Prince et al.; Otolith analysis of Makaira nigricans age and growth 



443 



14853-6401, pers. commun., 14 Aug. 1987). When sex 

 could not be determined, specimens were designated 

 as unknown sex. Otoliths (sagittae) were removed from 

 the craniums using extraction procedures of Radtke 

 (1983a). 



Larvae Istiophorid larvae were collected 25-26 Aug- 

 ust 1982 at 14 stations off Miami, Florida, during a two- 

 day cruise of the RV Virginia Key. Surface tows were 

 made at the western edge and in the axis of the Gulf 

 Stream using either aim conical plankton net or a 

 1 x 2m neuston sampler, both with 0.947-mm mesh size. 

 Larval istiophorids were separated from the other 

 plankton, and their numbers represented about 5% of 

 the fish in the samples. All larval samples were pre- 

 served in 95% ethanol. 



Preserved istiophorid larvae were soaked in water 

 for several minutes before measurements were re- 

 corded and otoliths extracted. This reduced some of 

 the shrinkage caused by the alcohol and tended to 

 straighten and soften the bodies. Theilacker (1980) 

 reports that shrinkage of larvae caused by net-handling 

 decreases with size while that due to preservation alone 

 is constant. Since all the larvae were nearly the same 

 size (5-10mm), we assumed shrinkage to be an undeter- 

 mined constant proportion. 



Larvae were measured with a dissecting scope to the 

 nearest 0. 1 mm from the tip of the lower jaw to the tip 

 of the notochord (NL), or to the developing hypural 

 plate (SL). Otoliths were removed from larvae using 

 the methods of Brothers and McFarland (1981). Istio- 

 phorid larvae were then cleared and stained according 

 to methods of Potthoff (1986) so vertebral counts could 

 be made. The blue marlin larvae were distinguished 

 from the Istiophorus-Tetrapturus group based on 

 vertebral counts. 



Otolith preparation 



and microstructural analysis 



The general approach of Brothers et al. (1983) for 

 otolith microstructure analysis of larval and juvenile 

 bluefin tuna was adopted for this study. Otolith mass 

 for all blue marlin ^4.3 cm was measured on a micro- 

 balance to the nearest 0.01 mg. The extremely small 

 size of sagittae from blue marlin < 4.3 cm precluded 

 measurements of otolith mass for this size category. 

 The transparency and shape of otoliths from larvae and 

 small juveniles (<23cm) allowed their examination, 

 without further preparation, with a compound light 

 (polarized) microscope adapted for video viewing. 

 Because of the change in mass and configuration of 



sagittae from larger fish (>23cm), preparation of these 

 otoliths included breaking them along the sulcus by 

 light pressure with a scalpel. The medial surface of the 

 dorsal lobe was ground on a glass plate with a mineral 

 oil slurry of 600-grit silicon dioxide to slightly thin the 

 fragment and give it a flat surface on which to rest. 

 The distal surface was then ground with the 600-grit 

 to a point just short of reaching the core region of the 

 otolith. Fine emery paper or diamond compound (3^m) 

 was then used to polish the surface. 



The best counting paths were found to be on either 

 the anterior (antirostrum) or posterior axis of the dorsal 

 lobe (Fig. 1A). Counts and photographs of the video 

 image of "primary" microstructural increments (Gef- 

 fen 1987) are from the dorsal lobe and, where possible, 

 along the anterior axis (Figs. 1B-D). Alternatively, due 

 to lack of specimen clarity or poor preparation, counts 

 were made along the posterior axis. Counts started at 

 the first visible increment outside the core (Fig. 1C) 

 and continued to the margin of the structure (Fig. ID). 

 Increment counts and measurements were made at 

 magnifications ranging from 100 to 2500 x. 



Increment counts for larval, juvenile, and young 

 adult/adult blue marlin otoliths include only primary 

 increments. Fine increments, provisionally identified 

 as "subdaily" (Figs. 2A,B), were often observed in the 

 otolith region corresponding to larval and early juvenile 

 growth. These subdaily increments were easily iden- 

 tified by their vague appearance and regular cluster- 

 ing within the more prominent primary units (Fig. 2), 

 and were not tallied. 



Counts were not corrected for age at first increment 

 formation because known age larvae were not avail- 

 able. Back-calculated spawning dates were computed 

 by subtracting the total count of primary increments 

 for each sample from the date of capture. 



Preparation of otolith sections for scanning electron 

 microscope (SEM) examination followed methods 

 described by Brothers et al. (1983), Brothers and 

 Mathews (1986), Brothers (1987), and Jones and 

 Brothers (1987). Some otoliths were sectioned and 

 rough polished according to the methods of Wilson 

 (1984). The majority of increment counts were made 

 on lateral views of whole otoliths or broken sagittae, 

 but a limited number of samples (9) were available in 

 which counts could be made from transverse sections 

 and whole sagittae from the same fish. 



Otoliths that were found to be overground, eroded, 

 decalcified, or which had an irregular, disrupted, or 

 unusual microstructural record were excluded from the 

 ageing analysis. 



