The length at which 50% of females of Decapterus russelli attain maturity was estimated at 153 mm. The presence of mature, ripe and spent gonads was observed from June to October and December to April indicating prolonged spawning season. The sex-ratio (male: female) varied from 1:0.43 in 140-149 mm to 1:1 in 110-119 mm length groups. Month-wise distribution of sex-ratio indicated significant predominance of male at 5% in April, May and November. The absolute fecundity ranged from 29,986 to 1,52,123 eggs. The relative condition factor value was highest in January which indicates the peak spawning month. Lengthwise condition factor of males and females were maximum in the length range of 100-229 mm. In the present study size of ova diameter ranged from 0.01 to 0.97 mm.
The relationship between dorsal mantle length (DML) and total body weight (TBW) for males, TBW = 0.0008414 * DML 2.57989 and for females, TBW = 0.0009723 * DML 2.55201 . Fish formed the major food item in both the sexes (87.1% in males and 86% in females) followed by prawns. Females were dominant in the catch with a sex-ratio of 1:1.24. Maximum numbers of males were mature (43.6%) while maximum numbers of females (37.2%) were spawning. The size at 50% maturity for females was estimated as 153 mm. The major peak spawning season for the species is during February to May.
Economic assessment of juvenile landings of 5 dominant cephalopods at New Ferry Wharf (NFW) landing centre, Mumbai was carried out during January to December, 2013. Dominant cephalopod fishery recorded include one species of squid, Uroteuthis (P) duvaucelii, three species of cuttlefishes, Sepia elliptica, Sepia pharaonis, Sepiella inermis and a species of octopus, Cistopus indicus together contributing 98% of total cephalopod landings. Estimated total annual economic gain calculated using bioeconomic model was Rs. 33.22 crores with estimated biomass gain of 4995 t per annum, if juveniles are allowed to grow up to length at first maturity (Lm)/Lmean. Among five species, C. indicus contribute maximum with 64.07 % of average juvenile catch followed by U. (P) duvaucelii (26 %), S. elliptica (23.63 %), S. inermis (23.27 %), and S. pharaonis (12.85 %). On these resources, while in S. inermis, both LM equals Lmean indicates breakeven point for the species. The study revealed that peak spawning season of these species coincides with peak juvenile landings which may result in reduction of overall size range thus will lead to loss of fishery in economic as well as ecosystem regime. The study indicates the improvement of harvest biomass by 2.95 times which would result in generating additional revenue to the fishers by a margin of 3.71 times; if juveniles are allowed to grow up to Lm/ Lmean whichever is greater.
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