Wyanski et al : Growth, population age structure, and aspects of the reproductive biology of Epinephelus niveatus 



215 



We feel confident that our assessment of the age 

 structure in the snowy grouper population off the 

 Carolinas was accurate, even though interpretation 

 of growth increments was difficult and a minimal 

 number of YOY specimens was available. The dif- 

 ficulty of assigning an age to the sagittae of snowy 

 grouper had not been reported by other investiga- 

 tors, although it has been reported for other deepwa- 

 ter species of continental slopes. The clarity of the 

 hyaline and opaque zones in otoliths (presumably 

 sagittae) from hoki, Macruroniis novaezelandidae, 

 off New Zealand is highly variable and is divided 

 into six categories based on internal features which 

 are related to the ease of counting increments (Kuo 

 and Tanaka, 1984). When otoliths are difficult to 

 interpret, one option is to base population age struc- 

 ture only on the specimens for which age is easily 

 assessed. Alternatively, ages can be estimated for 

 nearly all specimens despite the difficulties, as we 

 did in our study, with the assumption that the larger 

 sample will represent the population better. We found 

 that limiting the data set to only those specimens 

 for which the difference in counts between readers 

 was 0-1 increments did not improve the accuracy of 

 the age-length key for specimens caught with long- 

 lines (Table 6). Thus, we advocate using the entire 

 sample of specimens assigned an age. Crabtree and 

 Bullock ( 1998) found that rejected otoliths tend to be 

 from slower-growing older specimens, which could 

 introduce bias into analyses. This bias was minimal 

 in their study of growth in black grouper, M. bonaci, 

 where each otolith was examined three times by two 

 independent readers. Parameter estimates for the von 

 Bertalanffy model based on all black grouper with 

 ages were within one standard error of those based 

 only on specimens for which the coefficient of varia- 

 tion of the six readings was <12%. 



An important consideration in age determination 

 is positive identification of the first annulus and 

 any settling mark that may be deposited prior it. 

 We believe that we have identified the first annulus 

 because the largest YOY specimen (172 mm) had 

 an estimated age of 191-291 days and the measure- 

 ments of otolith radius for all YOY in -3) were less 

 than radial measurements to the first annulus in 

 a subsample of 23 specimens that were age 1. Evi- 

 dence to support our conclusion that these three 

 specimens were YOY was found in another sam- 

 pling effort, where six specimens 4-5 cm in length 

 were caught with a trawl during August and Sep- 

 tember (Machowski'^), the last two months of the 



^ Machowski, D. 1998. S. Carolina Department of Natural 

 Resources, P.O. Box 125.59. Charleston, SC, 29422. Personal 

 commun. 



spawning season. Moore and Labisky (1984) consid- 

 ered 150-175 mm specimens to be YOY, although 

 they did not examine daily increments. 



Validation of a technique for aging snowy grouper 

 with otoliths has been weakly supported by pre- 

 vious studies. We found that marginal increments 

 form annually and there is a peak in April and May 

 that corresponds to the beginning of the spawning 

 season. This finding concurs with the limited results 

 of Matheson and Huntsman (1984) and Moore and 

 Labisky (1984) who found that increment formation 

 appeared to begin in April and peaked in June. Mathe- 

 son and Huntsman (1984) measured marginal incre- 

 ments in 18 specimens collected during April through 

 October and Moore and Labisky (1984) examined 

 specimens collected during March through July in not 

 reported). Waltz"* found a wider period of increment 

 formation, April through September, although he was 

 not able to conclude that increments form annually 

 because samples were lacking for four months. 



Reproduction 



The reproductive pattern of snowy grouper needs 

 to be investigated more comprehensively and the 

 sex ratio should be assessed again, given the small 

 sample size in 1993-94 («=82), because there is evi- 

 dence that reef fish species, particularly grouper, 

 which change sex and aggregate to spawn, are more 

 susceptible to size-selective mortality and overex- 

 ploitation (Bannerot, 1987; Huntsman and Schaaf, 

 1994; Coleman et al., 1996). The capture of only one 

 male in the 1993-94 samples, which appeared to be 

 representative of the population based on commer- 

 cial landings, is reason for concern because the per- 

 centage of males has apparently decreased from the 

 7-23*^ for samples collected with three gear types in 

 the 1970s and 1980s (Table 7). 



Large decreases in the number of males have been 

 documented for two other grouper species in the 

 southeast region. Percentages of males in popula- 

 tions of gag and scamp in the Gulf of Mexico, grou- 

 pers that are known to form small spawning (lO's 

 to lOO's of individuals) aggregations, decreased from 

 17% to 19c and 38% to 18%, respectively, between 

 the 1970s and early 1990s (Coleman et al., 1996). A 

 similar decrease, from 20% to 6%, was noted for gag 

 along the Atlantic coast of the southeastern United 

 States during the same period (McGovern et al., 

 1998). The resultant decrease in genetic diversity 

 has been documented for gag (Chapman etal., 1999), 

 and its ramifications are currently of great concern 

 to many fishery scientists in the southeast region. 



The size (767-1090 mm) and age (8-29 yr) of 97 

 male specimens in the present study and the capture 



