Summary of purse seine fishery bycatch at a regional scale 2003-2016 WCPFC-SC13-2017/ST-WP-05

Citation
Peatman T, Allain V, Caillot S, et al (2017) Summary of purse seine fishery bycatch at a regional scale 2003-2016 WCPFC-SC13-2017/ST-WP-05. In: WCPFC Scientific Committee 13th Regular Session. WCPFC-SC13-2017/ST-WP-05, Rarotonga, Cook Islands
Abstract

The Western and Central Pacific Fisheries Commission (WCPFC) has a responsibility to assess the impact of fishing on non-target species. In this report, we estimate the bycatch of the large-scale purse seine fishery operating primarily in tropical waters of the WCPFC Area east of 140°E. These large vessels, typically greater than 500 tonnes carrying capacity, have been responsible for approximately 83% of the purse seine catch of the main tuna target species, skipjack, yellowfin and bigeye tuna, in recent years, a catch which has varied between 1.5 and 2 million tonnes annually since 2010.
Data are collected from large-scale purse seine vessels through logsheets completed by vessel operators, in which catch, primarily of the target species, and other information are recorded for each purse seine set. Since the mid-1990s, some vessels have had observers on board to collect more detailed information on fishing operations, including the quantities of bycatch. Observer coverage of the purse seine fleet was modest (<10%) prior to 2010, after which a new rule that all purse seine vessels should carry observers was agreed by WCPFC and implemented by its members, most notably those coastal states who are members of the Parties to the Nauru Agreement (PNA). Therefore, from 2010, coverage of available observer data increased dramatically to around 60-80% per year. Note that smaller purse seine vessels operate in largely coastal waters of Indonesia, Philippines and Vietnam to the west, Japan to the north and New Zealand to the south. Observer data are not currently available for these fleets, which have made up approximately 17% of the WCPFC tuna purse seine catch in recent years. This report therefore focuses on estimating bycatch taken by the fleet of large purse seiners operating in tropical waters, using available observer data.
Species/species-group specific bycatch of large-scale purse seine fleets was estimated using the following approach:
1. Estimate bycatch rates by strata (year, quarter and association type) using available observer data.
2. Use observer data and aggregate catch/effort data to determine the number of unobserved sets by strata.
3. Apply observed strata-specific bycatch rates (bycatch per set) to the number of unobserved sets, to estimate strata-specific unobserved bycatch.
4. Aggregate observed bycatch data to obtain strata-specific observed bycatch.
5. Combine observed bycatch with estimates of unobserved bycatch to estimate total bycatch.
For a given species/species-group, estimates of bycatch rates for unobserved sets were based on the product of: the estimated proportion of sets where the bycatch species/species group was present; and, estimated bycatch levels for sets where the bycatch species/species group occurred. The presence/absence of bycatch was based on a binomial model for commonly encountered species/species groups, and a non-parametric bootstrap procedure for all other species/species groups. Estimates of bycatch levels when present were based on a non-parametric bootstrap procedure for all species/species groups. Bycatch rates were estimated in number of specimens per set for large pelagic species (i.e. sharks, billfishes, turtles, marine mammals) and in weight per set for small pelagic species (finfish).