA stock is not in an overfished condition and that overfishing is not occurring relative to MSY based reference points; and
5. Several uncertainties may limit the interpretation of the assessment results including uncertainty in catch (historical and modeled period) and the biology and reproductive dynamics of the stock, and the lack of CPUE indices that fully index the stock.

Conservation Information

Stock projections of depletion and catch of North Pacific SMA from 2023 to 2032 were performed assuming four different harvest policies: 𝑈_2018-2021, 𝑈_MSY, 𝑈_2018-2021+20%, and 𝑈_2018-2021−20% and evaluated relative to MSY-based reference points. Based on these findings, the following conservation information is provided:

1. Future projections in three of the four harvest scenarios (𝑼_{𝟐𝟎𝟏𝟖-𝟐𝟎𝟐𝟏}, 𝑼_{𝟐𝟎𝟏𝟖-𝟐𝟎𝟐𝟏} + 𝟐𝟎%, and 𝑼_{𝟐𝟎𝟏𝟖-𝟐𝟎𝟐1} −𝟐𝟎%) showed that median depletion in the North Pacific Ocean will likely (›50% probability) increase; only the U_MSY harvest scenario led to a decrease in median depletion.
2. Median estimated depletion of SMA in the North Pacific Ocean will likely (›50% probability) remain above 𝑫_𝑴𝑺𝒀 in the next 10 years for all scenarios except 𝑼_𝑴𝑺𝒀; harvesting at 𝑼_𝑴𝑺𝒀 decreases D towards 𝑫_𝑴𝑺𝒀.
3. Model projections using a surplus production model may oversimplify the age structured population dynamics and as a result could be overly optimistic.