Biological Aspects and Catch Trends of Elasmobranchs in the Inshore Waters of Goa, West Coast of India  

Chandrashekher U. Rivonker , V. P. Padate , M. R. Hegde
Department of Marine Sciences, Goa University, Taleigao Plateau, Goa 403206, India
Author    Correspondence author
International Journal of Marine Science, 2014, Vol. 4, No. 46   doi: 10.5376/ijms.2014.04.0046
Received: 09 May, 2014    Accepted: 13 Jun., 2014    Published: 11 Aug., 2014
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Hegde et al., 2014, Biological Aspects and Catch Trends of Elasmobranchs in the Inshore Waters of Goa, West Coast of India, International Journal of Marine Science, Vol.4, No.46 1-12 (doi: 10.5376/ijms.2014.04.0046)

Abstract

Despite being the top predators Elasmobranchs are dwindling due to excessive fishing pressure. However, very few studies along Indian coasts have focused on their eco-biological aspects. The present investigation comprising 158 trawl samples (220 h effort) along the nearshore fishing grounds of Goa revealed that the elasmobranch population comprised 10 species (2 sharks, 6 rays and 2 skates). Analysis of spatial variation revealed significant variations between the regions (abundance, α = 0.001, P= 0.000191; weight, α = 0.001, P = 2.14E-08) suggesting high catches along southern region, owing to lesser fresh water discharge due to absence of major estuarine system. Assessment of size class indicated that juveniles dominated the elasmobranch population with few stray occurrences of adults. Dietary analysis of the three commonly observed species revealed the dominance of teleosts (45.95% IRI), followed by crustaceans (40.19% IRI). Analysis of the catch trends (1969–2004) of elasmobranchs in this region indicated meagre contribution (0.05–5.04%) to the total marine fish landings of Goa. Further, the catch trends displayed decrease in recent times suggesting reduction in trophic level of the regional fishery perhaps caused by fishing out of carnivores coupled with increased catches of low trophic level fishes as evidenced in the present study. These findings have implications for the trophic web dynamics of the coastal waters, which in turn affect the coastal fisheries of the region.

Keywords
Shark fisheries; Spatial variation; Temporal variations; Diets; Goa; India

Goa situated on the central west coast of India with 105 km long coastline and about 10,000 km2 shelf areas (Kurup et al., 1987) supports a wide array of demersal and pelagic ichthyofaunal diversity including elasmobranchs. Traditionally, the elasmobranch fisheries of Goan coast consisted of catches taken with beach seines (inshore), gill nets and hook-and-line (offshore). Mechanization of fishing vessels (1963) led to exploitation of bulk of the elasmobranchs as by-catch of bottom trawlers operating in the nearshore and offshore waters off Goa. Published literature (Raje et al., 2007) suggests that 38 species including 26 species of sharks, nine species of rays and three species of skates have contributed to the elasmobranch fishery of Goa during 1969–2004 (CMFRI, 1979; Kurup et al., 1987; Srinath et al., 2006) with an average annual landing of 461.78 mtper annum. Amongst the commercially exploited elasmobranchs, Scoliodon laticaudus (Müller and Henle, 1838)and Sphyrna zygaena (Linnaeus, 1758)attract lucrative markets for dried products (Hanfee, 1997), however only large sized batoids are locally consumed. Large-scale discarding of elasmobranch juveniles and small sized individuals by trawlers might be the reason for the inaccurate estimation of species abundance and diversity.
Elasmobranchs being top predators play a major role in regulating the population size and dynamics of lower trophic level (LTL) fishes (Wetherbee and Cortés, 2004; Séret et al., 2010). Targeted fishing for elasmobranchs due to high demand for their meat, fins, liver and other products has resulted in increased global landings to the tune of 760,000 mt per annum (Stevens et al., 2000). Further, intrinsic biological traits such as slow growth rate, low fecundity (Holden, 1974; Jennings et al., 1998; Ebert et al., 2008), high fishing mortality coupled with juvenile discard push some species to depletion, while endangering others (Stevens et al., 2000). Although elasmobranchs have been traditionally exploited, the present rate of elasmobranchs’ exploitation seems to be highly unsustainable as there are serious concerns owing to drastically declining populations (Séret et al., 2010).
In response to global concerns over dwindling stocks of elasmobranchs as a result of overexploitation, studies pertaining to the biological traits of elasmobranchs (Cortés, 2000; Ebert et al., 2008; Abdurahiman et al., 2010), their population dynamics (Walker and Heessen, 1996; Walker and Hislop, 1998), status of exploitation (Compagno, 1990; Bonfil, 1994) and its effects on their stocks (Stevens et al., 2000; Stobutzki et al., 2002) have received attention of fisheries researchers recently (Stevens et al., 2000).
Rapid changes in demersal fishing effort as well as catch trends in the recent years along with alterations in water quality in the coastal waters and increased removal of elasmobranchs and their prey species have exacerbated the deleterious effects on the elasmobranchs populations. Hence, it was pertinent to evaluate the status of elasmobranch populations through continuous monitoring. Moreover, the elasmobranchs catch and their prey items would enable to assess the changes in their populations and its dependence on prey items. Against this background, an attempt have been made to provide baseline information on species composition, spatio-temporal variations in occurrence, size class and diet of dominant elasmobranch species collected from the fishing grounds of Goa in this communication. Further, an attempt has also been made to provide a better insight into the status of exploitation and utilization of these resources based on elasmobranch landings of Goa during 1969–2004.
1 Results
1.1 Environmental variables
Sea Surface Temperature (SST) range during the entire study period was 26.02 – 31.03 with a mean value of 28.86 ± 1.14.
1.2 Biological aspects
1.2.1 Species composition
A total of 10 elasmobranch species (Table 1) were observed in the inshore trawl catches. Among these, only three namely C. griseum, H. walga and S. laticaudus were found to occur in 16, 15 and 10 % of the trawl hauls, respectively (Table 1) indicating a sizeable contribution of these species to the total elasmobranch catch. However, the contribution of the other seven species was negligible (<0.05 %; Table 1) highlighting the rarity of their occurrence. Further, only seven species namely S. laticaudus, C. griseum, H. walga, Himatura gerrardi (Gmelin, 1789), Aetobatus flagellum (Bloch and Schneider, 1801),Glaucostegus granulatus (Cuvier, 1829) andRhinobatus obtusus Müller and Henle, 1841 were found to occur off both North and South Goa; two others namely Neotrygon kuhlii (Müller and Henle, 1841),Pastinachus sephen (Forskål, 1775)were found off North and Himantura uarnak (Gmelin, 1789)was found only off South Goa.


Table 1 Species composition, occurrence and size range of elasmobranch species examined during the present study


Quantitative analysis of trawl catch data collected during the present study revealed a meagre contribution from elasmobranchs (0.42 and 0.97 % by abundance and weight, respectively). Subsequently, the above data was assorted to represent ‘pre-monsoon’ and ‘post-monsoon’ seasons. The temporal trends revealed no marked differences between the seasons(Figure 1 a, b). Further, analysis of elasmobranch abundance and weight data between the two sites (North Goa and South Goa) revealed significant variations in both abundance (α = 0.001, P = 0.000191) and weight (α = 0.001, P = 2.14E-08). Similarly, annual landings of Goa (2006–2010; Directorate of Fisheries, Government of Goa; Figure 2) indicated greater contribution from South Goa (α = 0.01, P = 0.00295).


Figure 1 Seasonal variations in elasmobranch abundance (a) and weight (b)



Figure 2 Elasmobranch landings along North Goa and South Goa) during 2006-2010


1.2.2
Size class and life stages
The observations on the size and their comparison with Lm values (Froese and Pauly, 2011) indicated that 72% specimens were juveniles, whereas only 28% were adults (Table 1). Species wise data indicated that S. laticaudus, H. walga, A. flagellum and R. obtusus were dominated by juveniles, C. griseum was represented equally by juveniles and adults, and the other five species were represented exclusiv
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