Results and discussion

Altogether 77 fish species from 47 families (Table 1) were recorded. Of them 17% were basically marine, 57% marine-estuarine and 24% from wide ranging habitats, sharing even fresh water. (Figure 2)  The pearl-spot (Etroplus suratensis) was the only true estuarine species. The Asian swamp-eel (Monopterus albus) that shift between fresh water and estuary was another exception. Zone I, Aghanashini, was the richest with all 77 species recorded in the course of the study. Some fishes, notably mangrove snappers, seabass, ponyfish, perchlet, mullets, eels etc. inhabited here throughout–monsoon to pre-monsoon. The marine-estuarine group, mainly anchovis, croakers, barracudas, snappers, sillagos, black pomfrets, soles etc kept away from zone I during monsoon.  Such euryhaline fishes entered the zone from the sea only from post-monsoon with rising salinity. The stenohaline marine fishes, notably mackerels, silver pomfrets and cods appeared in Aghanashini when salinity level peaked in the pre-monsoon months. The mid-estuarine zone II (Kodkani) had 67 species, most of them euryhaline. Stenohaline marine fish avoided this medium salinity zone. Only 39 species occurred in zone III (Divgi), the interphase with fresh water. They were mostly a subset of zone II and moved freely between the sea and estuary; some even entered fresh water. Exclusive fresh water were absent in the estuary even during the rainy season.

Table 1.Checklist of Ichthyofauna observed in Aghanashini estuary (categorization based on fish base.com accessed on 19/1/2014)

Figure 2: Habitat combinations of estuarine fishes of Aghanashini (Based on www. fishbase. org. website accessed on 19 January2014)
(M-Marine; M, E-Marine; Estuarine; E-Estuarine; E, F-Estuarine, Fresh; M, E, F- Marine, Estuarine, Fresh)

The families Carangidae and Engraulidae had five species each; Sciaenidae had four species Clupeidae, Cynoglossidae, Gobiidae, Leiognathidae, and Lutjanidae had three species each. Eleven families were represented by two species while 28 had only one each. A comparable study from Ponnani estuary in Kerala had 112 species from 53 families15. Carangidae with   eight, Leiognathidae with seven and Engraulidae with six species each followed. From Kodungallur-Azhikode estuary of Kerala was reported 63 fishes of 37 families. Mugilidae had five and Engraulidae, Carangidae Cyprinidae, Cichilidae, Gerridae had three each16. From Yedayanthittu Estuary of Tamil Nadu was reported 75 species of 37 families17.

Monthly variations in environmental parameters salinity, pH, DO, water and air temperature of three zones are depicted in figures 3, 4 and 5 respectively. Heavy rains (2500-5000 mm), in catchment areas, caused salinity drop to nil or very low; from marginal rise in September to zone-wise peaks were attained in March - May. Similar pattern happened in the Naaf estuary of Bangladesh, when July rainfall (1,159 mm) caused salinity decline to 0-8.6 ppt18.

Figure 3: Hydrological parameters observed at zone I (Aghanashini) during June 2011-May 2012

 Figure 4:  Hydrological parameters observed at zone II (Kodkani) during June 2011-May 2012

Figure 5:  Hydrological parameters observed at zone III (Divgi) during June 2011-May 2012

Aghanashini estuary had maximum DO (6.32mg/l) during monsoon to early post-monsoon. It was similar in Vellar of Tamil Nadu19. Pre-monsoon saw highest water temperature in Aghanashini (31.5˚C) and lowest (25.6˚C) was in monsoon. Water temperature was lower than air temperature, which was higher in October, April and May.

When 77 species and five environmental variables from Aghanashini estuary were selected for CCA analysis (figure 6) the first two components had Eigen value (λ1= 0.119, λ2 =0.051). First axis accounted for 55.32% of the cumulative percentage of variance in species abundance with environmental parameters and was positively correlated with salinity, water temperature and air temperature and negatively correlated with DO and pH. Axis 2 explained 24.1% variance and was positively correlated with salinity, pH and air temperature, and negatively with DO and water temperature. CCA showed most fishes exhibiting positive correlation with salinity. Few like Strongylura leiura, Hemirhampus far and parrot fish showed negative correlation with salinity as these occurred mainly during monsoon.

 Figure  6: CCA of Aghanashini estuary on fish species numbers and month-wise environmental parameters

JN (January), FB (February), MR (March), AP (April), MY (May), JU (June), JL (July), AU (August), SP (September), OC (October), NV (November), DC (December). Square = zone I with corresponding month; Diamond = zone II with corresponding month; Triangle = zone III with corresponding month; Numbers = fish species; Line = Environmental parameters

Bray Curtis analysis of monthly fish assemblage distinguished two (A & B) major groups (figure 7). Cluster A was grouped in to all the seasons of zone III (Divgi) and three months (October, November, and December) of zone II (Kodkani).  In this group fishes such as Sillago sihama, Otolithes ruber, Lutjanus johni, Lutjanus ruselli, Lutjanus argentimaculatus, Etroplus suratensis, Glossogobius giuris, Gerres filamentosus, Mugil cephalus, Terapon jarbua are shared between zone II and zone III. These are among well-known indicators of highly variable estuarine salinity conditions. Cluster B consists of two sub groups namely cluster B1 and B2; B1 is further sub-divided into B1-I and B1-II.  B1-I has clustered with post monsoon seasons of zone I (Aghanashini) and premonsoon season of zone II. The clustering of this nature reveals that many estuarine fishes have their own preferable salinity regimes necessitating constant movements of fishes within estuary in adjustment with dynamic salinity conditions depending on mixing of fresh and salt waters. The cluster B1-II shows that the fish assemblages are 92% similar and closely related to the pre-monsoon of (April and May) of Zone I. Predominantly marine fishes like Rastrelliger kanagurta, Scomberomorus commerson, and Pampus argenteus get into this high salinity (34 ppt) assemblage. However, 20 had reported mackerels (Rastrelliger kanagurta) from Netravati estuary of Mangalore during Janaury in lesser salinity (14.10 - 23.50 ppt). 

Figure 7:  Bray Curtis clustering of fish species in Aghanashini estuary

Estuarine life is more challenging than marine mainly because of fluctuating salinity. Fishes of specific salinity ranges keep shifting positions within their respective ranges. The osmoregulation mechanism in fishes and species-specific operating ranges in relation to salinity are of great interest to fish physiologists21,22. Through osmoregulation the fish maintains an internal balance of salt and water within the cells when there is difference in salinity between internal and external conditions. Various cellular level mechanisms exist safeguarding fishes from high salinity and fluctuations in salinity21-25.

The estuarine-fresh water fish Seabass (Lates calcarifer) requires greater depths (10-15 m), hardly available in the estuary, and higher salinity conditions (30-32 ppt) for gonadial maturity, making it migrate from the river into the sea for spawning during monsoon. The spawning happens in September and the larvae move into the estuary for further development26.

The Aghanashini study has notable similarity with  works in the Caete River estuary27 of Brazil. Both the basins have hot and humid weather, five to six rainy months and annual rainfall exceeding 2,500 mm. In both seasonal salinity fluctuations appeared to be the main factor that structured fish assemblage in the entire estuarine system. At least 85% of the 82 species captured by the artisanal fishers of the adjoining Brazilian coast required estuarine conditions to complete their life cycle. While the study reveals that 74 of the 77 species recorded from Aghanashini used sea as a common habitat, the estuary plays a significant role in their lives highlighting the importance of its conservation.