<I>Kajikia Albida</I> from Roundscale Spearfish

BULLETIN OF MARINE SCIENCE. 87(1):147–153. 2011 doi:10.5343/bms.2010.1036 NOTE USING HEAD MEASUREMENTS TO DISTINGUISH WHITE MARLIN KAJIKIA ALBIDA FROM ROUNDSCALE SPEARFISH TETRAPTURUS GEORGII IN THE WESTERN NORTH ATLANTIC Lawrence R Beerkircher and Joseph E Serafy ABSTRACT The morphometric characteristics that separate white marlin Kajikia albida (Poey, 1860) from roundscale spearfish Tetrapturus georgii Lowe, 1841 can be difficult for non-experts to distinguish, particularly when the animals are in the water. Using the measurements of the lower jaw tip to the longest branchiostegal ray (LJBR) and the opercle (LJOP), a discriminant function is presented that will help field observers separate the two species. The function DR−W = LJBR (3.31861) + LJOP (−2.84856) − 6.37198 classified specimens as roundscale spearfish when DR−W was > 0 and as white marlin when DR−W < 0 with a 95% correct classification rate overall. Since only the head of a specimen needs to be closely observed, the method has particular utility for tournament anglers during the hook removal process. Billfishes I( stiophoridae) are large, cosmopolitan apex predators captured by both commercial and recreational fisheries. In the Atlantic Ocean, several billfish popula- tions are considered overexploited (ICCAT 2003) and are subject to international re- building efforts. One of these species, the white marlin Kajikia albida (Poey, 1860), is morphologically very similar to the co-occurring roundscale spearfish Tetrapturus georgii Lowe, 1841, raising concerns that misidentification has affected the accuracy of past stock assessments (Beerkircher et al. 2009). Differentiating white marlin from roundscale spearfish when comparing dead animals onboard vessels, dockside, or in the laboratory has been made simpler by the use of scale morphology and position of the anal opening (Beerkircher et al. 2008a). Roundscale spearfish have characteristic lateral scales with wide, rounded anterior edges and, normally, multiple points; white marlin lateral scales are narrow, with usually only a single point. The position of the anal opening on roundscale spearfish is located in advance of the anal fin a distance (roughly) greater than half to three- quarters the height of the anal fin; in white marlin, the anal opening is located a distance (roughly) less than half the height of the anal fin (see Beerkircher et al. 2008a for precise relationships). These characteristics (scale and anal opening morphology) are easily learned by both field samplers and the general public, and have ready ap- plication in situations where circumstances offer the opportunity for close inspection of billfishes (such as landings at tournaments or onboard research vessels with the permits required to board these animals). However, current US regulations do not allow commercial fishermen to board billfishes (alive or dead) and recreational fishermen are required to release billfishes without removing them from the water if the fish is undersized or otherwise not intended for legal possession (NMFS 2006). In many situations, this prevents vessel personnel from closely examining hooked fish, thus precluding species identification based on the above characteristics. Best Bulletin of Marine Science 147 © 2011 Rosenstiel School of Marine and Atmospheric Science of the University of Miami 148 BULLETIN OF MARINE SCIENCE. VOL 87, NO 1. 2011 release practices encourage fishermen to remove gear from billfishes and other large pelagics prior to release (Prince et al. 2002). A morphometric characteristic useful in differentiating white marlin from roundscale spearfish, and which may be easily observed during the hook removal process, would help fishery participants to more accurately report billfish catches in self-reported or survey data collection programs. Based on examination of photographs taken by fisheries observers in the north- eastern Atlantic, Beerkircher et al. (2008b) suggested that branchiostegal length, relative to other reference points on the head of a specimen, was a diagnostic character, but offered no statistical basis for this conclusion due to small sample sizes available at that time. Since (in most cases) this characteristic can be easily observed and pho- tographed while removing a hook from specimens, relative branchiostegal length has potential utility for use by the public and researchers to differentiate between white marlin and roundscale spearfish. Here, we examine white marlin and roundscale spearfish relative branchiostegal lengths to determine the extent to which this characteristic offers an alternative diagnostic approach for these two species. Methods Measurement of Specimens.—Fishery observers deployed onboard US commercial pe- lagic longline vessels fishing in the NW Atlantic, Gulf of Mexico, and Caribbean (see Keene et al. 2007 for observer program details) opportunistically collected morphometric information from all boated specimens in the white marlin/roundscale spearfish complex; specimens were identified to this level using diagnostic information given in Nakamura (1985). All boated billfishes were dead, which allowed rapid inspection and measurement. Following Beerkircher (2008b), observers measured lower jaw fork length, height of the first anal fin, and distance between the posterior edge of the anal opening to the anterior edge of the anal fin. Observers also measured the distance from the tip of the lower jaw to the distal end of the longest branchiostegal ray (hereafter referred to as LJBR), and the distance between the tip of the lower jaw to the edge of the opercle following an axial line (hereafter referred to as LJOP; Fig. 1). All Figure 1. Measurement of the distance between the tip of the lower jaw and the longest branchiostegal ray (LJBR) and the distance between the tip of the lower jaw and the opercle (LJOP) of a billfish. NOTE 149 Table 1. Mean and ranges (in parentheses) for the following distances (in cm) measured from the tip of the lower jaw: fork length (LJFL), longest branchiostegal ray (LJBR), and opercle (LJOP) for white marlin and roundscale spearfish (n = total number of fish) observed in the northwestern Atlantic. Ratio means and ranges for LJBR:LJOP calculated for each species are also given in the last column. Species LJFL LJBR LJOP LJBR:LJOP White marlin (n = 140) 158 (133–202) 35 (29–43) 41 (34–50) 0.87 (0.79–0.97) Roundscale spearfish (n = 82) 162 (126–189) 37 (31–42) 39 (33–44) 0.95 (0.88–1.00) measurements were taken by tape measure to the nearest cm, while the mouth of the specimen was held shut, usually by use of a nylon cable tie. Beerkircher et al. (2008b) found that the distance between the anal opening and the first anal fin AFA( ) divided by the height of the first of the anal finAFH ( ) accurately distinguished genetically-confirmed roundscale spearfish and white marlin specimens. They observed no overlap in AFA:AFH ratios between the two species, with values ≥ 0.48 being 100% diagnostic of roundscale spearfish. Therefore, in the present study, we used the above cut-off to as- sign species identity to each specimen that had been initially assigned into the white marlin/ roundscale spearfish complex. Discriminant Function Analyses.—Evaluation of the discriminatory power of branchiostegal ray length and opercle length for differentiating the two species was achieved via discriminant function analysis (DFA) using SAS (1990) statistical software. The DFA pro- vided: (1) assessment of the statistical significance of the head measurements based on F-tests of Wilks’ Lambda; (2) linear functions for calculation of discriminant scores (D values) for each specimen; (3) matrices of classification error rates with a resubstitution analysis and leave-one-out cross validation procedure in SAS (1990); and (4) posterior probabilities of correct assignment of species membership for each specimen. This allowed us to calculate cut-off points for achieving ≥ 75% probability of correct classification of an individual being a white marlin or roundscale spearfish based on the two head measurements. Finally, to accommo- date fishers that may have difficulty with discriminant score calculations, we also conducted a simplified univariate DFA to assess the power of the LJBR:LJOP ratio to distinguish the two species. Results In total, 222 individuals were measured for AFA, AFH, LJBR, and LJOP, and ratios of AFA:AFH and LJBR:LJOP were calculated for each specimen (Table 1). Based on AFA:AFH values, our sample was composed of 140 white marlin and 82 roundscale spearfish.T ogether, the two head measurements were highly effective for separating white marlin from roundscale spearfish (Wilks’ Lambda = 0.29, F2,219 = 266.51, P < 0.0001). The discriminant function was: DR−W = LJBR(3.31861) + LJOP(−2.84856) − 6.37198 (Fig. 2); such that fishes were classified as roundscale spearfish whenD R−W was > 0 and as white marlin when DR−W < 0. The discriminant function with these two morphometrics correctly classified 94% of known roundscale spearfish and 96% of known white marlin (95% correct classification rate overall). The leave-one- out cross validation test also correctly classified species for 95% of the fishes. This analysis indicated > 75% accuracy in species assignment when DR−W < −1.1 for white marlin and > 1.1 for roundscale spearfish F( ig. 3). The simplified discriminant function using only the LJBR:LJOP ratio was also effective in classifying species (Wilks’ Lambda = 0.28, F1,220 = 553.63, P < 0.0001), al- 150 BULLETIN OF MARINE SCIENCE.

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