Analyses of catch data for Oceanic whitetip and silky sharks reported by fishery observers in the Hawaii-based longline fishery in 1996-2010
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Analyses of catch data for Oceanic whitetip and silky sharks reported by fishery observers in the Hawaii-based longline fishery in 1996-2010
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  • Alternative Title:
    Catch data for oceanic Whitetip and Silky sharks reported by fishery observers in the Hawaii-based longline fishery in 1996-2010
  • Description:
    This report presents descriptive statistical summaries and generalized linear model (GLM) analyses of catch data for oceanic whitetip shark, Carcharhinus longimanus, and silky shark, C. falciformis, in the Hawaii-based pelagic longline fishery. This paper is a collaborative effort begun at the Secretariat of the Pacific Community in New Caledonia and completed at the NOAA Fisheries Pacific Islands Fisheries Science Center in Hawaii. The data were collected by fishery observers aboard commercial vessels in 1995 - 2010. Oceanic whitetip shark mean annual nominal catch-per-unit effort (CPUE) decreased significantly from 0.428/1000 hooks in 1995 to 0.036/1000 hooks in 2010. This reflected a significant decrease in nominal CPUE on longline sets with positive catch from 1.690/1000 hooks to 0.773/1000 hooks, and a significant increase in longline sets with zero catches from 74.7 % in 1995 to 95.3 % in 2010. Oceanic whitetip shark CPUE was standardized by delta-lognormal and zero-inflated Poisson GLM methods. The latter method was employed because 90 .1 % of the longline sets caught zero oceanic whitetip sharks. Four factors (16 haul years, calendar quarters, deep-and shallow-set fishery sectors, 8 fishing regions) were significant explanatory variables in these analyses. Sea surface temperature was a significant continuous explanatory variable in a binomial GLM of the presence or absence of oceanic whitetip shark catches. The haul-year effect coefficients from these models were used to compute indices of relative abundance. The setime series were highly correlated, and each was also highly correlated with the time series of nominal CPUE. The silky shark catch data differed from the oceanic whitetip shark data in four major respects. The first was that nearly all silky sharks are caught on deep sets. The second was that most (62.5%) of the silky shark catch was taken from 0 ⁰ to 10 ⁰ N, although only 3.4 % of the observed fishing occurred in those latitudes. The third difference was that sample sizes were very small prior to 2000. Finally, although 46.3 % of the longline sets from 0 ⁰ to 10 ⁰ N caught zero silky sharks, 54.5% of the silky shark catch in these waters was taken on 11.5% of the longline sets, which caught e 5 silky sharks. These differences led to use of the data from 0 ⁰ to 10 ⁰ N in the deep sector from 2000 to 2010 in the GLM analyses, which were fitted by delta-lognormal and quasi-Poisson (i.e., overdispersed) methods. These GLM analyses had low explanatory power. Silky shark CPUE has ranged from 0.034/1000 hooks to 1.840/1000 hooks, but with no significant trend. Therefore, it is concluded that the relative abundance of silky shark in tropical waters exploited by this fishery, particularly near the Line Islands, has remained fairly stable since 2000. This was not the case with oceanic whitetip shark, which has apparently undergone a highly significant decline in relative abundance in this fishery since 1995.
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