Estimates of the mortality of non-target species with an initial focus on seabirds, turtles and sharks in longline fisheries of the western and central Pacific Ocean, 1990–2004

Molony B (2005) Estimates of the mortality of non-target species with an initial focus on seabirds, turtles and sharks in longline fisheries of the western and central Pacific Ocean, 1990–2004. WCPFC-SC1-EB-WP-01, Noumea, New Caledonia, p 84

Total numbers of individuals captured and the total number of mortalities of birds, sharks and turtles were estimated for the central region of Western and Central Pacific Fisheries Commission (WCPFC) area. In addition, total catches of marine mammals were also estimated.
While abundant logsheet data exists, the reporting rates of these four taxa are relatively low and observer data were used in order to generate estimates. Observer coverage of the WCPFC region varies among flags, fleets and areas and observer data for the WCPFC region is not centrally available from a single location. As a result observer data held at the Secretariat of the Pacific Community (SPC) were used.
Four fisheries were defined for the region of the WCPFC between 15 degrees N - 31 degrees S; tropical shallow longline (TSL, 15 degrees N - 10 degrees S, less than 10 hooks between floats (HBF)), tropical deep longline (TDL, 15 degrees N - 10 degrees S, 10 or more HBF), temperate albacore longline (TAL, 10 degrees S - 31 degrees S) and a single purse-seine fishery. Annual catches and mortality of each taxa for each of the four fisheries were estimated and raised by the estimated total effort in these fisheries to generate total annual catches and mortalities for each taxa.
Relatively few observer records of birds existed for the fisheries examined. Total annual catches of birds by these fisheries were less than 1,593 plus or minus 8,714 (95% confidence intervals (CI)) birds per year between 1990 and 2004, with most birds suffering mortality. Most birds were reported from the TAL with fewer records of bird catches reported from the other fisheries. However, less than 100 birds per year were estimated to be captured by these four fisheries since 1998. Few birds were identified to species.
Fewer mammals were reported by observers in these fisheries and total annual catches were less than 1,362 plus or minus 87,352 mammals per year during the period 1990-2004. In contrast to birds, most mammals were released alive by all fisheries, with annual mortalities estimated at 300 plus or minus 3,986 mammals per year. The highest catches were reported from the TSL and purse-seine fisheries. Most fishery-mammal interactions in the purse-seine fishery were a result of deliberately setting upon whales in order to capture associated tuna schools. Most mammals were not identified to species.
As expected, the total annual catches of sharks were much higher than for the other taxa examined due to the high number of shark species, relatively high abundance of sharks compared to the other taxa, the existence of dedicated shark longline fisheries and that sharks and shark products (e.g. fins) are part of the commercial catch of all fleets. An annual estimated catch of 696,401 plus or minus 907,848 sharks per year were captured by these four fisheries between 1990 and 2004, mainly by the TSL fishery. Annual estimated mortalities were relatively low but were likely to be underestimated due to the relatively low levels of observers reporting condition and fate of sharks. It is likely that estimated total shark mortalities for these four fisheries were similar to the estimated total catches. Most sharks were identified to species and catches were dominated by blue sharks, silky sharks, oceanic whitetip sharks and pelagic sting rays, although the relative abundances of shark species varied among fisheries and years.

An estimated 6,962 plus or minus 22,567 turtles were captured by the four fisheries each year between 1990 and 2004, with an estimated total annual mortality of 931 plus or minus 7,392 turtles per year. The highest catches were estimated from the TSL fishery as most turtles spend their time in the upper regions of the water column (less than 120 m). However, the highest turtle mortalities were estimated for the TDL fishery, likely a result of turtles being unable to surface if hooked on this deeper gear. Most turtles were not identified to species but a high proportion of olive ridley turtles were reported by observers.
Purse-seine set type was a major factor influencing catch rates of all taxa examined. Higher CPUEs of mammals, sharks and turtles were estimated from sets upon floating objects (i.e. associated sets), especially sets on logs and aFADS. Any management measures designed to reduce catches and mortalities of these taxa by the purse-seine fishery of the WCPFC should consider the influence of set-type.
While total annual catches and moralities were estimated for all taxa, confidence intervals around each estimate were relatively large. This is a result of the small number of records for each taxa (especially birds, mammals, turtles and individual species of sharks) and due low observer coverage rates. Increasing observer coverage rates for all fleets would result in more- robust estimates of catches and mortalities. Additionally, improving the rate of identification to the level of species and increasing the rates of observers reporting condition and fate of captured animals would also assist in the generation of more robust estimates of mortality. Additionally, centralising all observer data would provide a larger dataset in order to better estimate total catches and mortalities of all taxa.
Current observer programmes are primarily designed to record information on tuna catches. In future, specific observer programmes should be designed to address specific catch and bycatch issues, as has been done in other areas. For example, specific observer programmes could be designed to address the issues of interactions between birds, mammals and turtles with the newly developed shark and swordfish fisheries within the WCPFC area.
Finally, analyses of shark data were complicated by the large number of shark species and identifying target and non-target species of sharks for each of the four fisheries. More thorough research could be achieved by identifying and prioritising specific species of sharks important within each fishery, either by re viewing the shark species of Annex 1 of UNCLOS and/or by prioritising the list of species.