Indicator-based Analysis of the Status of Shortfin Mako Shark in the North Pacific Ocean

International Scientific Committee for Tuna and Tuna-like Species in the North Pacific Ocean (2015) Indicator-based Analysis of the Status of Shortfin Mako Shark in the North Pacific Ocean. WCPFC, Pohnpei, Federated States of Micronesia

Stock Identification and Distribution Shortfin makos are distributed throughout the pelagic, temperate North Pacific. Nursery areas are found along the continental margins in both the western and eastern Pacific, and larger subadults and adults are observed in greater proportions in the Central Pacific. A single stock of shortfin mako sharks is assumed in the North Pacific Ocean based on evidence from genetics, tagging studies, and lower catch rates of shortfin makos near the equator than in temperate areas. However, within the North Pacific some regional substructure is apparent as the majority of tagged makos have been recaptured within the same region where they were originally tagged, and examination of catch records by size and sex demonstrates some regional and seasonal segregation across the North Pacific.
2. Catch
Catch was estimated for many fleets and nations based on the best available information. Catch estimates for each fishery were made based on effort, knowledge of the species composition of catch, estimated catch per effort, and scientific knowledge of the operations and catch history. These time series provide an idea of recent catch history for many of the main fleets, but estimates of total catch for shortfin mako sharks in the North Pacific are incomplete. Data are lacking for several significant fishing nations (e.g. Korea and China) and fleets (e.g. Taiwan small-scale longline, Japan deep-set longline and Japan training vessel fleets). Estimates are difficult to derive because discards are often not recorded and retained catch data are available with low quality. Given that trends in catch cannot be derived from the incomplete catch information provided, the catch time series were not considered for the purposes of providing stock status information.
3. Indicator Data and Analysis
Simulation analyses were conducted to examine the effects of CPUE time series of varying lengths and precision, of CPUE time series from predominately adult versus juvenile areas, and of the contribution of trends in mean size versus CPUE in determining stock status. Results from the simulations showed that time series of mean size are less informative regarding the current stock condition (Bcur/Bmsy) than CPUE indices. Simulation results also showed that CPUE indices that are derived from predominately adult areas provide better information on current stock status than CPUE indices from recruitment areas.
Four types of indicators were developed for the north Pacific shortfin mako shark: proportion of positive sets, abundance (CPUE) indices, sex-ratio and size compositions. The proportion of positive sets, defined as set/trip where at least one shortfin mako sharks is caught, is calculated for major fisheries. The trends for proportion of positive sets varied across fisheries with the Japanese shallow-set longline fishery having the highest proportion of positive catch sets (approximately 75% in 2013, with the rate nearly tripling over the time series).
Indices of shortfin mako relative abundance were developed from eight fisheries or surveys ranging from 1985-2014 and covering different areas across most of the North Pacific. All indices were reviewed by the SHARKWG, and three were selected as the most plausible indicators of abundance based on their spatial and temporal coverage, size of sharks, data quality, and model diagnostics (Figure E1). The Japanese shallow-set longline index was considered to be the best abundance indicator candidate. The standardized index showed a flat or slightly increasing trend from 1994-2004, before a substantial increase from 2005-2013. Abundance indices developed from the Hawaii-based deep-set and shallow-set longline fisheries were also both considered to be plausible indicators of stock abundance. Trends in abundance showed some variability for the two fishery sectors between 2004 and 2012. The standardized CPUE trends moved in opposite directions, with the trend for the shallow-set sector showing a slight decrease, while the trend for the deep-set sector increased overtime. Figure E1. Standardized indices of abundance by fishery for shortfin mako sharks. While all of the available independent information was examined to draw conclusions about the stock, these three indices were considered to have the greatest value in determining stock status.
Overall, no trends in sex-ratio are apparent through time across fisheries, although sample sizes are generally low. It would probably be difficult to interpret any trends in sex-ratio because there is not a good understanding of population movement by size and sex through time. Thus, the SHARKWG considered sex-ratios to be of little value as indicators in this analysis of stock status.
The annual median and quartile percentiles of catch at size for shortfin mako sharks caught by the various fleets were examined. In general sizes remained relatively stable for all fleets. Larger sizes were recorded for the deep-set sector of the Hawaiian fleet and the Japan research and training longline vessels, while smaller individuals were more common in the U.S. juvenile longline survey, U.S. drift gillnet,
and Japan longline survey.
4. Summary of Indicators
Although our knowledge about shortfin mako life history is not complete, a general picture of the dynamics of the assumed north Pacific stock has emerged: juvenile sharks are found in both the eastern and western areas of the Pacific Ocean near continental land masses while older sharks are found in greater numbers in the central Pacific. Like our knowledge of life history, our compilation of fishery data is also incomplete.
Effort for the fisheries examined that catch shortfin makos throughout the North Pacific seems to have declined overall, although effort estimates for several fisheries and nations were unavailable. Of the available indices of relative abundance (CPUE), the Japanese shallow-set longline and both Hawaiian longline indices (shallow-and deep-set) are considered good candidates for representing stock trends. All three indices cover a large part of the Pacific where mature sharks are found. Both the Japanese shallow-set and the Hawaiian deep-set indices indicate non-negative trends and the Hawaiian shallow-set a negative trend. Although the negative trend in the Hawaiian shallow-set should not be completely discounted, the variability in the annual length distributions from this fishery suggests that the index selectivity/catchability fluctuates which may invalidate the proportionality assumption. It should also be considered that the rate of population increase described by the Japanese shallow-set post 2000 is likely too steep to solely reflect the response of a Lamnidae species to a relaxation of fishing pressure.
5. Stock Status and Conservation Advice
Shortfin mako is a data poor species. Recognizing that information on important fisheries is missing, the untested validity of indicators for determining stock status, and conflicts in the available data, stock status (overfishing and overfished) could not be determined. Managers should consider the undetermined stock status of shortfin mako shark in the North Pacific when developing and implementing management measures.
The SHARKWG reviewed a suite of information to determine the stock status of shortfin mako shark in the North Pacific. Of the three indices considered to have the greatest value in providing stock status information, abundance trends in two of the series appear to be stable or increasing, while the abundance trend in the third series appears to be declining.
It is recommended that data for missing fleets be developed for use in the next stock assessment scheduled for 2018 and that available catch and CPUE data be monitored for changes in trends. It is further recommended that data collection programs be implemented or improved to provide species-specific shark catch data for fisheries in the North Pacific.