Understanding Vascularization in Fin Spines of White Marlin (<I>Tetrapturus Albidus</I>)

BULLETIN OF MARINE SCIENCE, 79(3): 847–852, 2006

NOTE Understanding vascularization in fin spines of white marlin (Tetrapturus albidus)

Katherine Drew, David J. Die, and Freddy Arocha

A recent stock assessment of white marlin (Tetrapturus albidus Poey, 1860) indicated that the species is significantly overfished and continues to suffer overfishing; however, the assessment had a high degree of uncertainty in the estimates of current stock status (ICCAT, 2003). Management could be more effective if this uncertainty is reduced. One of the major sources of uncertainty is the lack of basic biological data, including life-history parameters such as size-at-age and growth rates. A vali- dated age and growth model would fill these gaps, but none exist for Atlantic marlin. Prince et al. (1991) developed an age and growth model for young blue marlin (Mak- aira nigricans Lacépède 1802) from the Atlantic Ocean but were unable to validate their aging method for adult fish. Nothing similar has been published for white marlin. Due the nature of this family, billfish present unique challenges in the determination of age, including the choice of hard part for aging, collection of adequate samples, and validation of ages. Preliminary work has been done investigating hard parts (otoliths and fin spines) for use in aging blue and white marlin (Prince et al., 1984; Hill et al., 1989); both of these studies found significant relationships between length and both the radius and ring count of the hard part. Otoliths are generally the preferred hard part for aging fish; unfortunately, the otoliths of billfish are very small and fragile, making them difficult to extract and readH ( ill et al., 1989). Instead, most age and growth models for billfish have been done with fin spines: swordfish (Ehrhardt, 1992; Sun et al., 2002), sailfish H( edgepeth and Jolley Jr., 1983; Chiang et al., 2004), black marlin (Speare, 2003), and striped marlin (Melo-Barrera et al., 2003). A study of Pacific blue marlin indicated that anal fin spines provide more precise age estimates and are easier to collect and process than the saggital otoliths (Hill et al., 1989). Ease of collection is particularly important when the sampling is being done on-board fishing vessels or in port when the catch is being off-loaded. In these situations, observers do not have the time to perform the delicate procedure of removing the otoliths. Also, in some artisanal fisheries, the head has value and damaging it would require payment, whereas removing the anal fin does not affect the value of the fish (F. Arocha, pers. obs.). The drawback of using fin spines is that over time the material of the spine that forms the rings is replaced by vascular tissue. This process starts at the center of the spine and progresses outward. As a result, the earliest rings can be lost. Understand- ing patterns of vascularization in fin spines is thus a necessary step in estimating the number of obscured rings and calculating the age of a particular fish. Here we present preliminary observations on vascularization in the anal fin spines of white marlin, including a description of how vascularization relates to fish size.

Methods

Sampling began in the last quarter of 2003 and occurred exclusively in waters fished by the Venezuelan fleet until 2005, when sampling was expanded to cover fleets operating in

Figure 1. Location of spine sections taken from white marlin. other parts of the Atlantic and in the Caribbean. Observers—either aboard the vessels or at port, depending on the fishery—removed the anal fin from all dead marlin and recorded the lower jaw fork length (LJFL) and sex (determined from gonad morphology) for each individual, along with the date and location of capture. The fins were kept frozen until they could be cleaned and sectioned in the lab. Fin spine samples were processed according to the method detailed by Drew et al. (2006). Briefly, three 0.45 mm-thick sections were taken at a distance of one-half the length of the condile base (Fig. 1) and magnified photographs were taken of each section. Measurements of the cross-sectional area, the radius of the spine, and the area of vascularized tissue were made from the digital images using Sigma Scan Pro software (Systat Software Inc, Richmond, CA). The area measurements were collected from the images by tracing the perimeter of the spine and outlining the area of vascular tissue (Fig. 2). Images were calibrated based on the magnification under which the spines were photographed so that the program could convert pixels to distance and area. The extent of vascularization in the second and third anal fin spines was compared in 278 fish, using paired t-tests. The absolute area of vascularization and the percent of the cross- sectional area of the spine that was vascularized were regressed against lower jaw-fork length, using SAS v 9.1.3 software. Regressions were performed for the second and third fin spines separately, with the data both pooled and separated by sex. Several linear (first order, quadratic) and nonlinear (power, logistic) models were tested, along with log transforms of both the dependent and independent variables.

Results And Discussion

In total, 1051 white marlin (443 males, 559 females, and 9 of unknown sex) were sampled, including two that were recovered with tags. A subset of these samples (N = 409) was processed for this study and covered a restricted range of sizes. Ninety percent of the fish collected were between 150 and 180 cm fork length, a result of popu- lation demographics at the initial sampling site (Arocha and Marcano, in press). The cross-sectional area of the vascularized tissue (and the percentage of area vascularized) in the second spine was greater than that in the third spine (paired t-test, P < 0.001), making the third spine the preferred structure for aging. Both the area of vascularized tissue in the spine and the percentage of the cross-sectional area of the spine obscured by vascularization increased as the length of the fish increased, but there was a high degree of variation among individuals (Figs. 3, 4). The increasing trend in the percentage of the spine area that was vascularized suggested that the NOTES 849

Figure 2. (A) Spine section from white marlin and (B) same section with image measurement overlay indicating area of vascularization (black) and total area (gray).

addition of new spine tissue and the vascularization of old tissue did not occur at the same rate. Partial F-tests showed that there was no significant differenceP ( > 0.05) between the coefficients of the regression lines for each sex, so data were subsequently pooled for both sexes to describe the relationship between lower jaw-fork length (L) and

cross-sectional area of vascularized tissue (Av ) for each spine.

For the third spine, the quadratic form had the highest r-squared value (Av = −133.56 + 1.577 × L − 0.0042 × L2; r2 = 0.31), while the linear model had a slightly 2 lower r-squared value; (Av = −26.34 + 0.234 × L; r = 0.30). For the second spine, the quadratic term was not significant P( = 0.112), resulting in a linear relationship; (Av = −31.49 + 0.274 × L; r2 = 0.34). Although the quadratic model appeared to be trying to capture a decrease in the growth rate of vascularization at larger lengths, the nonlinear models had difficulty converging, and often converged to parameters which included zero in their confi- dence intervals. With more data points at the extremes of the size ranges to anchor the regression, nonlinear models are likely to provide better fits; otherwise, a linear 850 BULLETIN OF MARINE SCIENCE, VOL. 79, NO. 3, 2006

Figure 3. Area of vascularization vs lower jaw fork length for (A) the second anal fin spine and (B) the third anal fin spine. Hollow squares ®( ) are females; filled circles •( ) males. The solid lines are the best-fit regression curves. N = 409 for each spine. model may be a better choice for data sets restricted to a narrow range of fish sizes, such as ours. Our preliminary work on the radius of visible rings seen in the spines of white marlin suggests that vascularization leads to ring loss. The radius of rings seen in the smallest fish is smaller than the radius of the vascularised area observed in large fish. Understanding the process of vascularization is necessary to estimate the number of rings lost in the spines of larger fish, and thus calculate the correct age. This study has shown that the high degree of individual variability in the process of vascularization makes this estimation difficult, but not impossible, because the relationship between fish size and the extent of vascularization can be described mathematically. With samples from a larger range of sizes, which we are in the process of obtaining, we expect to be able to develop a more robust statistical model relating fish size and vascularization. Although aging billfish with spines presents a number of challenges, with a comprehensive sample of fish sizes, a thorough understanding of the process of histological development of spine tissue, and an appropriate statistical method, we can overcome these difficulties. NOTES 851

Figure 4. Percent of the cross-sectional area of spine sections from (A) the second anal fin spine and (B) the third anal fin spine in white marlin that has been vascularized vs lower jaw fork length. Hollow squares (®) are females; filled circles •( ) males. N = 409 for each spine.

Acknowledgments

This research would not be possible without the efforts of those researchers who have helped coordinate our Atlantic-wide sampling: E. Prince in the US, F. Hazin in Brazil, J. Ariz and I. Mosqueira in Spain, and L. Reynal in Martinique. Additionally, A. Barrios in Venezuela has done exceptional work in preparing the samples and images. This work has been funded by the Center for Sustainable Fisheries, the US National Oceanographic and Atmospheric Administration, the Gulf States Marine Fisheries Commission, and the Yamaha Contender Miami Billfish Tournament.

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Addresses: (K.D., D.D.) University of Miami, RSMAS, Division of Marine Biology and Fish- eries, 4600 Rickenbacker Causeway, Miami, Florida 33149. (F.A.) Instituto Oceanográfico de Venezuela, Universidad de Oriente, Apartado de Correos No. 204, Cumaná 6101, Venezuela. Corresponding Author: (K.D.) E-mail: .