Biological analysis of the nearshore reef fish fishery of Saipan and Tinian

Graham, T.

The results of this study are summarized below within the context of the three objectives set for the project for the 1993-1994 period. The period of study reported here is February through October 1993. 1) To characterize the Saipan/Tinian fishery in terms of species and length compositions, seasonality, and maturity and first size of reproduction. - The spearfishing catches of a day-active freedive operation (1215 fish sampled) and a night-active hookah operation (9202 fish) were sampled over a nine-month period and characterized in terms of species composition and, for the night operation only, length distributions. - A total of 107 species were encountered. The Acanthuridae (43% by weight) and Scaridae (20%) families dominated the night operation's catch. Acanthuridae (26%) and Kyphosidae (24%) dominated the day operation's catch, followed by Scaridae at 22 percent. -For the night operation, significant differences (alpha =0.05) in mean fish size were found among several fishing areas for eleven species: Myripristis berndti, Sargocentron tiere, Monotaxis grandoculus, Scarus microrhinos, Scarus psittacus, Scarus rubroviolaceus, Scarus schlegeli, Acanthurus lineatus, Ancanthurus nigricauda, Naso literatus, and Siganus punctatus. Temporal differences were observed for only one species, Siganus punctatus. -The parameters of the following regression models were computed for 59 species: weight-on-standard length, weight-on-fork length, and fork length-on-standard length. 2) To determine if, to what extent, and in what ways the reef fish of Saipan and Tinian are being overfished (including identification of those species that might require special attention and/or protection). - Catch-per-unit-effort (CPUE) data from the day (102 trips) and night (173 trips) operations were examined for use as indicators of fish density. The nighttime mean CPUE was 8.7 kg/spear-hour. The daytime mean was 4.1kg/sp-hr. Significant differences (alpha = .05) in CPUE were found between fishing areas, including between Saipan and Tinian. After adjusting for the significant effects of the number of fishers and lunar phase (night only), the differences in CPUE among areas were less pronounced for the night operation and the differences bewteen islands more pronounced for the day operation. The daytime adjusted mean CPUE for Tinian was about 1kg/sp-hr higher than for Saipan. Significant temporal differences (alpha = .05) in CPUE were observed for the night operation, even after adjusting for the effects of the number of fishers, lunar phase, and area fished. June and July exhibited the highest (adjusted) catch rates, at 10.2 and 10.8 kg/sp-hr, respectively, compared with a low of 7.3 kg/sp-hr in April. It appears unlikely that this variation was due to changes in fish density, expecially since no significant temporal differences in CPUE were observed for the day operation. Instead, hidden factors, perhaps having to do with the motivations of the fishers, may have had strong influences on fishing success and may have caused most of the observed variation. -Based on species composition and length distributions, 19 species were tentatively identified as being useful as subjects of more detailed investigations. 3) To assess the accuracy of the size restrictions proposed by the DFW in 1990, and, if necessary, to develop new management strategies to prevent overfishing. -Based on the length distributions of the night operation's catch, a set of modified size limits was developed. The modifications were aimed at both simplifying the regulations (e.g. reducing the problem of having to identify numerous species) and making them more realistic in terms of the biology of the species. It was recommended that this modified set of restrictions be adopted or that the size restrictions be dropped altogether from the regulations. Under the former scenario, it was recommended that only commercially sold fish be subject to the size restrictions.

Biological analysis of the nearshore reef fish fishery of Saipan and Tinian

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