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Fishery Bulletin 89(3). 1991 



marlin Tetrapturus albidus. An initial report (Prince 

 et al. 1984) indicated that otoliths and dorsal spines 

 were the most promising structures examined, but the 

 best approach for determining the accuracy of these 

 two ageing methods was unclear. Marginal increment 

 analysis (MIA) of 328 dorsal spine sections (Prince 

 et al. 1987) failed to provide evidence of the temporal 

 periodicity (i.e., regularity) of annulus formation, due 

 in part to the large amount of variability in these 

 measurements. 



Estimates of age and growth rates of young fish are 

 generally more reliable than for adults because micro- 

 structural increments on otoliths of fast-growing juve- 

 niles are fewer in number and are often easier to count 

 or measure accurately (Casselman 1983, Prince et al. 

 1987, Summerfelt and Hall 1987). In addition, otolith 

 microstructural analysis for ageing young fish is in- 

 herently more precise than counts of annual marks on 

 structures of adults since errors in counting increments 

 are made in days, not years. Following the recommen- 

 dations of Prince et al. (1987), otolith microstructural 

 analysis of young blue marlin was selected as the 

 method of choice for improving knowledge of age and 

 growth of this species. The small otoliths, narrow in- 

 crement widths, and longevity of blue marlin (25-30 

 years) were expected to limit the microstructural 

 method to the first few years of life. A few researchers, 

 however, have reported success in counting microstruc- 

 tural increments between presumed annual marks on 

 sagittae from older adult temperate and tropical spe- 

 cies (Pannella 1971, Radtke 1984, Brothers and 

 Mathews 1986) as a means of determining their 

 periodicity in older fish. 



Objectives of this study were to (1) estimate the age 

 and growth of young Atlantic blue marlin from otolith 

 microstructural analysis, (2) determine the periodicity 

 of increment formation by (i) comparing the distribu- 

 tion of back-calculated spawning dates with the spawn- 

 ing season of Atlantic blue marlin reported in the 

 literature, and by (ii) comparing microstructure char- 

 acteristics of increments on larval blue marlin otoliths 

 with those found in other species where increment 

 deposition rate has been established, (3) determine the 

 precision of the ageing method, (4) fit the ageing data 

 to an appropriate growth equation(s) so daily growth 

 rates could be compared with other fast-growing 

 species, and (5) determine whether counts of micro- 

 structural increments between presumed annuli in 

 otoliths of adult blue marlin are consistent with the 

 hypotheses that microstructural increments are 

 deposited daily and gross zonation is annual. 



Methods 



Data used in this study cover several distinct life- 

 history stages, i.e., larvae, juveniles, young adults, and 

 adults. Although our life-history partitions are some- 

 v what arbitrary, they are defined here to minimize 

 confusion related to their use in various analyses 

 described below. All references to length for juvenile, 

 young adult, and adult blue marlin in the remainder 

 of the paper are lower jaw fork length (LJFL), while 

 length for larvae are notochord (NL) or standard length 

 (SL). Adults were separated from younger stages at 

 about 110cm, based on changes in form of the length- 

 at-age relationship (discussed later). All planktonic blue 

 marlin larvae in our sample were <llmm. Juvenile 

 blue marlin 4.3-110cm were always sexually immature 

 and did not have the full array of adult morphological 

 characters. A few young adult blue marlin > 110-140 

 cm had the full array of adult morphological character- 

 istics but most in this size range were sexually imma- 

 ture. Sexually mature adult blue marlin were nearly 

 all over 140 cm and had the full array of adult morpho- 

 logical characteristics. 



Data collection 



Juveniles, young adults, adults During 1980-83, 

 juvenile through adult Atlantic blue marlin were col- 

 lected directly by NMFS samplers from taxidermists, 

 commercial and recreational fishermen, and at billfish 

 tournaments or ports in the Gulf of Mexico, Caribbean 

 Sea, and northwestern Atlantic Ocean. All size cate- 

 gories were initially targeted and 3-5 hard structures, 

 including sagittae, were collected from each specimen. 

 After 1983, a special "save it for science program" was 

 developed (Prince 1984) to obtain extreme size cate- 

 gories, since blue marlin under 100 pounds (45 kg) and 

 over 900 pounds (409 kg) are very rare in the north- 

 western Atlantic Ocean (Prince et al. 1984). 



Most blue marlin specimens used in this study were 

 caught on hook-and-line, but dipnets were also used for 

 smaller size categories. In addition, some specimens 

 were obtained from the stomachs of larger predators. 

 Fish samples were preserved by freezing or immersion 

 in 95% ethanol to reduce deterioration of the otoliths. 



When possible, the following supplemental data were 

 collected from each fish: (1) lower jaw fork length, (2) 

 round weight, (3) sex, and (4) date of capture. Length 

 measurements along the contour of the body were 

 made to the nearest centimeter (cm). Weight was 

 measured to the nearest pound and later converted to 

 kilograms (kg). Sex was determined by visual inspec- 

 tion or histological examination (M.J. Wolfe, Dep. 

 Avian and Animal Medicine, Cornell Univ., Ithaca, NY 



