STOCK ASSESSMENT OF SELECTED SCIAENID SPECIES OF THOOTHUKUDI COAST
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Date
2007
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Fisheries College and Research Institute, Thoothukudi, Tamil Nadu Dr.J.Jayalalithaa Fisheries University
Abstract
A detailed investigation was undertaken to study the stock assessment of selected sciaenid species of Thoothukudi coast. Specimens for the present investigation were collected six times in a month from the commercial trawl catches of Thoothukudi coast. The study was carried out for one year from July 2006 to June 2007.
Availability of sciaenid species in trawl net catch along Thoothukudi coast was recorded. Occurrence and abundance of Nibea maculata and Otolithes ruber were recorded. Data on catch composition and CPUE of N. maculata and O. ruber from trawl nets were collected. Average monthly catch effort and CPUE of N. maculata and O. ruber were recorded. Length frequency distributions of these species were studied. Length-weight relationship, growth parameters, mortality parameters were estimated for N. maculata and O. ruber using FAO-ICLARM Stock Assessment Tools (FiSAT) software. Recruitment pulse of N. maculata and O. ruber were analysed with time series length frequency data. Total stock, annual stock, MSY and exploitation ratio of N. maculata and O. ruber were estimated and yield isopleth was plotted for both the species.
Nibea maculata and O. ruber showed year round occurrence at Thoothukudi coast. The annual catch of N. maculata and O. ruber were 830.7877 tonnes and 93.7921 tonnes respectively. The major catch of N. maculata and O. ruber were recorded from June to August and June to July respectively. The catch per unit effort was found to be high in the month of June, July and August for N. maculata and June and July for O. ruber. The length-weight relationship of N. maculata showed isometric growth and the weight gain of O. ruber in relation to the total length was found to be lower in Thoothukudi coast. The estimated life span of N. maculata was 4.3 years and it was 11 years for O. ruber. Significant seasonal oscillation in growth was found in N. maculata and hence seasonalised von Bertalanffy’s growth equation was employed to fit the growth curve. There was no growth oscillation in O. ruber and hence generalised von Bertalanffy’s growth equation was employed to fit the growth curve. An inverse relationship was observed between growth co-efficient and asymptotic length for N. maculata and O. ruber. The ‘K’ value of N. maculata was high, inferring fast growth rate and it was low in O. ruber, inferring slow growth rate. The estimated total instantaneous mortality (Z) of N. maculata was 2.730 with low fishing mortality (1.0649) and which infers less exploitation in this species. The ‘Z’ value of O. ruber was found to be 2.450 with high fishing mortality (1.7373) and which infers slightly over exploitation in this species. Recruitment of N. maculata was recorded round the year with a peak in December and the same in O. ruber was also observed throughout the year with two peaks one during January and February and other in September. The Virtual Population Analysis revealed that fishing pressure
was more in length group of 15 cm to 20 cm for N. maculata and 18.5 cm to 25.5 cm for O. ruber. Based on the relationship between exploitation ratio and yield per recruit of N. maculata, it could be inferred that this species is less exploited and the present level of effort should be increased by 27.82 % to achieve the optimal MSY level. In the case of O. ruber, present exploitation was more than the MSY level and the present effort could be reduced to 29.27 % to sustain the stock.
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