Grandcourt et a\ Biology and assessment of Diogiamma pictum and Lethnnus nebulosus in the southern Arabian Gulf 



79 



losus). Values of the growth performance index 

 <P for growth in length were 2.81 for D. pictum 

 and 2.80 for L. nebulosus. 



The length-weight relationship provided a 

 good fit to length and weight data for D. pictum 

 (W=lxlO-5xL2 -'9) (r-'=0.994) and L. nebulosus 

 (W=3xlO-^xL-»») (r2 = 0.992). Ratios of total 

 length (Lj) to fork length (Lp) were 1.11:1.0 for 

 D. pictum and 1.07:1 for L. nebulosus. 



Reproduction 



The mean size at first sexual maturity (i,„a, ) for 

 D. pictum was 30.7 cm Lp for males (24.3-37.2 cm 

 95% CI) and 31.8 cm Lp for females (24.3-38.9 

 cm 95% CI). Those for L. nebulosus were 28.6 

 cm Lp (23.7-33.8 cm 95% CI) and 27.6 cm Lp 

 (19.6-35.6 cm 95% CI) for males and females, 

 respectively. 



There was a peak in the gonadosomatic index 

 for both D. pictum and L. nebulosus females in 

 April, the main spawning period lasted until the 

 end of May (Fig. 5), and the mean birth date 

 was estimated as 1 May. There was a significant 

 (P<0.05) female bias in the overall sex ratios 

 (male to female) of 1:2.8 for D. pictum and 1:2.6 

 for L. nebulosus. The results of chi-square good- 

 ness-of-fit tests conducted for the sex ratios in 

 both age and size categories revealed that the 

 female bias was consistent across all categories 

 for L. nebulosus although the bias was removed 

 in the oldest age and largest size classes of D. 

 pictum (Tables 2 and 3). 



The proportion of fish in aggregated size-fre- 

 quency samples that were below the mean size 

 at first sexual maturity for females (juvenile re- 

 tention rate) was 35.1% for D. pictum and 10.9% 

 for L. nebulosus. 



Mortality and selectivity 



Modal age groups in age-frequency distributions 

 derived from age-length keys and size-frequency data 

 were 3 years for D. pictum and 5 years for L. nebulosus 

 (Fig. 6). The proportion offish above the age at which 

 fish were fully recruited was 13.8% and 45.7% for D. 

 pictum and L. nebulosus, respectively. There were no 

 significant differences between the total mortality rate 

 estimates derived from age-based and length-converted 

 catch curves for D. pictum {t=0.81, P=0.43, 15 df) and 

 L. nebulosus (^=0.03, P=0.98, 11 df) (Fig. 7). Fishing 

 mortality rates were in excess of the natural mortality 

 rates, accounting for 79% and 64% of the total mortality 

 for D. pictum and L. nebulosus, respectively (Table 4). 



The selectivity range derived from plots of the prob- 

 ability of capture at size was 25.0 cm for D. pictum 

 (18.0-43.0 cm) and 34.0 cm for L. nebulosus (13.0-47.0 

 cm) (Fig. 8). Values of the sizes where the probability of 

 capture was 50% (Ljq), 75% (L-g), and 100% (Lj,,,,) are 

 given in Table 5. For both species, fish were recruited 



