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An exploratory analysis of trends in swordfish (Xiphias gladius) length composition data from the Hawaiian longline fishery
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2017
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Series: PIFSC working paper ; WP-17-002
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Description:Generalized additive models (GAMs) were applied to explore the relative influence of spatial, temporal, and environmental predictors on the distribution of swordfish (Xiphias gladius) fishery length frequency data from the Pacific Islands Regional Observer Program database on the Hawaiian longline fishery during 1994-2016. Spatial predictors were more generally influential on swordfish length than temporal factors. Environmental variables such as sea surface temperature, the Pacific Decadal Oscillation index, and the Southern Oscillation Index were sometimes significant but did not explain a substantial portion of the variance in the data. Models of the deep-set sector explained 30-50% of the deviance while models of the shallow-set sector explained less than 25% of the deviance. The deep-set sector does not target swordfish, primarily captures small young-of-the-year (<100cm) individuals, and had much higher annual variability in length. The shallow-set sector targets swordfish, catches primarily large swordfish, and has had relatively low annual variability. Spatial patterns indicate that large fish are caught in the more northern latitudes while smaller fish are caught closer to the equator. Overall, based on the significant differences between the length composition of the shallow-set and deep-set catch, the deep-set data is unlikely to be representative of the targeted swordfish fishery. However, the deep-set data did primarily consist of small fish with a median length of 82 cm and may contain information on annual recruitment strength. It would be useful to compare the patterns observed in this analysis with those from other fisheries to see if the conclusions drawn here are applicable to the entire North Pacific swordfish population. [doi:10.7289/V5/WP-PIFSC-17-002 (http://doi.org/10.7289/V5/WP-PIFSC-17-002)]
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Rights Information:CC0 Public Domain
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