New Study Focuses on Trends in Abundance of Oceanic Whitetip and Silky Sharks in the Central Pacific

A new study has been completed providing valuable information about trends in the abundance of oceanic whitetip shark Carcharhinus longimanus and silky shark C. falciformis in Pacific Ocean waters fished by the Hawaii-based pelagic longline fishery. The collaborative research project was begun in April 2011 at the Secretariat of the Pacific Community (SPC) in New Caledonia and completed in August at PIFSC. The study was conducted by William Walsh, a scientist with the University of Hawaii Joint Institute for Marine and Atmospheric Research affiliated working at PIFSC and his colleague Shelley Clarke of SPC. The duo analyzed catch and effort data collected by fishery observers aboard Hawaii commercial longline vessels in 1995–2010 and computed catch-per-unit-effort (CPUE) statistics for each species.

The mean annual nominal CPUE for oceanic whitetip shark 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. Walsh and Clarke considered oceanic whitetip shark CPUE time series standardized using delta-lognormal and zero-inflated Poisson GLM methods. The latter method was selected for use because 90.1% of the longline sets caught zero oceanic whitetip sharks. The model coefficients representing the haul-year effect on CPUE were used to compute indices of relative abundance. These time 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 4 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 in waters from latitude 0° to 10°N, although only 3.4% of the observed fishing occurred at these latitudes. The third difference was that sample sizes were very small prior to 2000. Finally, although 46.3% of the longline sets from 0° to 10°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 ≥ 5 silky sharks. These differences led to use of the data from 0°-10°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. Silky shark CPUE has ranged from 0.034/1000 hooks to 1.840/1000 hooks, but with no significant trend.

Walsh and Clarke concluded that the relative abundance of silky shark in tropical waters exploited by the Hawaii longline fishery, particularly near the Line Islands, has remained fairly stable since 2000. On the other hand, oceanic whitetip shark has apparently undergone a highly significant decline in relative abundance in this fishery since 1995.

Distribution of catches of oceanic whitetip shark (aggregated over 1995–2010) as reported by observers 
                 on Hawaii-based longline vessels. Distribution of catches of silky shark (aggregated over 2000–2010) as reported by observers on 
                 Hawaii-based longline vessels.
Distribution of catches of oceanic whitetip shark (left; aggregated over 1995–2010) and silky shark (right; 2000–2010) as reported by observers on Hawaii-based longline vessels.
Annual indices of relative abundance from the delta lognormal and zero-inflated Poisson analyses of 
                 oceanic whitetip shark CPUE in the Hawaii-based pelagic longline fishery in 1995–2010.  The nominal CPUE 
                 trend (solid line) is included for comparison.
Annual indices of relative abundance from the delta lognormal and zero-inflated Poisson analyses of oceanic whitetip shark CPUE in the Hawaii-based pelagic longline fishery in 1995–2010. The nominal CPUE trend (solid line) is included for comparison.
Annual indices of relative abundance from the delta lognormal and quasi-Poisson analyses of silky shark 
                 CPUE in the Hawaii-based pelagic longline fishery in 2000–2010.  Analysis was limited to sharks caught 
                 on deep sets from 0° to 10°N.  The nominal CPUE trend (solid line) is included for comparison.
Annual indices of relative abundance from the delta lognormal and quasi-Poisson analyses of silky shark CPUE in the Hawaii-based pelagic longline fishery in 2000–2010. Analysis was limited to sharks caught on deep sets from 0° to 10°N. The nominal CPUE trend (solid line) is included for comparison.