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 ani- mals 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 dis- tance (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 inspec- tion 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, rel- ative 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 char- acteristic 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 bill- fishes 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 bran- chiostegal 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 branchio- stegal 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 speci- men 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% diagnos- tic 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 bran- chiostegal 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 cor- rect 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 ef- fective in classifying species (Wilks’ Lambda = 0.28, F1,220 = 553.63, P < 0.0001), al- 150 BULLETIN OF MARINE SCIENCE. VOL 87, NO 1. 2011

Figure 2. Plot of the distance between the tip of the lower jaw and the opercle (LJOP) versus the distance between the tip of the lower jaw and the longest branchiostegal ray (LJBR) for white marlin (open circles) and roundscale spearfish (black points) showing discriminant function (black line). Area between gray lines indicates morphological overlap where the discriminant function had < 75% probability of correctly classifying species.

Figure 3. Probability of fish being a roundscale spearfish (RSS) in relation to the discriminant function scores. Black line indicates 50% probability of correct species assignment; hatched lines indicate 25% and 75% cut-off points for discriminant scores of −1.10 and 1.10. NOTE 151

Figure 4. Probability of fish being a roundscale spearfish (RSS) in relation to LJBR:LJOP val- ue. Black line indicates 50% probability of correct species assignment at LJBR:LJOP = 0.908; hatched lines indicate 25% and 75% cut-off points for LJBR:LJOP values of 0.8996 and 0.9161.

though slightly less so than the full DFA. This discriminant function was: DR−W = LJBR:LJOP (−120.67802) + 133; it correctly classified 90% of roundscale spearfish and 96% of white marlin for an overall correct classification rate of 93%. The leave-one- out cross validation test also correctly classified species for 93% of the fishes. This analysis indicated 50% accuracy in species assignment at a LJBR:LJOP value of 0.908 and > 75% accuracy in species assignment when LJBR:LJOP < 0.8996 for white marlin and > 0.9161 for roundscale spearfish F( ig. 4).

Discussion

Collection of morphometric data for billfishes in commercial fisheries presents a challenge. First, current regulations in the United States prevent the sacrifice of live animals or even the boarding of dead animals without special permits; second, the large size of these animals prevents easy collection, retention, and transport to laboratory facilities for detailed measurements; and third, observer coverage is ex- pensive, placing observers under a requirement to collect a diverse and aggressive suite of information on both catch and effort, making collection of detailed measure- ments on specific animals (for example, caliper measurements to the nearest mm) impractical. Even when considering catch from international fleets that allow reten- tion and sale of billfishes, the fishes are generally processed at sea in a way that makes dockside data collection of the morphological characters we investigated impossible. The methodology we used was developed to take maximal advantage of the sampling opportunities presented to observers. Previous authors have pointed out the drawbacks of using ratios when examining morphometric data (Atchley and Anderson 1978, Humphries et al. 1981); however, 152 BULLETIN OF MARINE SCIENCE. VOL 87, NO 1. 2011

Figure 5. Field photograph of the relationship between opercle and branchiostegal in roundscale spearfish (upper specimen) and white marlin (lower specimen). the alternative analytical procedures, such as multivariate approaches, require mea- surement of many more characters and thus have little practical application to use in the field by the lay public. Because fisheries management largely rests on information self-reported (either mandatory or voluntary) by the public and data collected by field personnel with limited time and materials to perform detailed measurements, we suggest that the approach used here presents the greatest utility under existing constraints. The results of our analyses indicate that branchiostegal length, compared to oper- cle length, is a valid and robust morphological character to differentiate white marlin from roundscale spearfish provided there is an initial determination that the speci- men in question must be either from one of these two species. Two simple measure- ments entered into a pocket calculator using the function we provide can be used to help identify the species. Alternatively, a photograph of the specimen’s head, taken at a right angle to the fish, can later be analyzed using the function. We are often asked to identify billfishes based on photographs; it is rare that the resolution of the photograph allows determination of the lateral scale morphology and equally rare that the angle of the photo shows the location of the anal opening, thus prevent- ing identification based on those characteristics. However, in many cases the lower jaw, branchiostegals, and opercle are clearly visible in photographs and use of the method we describe here would allow a greater number of billfishes to be accurately identified and therefore improve the quality of catch data from both recreational and commercial fisheries. Observation, measurement, or photographing of the LJBR and LJOP characteristic in many cases would be easily obtained during the hook removal process. Even a cursory look at the head of a putative specimen can quickly show whether or not the branchiostegals reach nearly as far back as the opercle (Fig. 5). The addition of this identification criterion to the existing characteristics that differenti- NOTE 153 ate white marlin from roundscale spearfi sh will enable collection of more accurate catch data as well as potentially allow for analysis of historical photographic records.

acKnoWledGments

twenty-eight diff erent fi sheries observers and researchers collected information used in this study; however, J rollo, J sheldon, and m tierney collected 64% of the data and deserve special thanks. we also thank J Javech for creating fig. 1.

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