Hydrography of the study region
The coastal waters of Goa, Central West Coast of India, hydrography experiences seasonal variability. The region remains well-oxygenated during the period between November and May (pre-and post-monsoon). While during June to September (southwest monsoon) freshwater flow leads to the stratified water column in upper layers and upwelling-driven nutrient-rich and relatively low oxygen water prevails below (~ 10mt below the surface) impacting the plankton community of the region (Gauns et al, 2020) and in turn fishery. However, to facilitate the breeding of fish during the latter months the annual monsoon trawling is banned is enforced in Goa since 1981 generally from June 1 which lasts for 61 days, ending on July 31. Further, fish is considered a staple diet for the majority of the Goan population (> 90%); Goa shares nearly 2% of the total marine fish production of our country.
The surface water temperature during the Post SWM was prominently warm (30oC). However, during winter season a gradual drop in SST was seen in the month of December, January and February(28-29oC) (Fig. 5a). The phytoplankton biomass (chlorophyll a; mgm− 3) also showed a monthly variation in the Surface water. However during the Post SWM high level of chl a was > 3mg m− 3 and in the month of January, February and March level of chl a was relatively low (≤ 3 mgm− 3) indicating the prevalence of oligotrophic condition (Fig. 5b).The sea water salinity also showed a distinct temporal Scale variation, where relatively high salinity (> 34) was observed in the month of December and January where as an intermittent salinity drop was observed in the month of February (32) (Fig. 5c) .
The distribution and habitat of the fish species:
Rastrelliger kanagurta (Cuvier, 1817) (Plate II: A)
Rastrelliger kanagurta (The Indian mackerel) is a commercially important pelagic resource found on India's east and west coasts, as well as in the Indo-West Pacific region (Jayasankar et al., 2004). It can be found all over the Indo-West Pacific, from South Africa and the Red Sea to Samoa, and from northern to southern Japan. (Luther, 1995). The habitat of this marine fish is shallow coastal waters with a depth range of 20–90 m (Pauly et al., 1996). Over the last four decades the Indian mackerel, R. kanagurta, was under investigation. Pradhan (1956) and Yohannah (1974) made the first attempt at studying its age and development. During 1984 to 1988, its contribution in total marine fish production ranged from 2 to 20% (Noble et al., 1992). Rastrelligar kanagurta was one of the most significant neritic pelagic species to be studied by many fishery biologists. Although this species supports a rich commercial fishery on the west coast of India, its distribution on the east coast is only of biological interest (Roa et al.,1964). Research examining food and feeding habits can shed more light on pelagic fish species' migratory and shoaling patterns and is especially important for high commercial value species like mackerel (Noble et al.,1964). Since 2016-17, landings (in tonnes/yr) of this species in Goa vary between 22531 and 37514 (avg. 30137) contributing most (~ 28.1%) of the total pelagic catch (Source: Department of Fisheries, Government of Goa,2022). (Fig:2)
Tenualosa ilisha (Valenciennes, 1847) (Plate II: B)
Tenualosa ilisha (Indian shad) belongs to the family Clupeidae, subfamily Alosinae. popularly known as Hilsa and freshwater rivers, estuaries, and marine environments are all inhabited to this species. Hilsa is a commercial Indo-Pacific fish that is distributed primarily in Bangladesh, India, and Myanmar. In the Bay of Bengal, Persian Gulf, Red Sea, Arabian Sea, Vietnam Sea, and China Sea, it is among the most important migratory fish. (Rahman, 2018; Sachinandan et al., 2013). Food and feeding strategies of these fishes varies in an assortment of environments as these fish expend their various stages of life in different habitats depending on the food composition, time of day, season, fish size, and ecological conditions such as the dry and rainy season, high and low salinity and temperatures (Hynes, 1950). Landing (in tonnes/yr) of this species in Goa is relatively less compared to other studied species that varied between 461 and 738 (avg. 580) contributing only ~ 8% of the total pelagic catch (Fig:2). However, this is yet another famous seafood delicacy from the Konkan kitchen.
Sardinella longiceps (Valenciennes, 1847) (Plate II:C)
Sardinella longiceps (The Indian Oil Sardine) is the country's main resource for neritic pelagic fishery. This has a wide distribution along the coast of Somalia, the predominant resource along the southwestern coast of Kerala, Karnataka, Goa, and southern Maharashtra, along the Indian Peninsula, South Africa, Vietnam, and Philippines. The fishing of oil sardines is known for its highly unpredictable and fluctuating nature and exhibits annual variations (Annigiri et al.,1992). Malabar Coast of Oil sardine fishing is notorious for being highly erratic and changeable, with annual fluctuations. Because of the nutritional benefits and industrial uses for sardine oil, fertilisers, and canning, the Malabar Coast of India oil sardine fishery is an economically important fishery. It makes a considerable contribution to India's marine fish landings and is a key component of Indian pelagic fishing along the West Coast. The catch of this fishery varies dramatically from year to year (Deshmukh et al., 2015). However, its landing (in tonnes/yr) in Goa for the last five years (2016–2021) is consistently high (5218 and 42818; avg.21975) and comparable to the landings of R. kanagurta, making up to ~ 12.4% of the total pelagic catch. (Fig:2)
Mugil cephalus (Linnaeus, 1758) (Plate II:D)
Mugil cephalus (Grey mullet), is a member of the Mugilidae family which occurs in the fresh, brackish, hypo-saline lagoons and shallow coastal waters of less than 20 meters (Ficher and Bianchi, 1984). It’s a state fish of Goa It forms one of the main tropical and subtropical fisheries. (Lawson et al.,2010). The name Grey Mullet is used for many species within the Mugilidae family and widely distributed community. One particular genus, M. Cephalus, from 42 N to 42 S, is common in all oceans and most commonly All grey mullet species are known. Thompson (1966) examined the group's biology and mainly dealt with the adults, on the whole, due to the lack of adequate information on the juveniles at the time. Most grey mullets, with a few exceptions including Rhinomugil corsula, is catadromous fish that migrate into the sea for spawning. However, very few studies concerning their breeding habits, ecology, and the size of the sea's spawning habitats are carried out (Thompson, 1966). Young ones continue to migrate into inshore coastal waters after hatching, metamorphosis, and a short period of development, mainly into lagoons and estuaries and they are also known to migrate along rivers, which is not typical of the grey mullet species. (De silva, 1979). Of the four studied species, in Goa landing of M. cephalus was much lower (28 and 446 tonnes/yr; avg. 310.4) accounting ~ 11.2% of the total pelagic catch. (Fig:2)
The present study shows the prey species richness and diversification in the guts of fish. Among all four fish, species Rastrilliger kanagurta were found with more diversified gut content followed by Tenualosa ilisha, Sardinella longiceps and Mugil cephalus. Our observation revealed that Rastrelligar kanagurta mainly feeds on bivalve larvae (30%) and copepod (intact:28% and semi-digested:18%) apart from diatom (10%), pyrocystis (4%) and others planktonic forms (8%) (Fig:3a). Shannon index value of species richness 0.767 indicates more diversified feeding behavior of Rastrelligar kanagurta. Tenualosa ilisha also showed bivalve larvae (29%) followed by copepods (intact copepod: 24% and semi-digested copepod:19%) as major food particles followed by diatoms (12%) and other planktonic groups (16%) in their gut (Fig:3b). Shannon index value of species richness for this species (0.76) also indicates diversified gut content. Likewise, Sardinella longiceps gut was contributed by bivalve larvae (28%) followed by copepod (intact:25% and semi-digested:18%), diatoms (18%) and other planktonic forms (11%) (Fig:3c). Shannon species diversity index value was comparable (0.746) to other species described above. Much to our surprise, the gut content of this fish in the month of February and March was found to be relatively empty. In contrast, Mugil cephalus, feeds mainly on diatom (40%) and copepod (Intact: 31% and semi-digested: 23%), while crustacean larvae (4%) and dinoflagellates (2%) were less abundant (Fig:3d). This is also reflected in the Shannon species diversity index value of 0.408 indicative of least diversified feeding behavior compared to all other species.
Overall, the analyses of the four fish species viz. Rastrelligar kanagurta, Tenualosa ilisha, Sardinella longiceps and Mugil cephalus were found with more diversified gut content. Among all four fish species, bivalve larvae and copepods have dominated and are more preferred group within the gut of these fishes, compared to all other groups. Our observations showed the high abundance of bivalve larvae within the stomach of fishes during post-monsoon, which supports the previous report by Hadfield et al., (1988) elaborating this may be due to bivalve spawning during post-monsoon and is associated with extreme availability of phytoplankton in the study area. Furthermore, it may also be associated with increased water temperature during summer (Hadfield et al., 1988). However, according to Nath (2015), Rastrelligar kanagurta mainly feeds on plankton i.e., phytoplankton and zooplankton; our data also depicts a similar observation with bivalve and copepods as the preferred food items in the region. Elsewhere, De and Datta (1990) and De et al., (2013) studied and observed that stomach content analysis of fry, juvenile and adult Hilsa showed Copepoda were the most important food items consumed by the fish of all sizes. The earlier researchers have recorded diatoms, Copepoda and dinoflagellates mainly forming the food of the oil sardine (Ahirwal et al., 2018), which is slightly different from our observations recording Sardinella longiceps consumes high number of bivalve larvae apart from copepods compared to diatoms and other planktonic groups. While our work on gut analysis of Mugil cephalus revealed that it mainly feeds on diatoms followed by copepods, other crustacean larvae and some dinoflagellates, which is also recorded by the other researchers (Bruthet, 1975; Rangaswamy,1973). Previously, the highest abundance of diatoms in M. cephalus was determined to be around 48 percent, whereas the proportion of other food particles was not significant, and their changes remained rather constant throughout the year. (Islam et al., 2009).