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Article Excerpt
Introduction
In the Hawaii commercial longline, troll, and handline fisheries for highly migratory pelagic species, both dolphinfish (mahimahi), Coryphaena hippurus, and wahoo (ono), Acanthocybium solandri, are incidentally caught and are of secondary importance to tunas, Scombridae, and billfishes, Istiophoridae. Their relative abundance, market demand, and relatively smaller size contribute to their secondary role. However, as people became more health conscious in the late 1970's, coupled with the decline of many fisheries, the demand for both dolphinfish and wahoo increased. (1) In 2003, the last year for which complete Hawaii state fishery landings were compiled, 596 t of dolphinfish and 446 t of wahoo were landed by the commercial fishery in Hawaii. (2)
As the focus of fishery management broadens to include a wider range of species in marine ecosystems, a growing need exists to evaluate catch data on species caught incidentally. Weight-on-length (W-L) and length-on-weight (L-W) relationships are needed to convert at-sea length measurements to weights, market weight data to length, and to examine patterns in fish condition that may provide insights into reproductive life history and ecology.
Many W-L predictors have been published for dolphinfish; these relationships have been described from the Mediterranean Sea (Bannister, 1976; Massuti et al., 1999); eastern tropical Atlantic (Castro et al., 1999), western tropical Atlantic and the Carribean (Oxenford and Hunte, 1986b; Oxenford, 1999), the Straits of Florida (Beardsley, 1967), Gulf of Mexico and the Gulf Stream (Gibbs and Collette, 1959), and North Carolina waters (Schuck, 1951; Rose and Hassler, 1968). In the Pacific, W-L predictors have been described for dolphinfish caught off Colombia and Panama (Lasso and Zapata, 1999), the entire Pacific Ocean (Takahashi and Mori, 1973); Hawaii (Tester and Nakamura, 1957); and Taiwan in the western Pacific (Wang, 1979). In the Indian Ocean, Chatterji and Ansari (1985) examined sexual dimorphism in W-L relationships for east African dolphinfish. Hence, most of these studies examined fish in populations distant from Hawaii. The lone Hawaiian study--fish caught off Kaneohe Bay, Oahu--was based on a relatively small number of measurements for mostly .juvenile fish (Tester and Nakamura, 1957), and it is inadequate for describing the full size range of dolphinfish that is commercially landed in Hawaii.
Where many W-L predictors for dolphinfish have been described globally, only three published reports (Iversen and Yoshida, 1957; Beardsley and Richards, 1970; Santana, et al. (3)) and a PhD. thesis (Hogan, 1976) have described W-L relationships for wahoo. A recently published review on wahoo from the western Central Atlantic region by Oxenford et al. (2003) mentioned three other studies in a table of various morphometric relationships. Beardsley and Richards (1970) provided a W-L relationship for wahoo from southeast Florida using data obtained from taxi-dermists. Hogarth (1976) described a W-L relationship for wahoo based on data from the sport fishery off the North Carolina coast. Santana et al. (3) described the W-L relationship of wahoo caught around the Canary Islands in the eastern Atlantic Ocean. In the Pacific Ocean, Iverson and Yoshida (1957) provided a W-L relationship for wahoo caught around the Line Islands in the equatorial central Pacific. There has not yet been a thorough evaluation on the W-L relationship for wahoo caught around the Hawaiian Islands.
Highly migratory management species (tunas, billfishes, dolphinfish, wahoo, and other incidentally caught species) are caught by the local fisheries around Hawaii and sold at the local fish auction. The Hawaii-based longline fishery has provided the majority of the dolphinfish and wahoo landed at the fish auction, but the troll fishery, bait boats, and the handline fishery (i.e. deep-sea handline, ikashibi, and palu-ahi) also have contributed significantly to total landings (NMFS, 2001). Wahoo landings peak in weight in May and are lowest in December-January. Dolphinfish landings peak in weight in the spring and again in the fall. (2)
Both dolphinfish and wahoo belong to the species complex of Pelagic Management Unit Species for the Western Pacific Regional Fishery Management Council (NMFS, 2001), where they represent an important component of the commercial landings. Stock assessment analyses and ecosystem models can be improved with more accurate morphometric relationships. Weight-length relationships are also useful for converting metrics in tag and recapture experiments and in databases lacking either metric, and they can be used to indicate body condition or robustness of fish in a stock (Le Cren, 1951). This study on dolphinfish and wahoo landed at the Honolulu fish auction during 1988-89 describes W-L predictor variables based on a large number of measurements covering a wide size range. The effects of sex and month on the variables were also examined.
Material and Methods
Dolphinfish morphometric data (4) were collected at the United Fishing Agency (5), Honolulu's public fish auction, from March 1988 through November 1989, except for December 1988 and June 1989. Wahoo morphometrics (4) were collected from July 1988 through November 1989, except for December 1988 and June 1989. Prior to the opening of the auction, fork-lengths (FL) of dolphinfish and wahoo were measured to the nearest millimeter using a meter-long fish caliper, and corresponding whole weights (W) were estimated using the auction scale. (6) When fish length exceeded 1 m, a mark was scratched on the skin at 1 m and the remainder of the fish length was measured to complete the measurement. Weights were recorded to the nearest 0.5 lb at the auction and later converted to kilograms. Small dolphinfish and wahoo were frequently sold in lots of 2-6 fish with a combined weight. These fish did not have individual weights and were not used in these analyses. The sex of dolphinfish was based on the pronounced forehead crest that is present in sexually maturing or mature male dolphinfish but absent from females (Fischer and Whitehead, 1974).
Data were first checked for outliers. All weight-length data for a species were first fitted to a (natural) log linear W-L regression, and outliers greater than [+ or -] 3 Studentized residuals were eliminated from the data file as it was believed that these were measurement or recording errors. Then sexual dimorphism and monthly effects were examined by comparing the slopes and intercepts by multiple regression analysis for the sexes or by a general...
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