Wells and Rooker: Distribution, age, and growth of young-of-the-year Senola dumenli 



549 



Validation of daily growth increments has been ob- 

 served in a similar study involving juvenile (0-60 

 days) Seriola quinqueradiata (Sakakura and Tsuka- 

 moto, 1997). 



Age of S. dumerili was similar between years; es- 

 timated ages ranged from 41 to 150 days (35 to 210 

 mm SL) in 2000 and from 35 to 120 days (33 to 198 

 mm SL) in 2001 (Fig. 6). Interannual differences in 

 growth were observed: 2000 (1.65 mm/d). 2001 (2.00 

 mm/d) (ANCOVA, slope, P<0.001) (Fig. 7). A signifi- 

 cant cohort effect was also observed; the late-season 

 (April) cohort experienced the fastest growth (ANCO- 

 VA, slopes, P<0.001) (Fig. 8). Average cohort-specific 

 growth rates of S. dumerili spawned in February, 

 March, and April of 2000 were 0.85 mm/d, 1.15 mm/d, 

 and 2.76 mm/d, respectively. In addition, a signifi- 

 cant difference in growth was observed for S. du- 

 merili collected from inshore (1.55 mm/d) and offshore 

 (1.65 mm/d) zones of 2000 (ANCOVA, slope, P<0.001) 

 (Fig. 9). Again, the lack of individuals within the in- 

 shore zone in 2001 precluded a comparison between 

 zones for that year. 



Mortality 



Owing to the limited number of S. dumerili collected 

 in 2001, a single catch curve was developed for the 

 2000 year class, and the mortality coefficient (Z) was 

 0.0045 (0.45%/d) for individuals between 40 and 139 

 days (Fig. 10). Cumulative mortality was estimated 

 for the 100-day period (40-139 days), resulting in an 

 overall mortality of 36%. 



20 



15 



10 



5 







1 A 



□ 2000 

  2001 



MAY 



JUNE 



JULY 



MAY 



JUNE 



JULY 



□ Offshore 

  Inshore 



r^i ^ 



MAY 



JUNE 

 Months 



JULY 



Figure 3 



Relative abundance (number per purse seine) (±1 SE) 

 of S. dumerili collected in association with Sargassum 

 mats: (A) 2000 and 2001: (B) 2000 by zones; (C) 2001 

 bv zones. 



Discussion 



The size range of S. dumerili collected in association 

 with Sargassum ranged from approximately 30 to 210 

 mm (SL), and these sizes are similar to those reported in 

 other studies investigating fish assemblages associated 

 with pelagic Sargassum. Bortone et al. (1977) collected 

 several small S. dumerili (12-72 mm SL) in the eastern 

 Gulf, whereas individuals collected in the western Atlan- 

 tic by Dooley (1972) ranged from 13 to 108 mm (SL). 

 Cho et al. (2001) found juvenile S. dumerili (35-120 mm 

 TL) associated with drifting Sargassum in the western 

 Pacific. Additionally, Sakakura and Tsukamoto (1997) 

 collected over 200 juvenile Japanese amberjack (S. quin- 

 queradiata) (18-114 mm TL) associated with pelagic 

 Sargassum in the East China Sea. Results of the present 

 study and others indicate that pelagic Sargassum mats 

 in the NW Gulf of Mexico serve as nursery habitat for 

 S. dumerili. 



The limited size range of S. dumerili associated with 

 pelagic Sargassum indicates that a shift in habitat use 

 may occur at approximately 5-6 months of age. Indi- 

 viduals greater than 210 mm (SL) have not been found 

 in association with pelagic Sargassum, and larger S. 

 dumerili (ca. 300 mm TL) are relatively common in the 

 recreational headboat fishery in the Gulf of Mexico (Ma- 



nooch and Potts, 1997a). As a consequence, S. dumerili 

 may transition from a pelagic to a demersal existence 

 at the late juvenile stage (between 200 mm SL and 

 300 mm TL). Pipitone and Andaloro (1995) found a shift 

 in the diet of S. dumerili, from a diet predominately 

 consisting of crustaceans toward one of fish >200 mm 

 (SL), further supporting this hypothesis. 



Seriola dumerili abundance was greater in the off- 

 shore zone than the inshore zone throughout the sam- 

 pling period. These patterns of habitat use are consis- 

 tent with earlier information that indicates S. dumerili 

 is an offshore species (Hildebrand and Cable, 1930). 

 The proximity to spawning grounds may contribute to 

 the observed spatial patterns because S. dumerili are 

 known to spawn in offshore areas (Fahay, 1975). Physi- 

 ological preferences may also contribute to the domi- 

 nance of S. dumerili in the offshore zone. In our study, 

 salinity values were higher in the offshore zone but 

 more variable within the inshore zone, suggesting that 

 freshwater inflow influences conditions within the in- 

 shore zone. Chen et al. (1997) determined that optimum 

 salinity conditions for S. dumerili larvae were between 

 32%<r and 35%t, and larvae remained inactive below a 

 salinity of 30%<r. Zonal differences in temperature and 

 dissolved oxygen were also observed. Tzeng et al. (1997) 



