Analyses of the potential influence of four gear factors (leader type, hook type, “shark” lines and bait type) on shark catch rates in WCPO tuna longline fisheries [EB WP 02] Don Bromhead, Joel Rice, and Shelton Harley

"WCPFC-SC9-2013/EB-WP-02 rev 1."

Not published by the SPC

Includes bibliographical references (p. 22-24)

"In reviewing the stock assessments conducted in 2012 for silky and oceanic whitetip sharks, SC8 noted both a) concerns over the status of the stocks, and b) the large impact that non-target longline fisheries are estimated to have. For these reasons, SC8 recommended consideration of mitigation measures as providing the best opportunity to improve their stock status. A number of factors related to longline fishing methods, such as leader type, hook type, shark lines and bait (amongst others) may influence shark catch rates and offer potential for developing mitigation options. This paper represents an extension of work presented in a preliminary analysis of wire trace effects on oceanic whitetip and silky shark provided to WCPFC9 in 2012. The key differences are that this paper, firstly, reviews regional observer data and determines if there is sufficient homogeneity and contrast throughout fishery/area/time strata in the key factors of interest to support integrated regional level analysis of leader, hook, shark line and bait effects. Secondly, it identifies strata within the observer database where data are relatively more concentrated and contain relatively high contrast (heterogeneity) in one or more of these factors through time and space. Finally it describes subregional models to assess the relative effect of wire trace, hook type, shark lines and bait categories (along with other environmental and fishing method) factors upon catches of oceanic whitetip and silky shark in fisheries within the WCPO. Key findings include: • A significant relationship between shark line use and increased catch of both species was identified in both the RMI/FSM and Fiji based tuna fisheries; • Wire trace was also estimated to have a positive relationship with silky shark catches in the Fiji fishery, but not in the Hawaii or RMI fisheries; • Shark bait was not, on its own, estimated to be related to shark catches (although it is uncertain to what degree it may be confounded in models with shark line use); and • Hook type (interacting with leader type) was assessed in the Hawaii fishery but no substantial difference in effects on catch of either shark species was estimated. However model diagnostics were particularly poor for this fishery, and there is uncertainty over whether some sets in fact constituted “mixed” hook type sets and further work on the models may be required. This information may, in combination with information on shark condition (See working paper SC9- EB-WP-06) and wire trace usage (WCPFC9-2012-IP-14) assist with the development of models to predict changes in oceanic whitetip and silky shark catch/mortality levels under different potential mitigation scenarios. To address some specific issues that involve interactions between factors, higher levels of observer coverage plus heterogeneity in fleet practices will be necessary otherwise it might be necessary to undertake specific experiments."