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Length–weight relationships (LWRs) of twelve fish species: Planilizaplaniceps, Planilizaparsia, Etroplussuratensis, Pseudetroplus maculatus, Oreochromis mossambicus, Gerres setifer, Scatophagus argus, Escualosathoracata, Carangoidspraeustus, Caranxheberi, Deveximentuminsidator, Nuchequllablochiiwere examined from Cochin estuary. Fish were caught between March 2019 to February 2020 by various gears like ring seine (mesh size of 8–26 mm), smaller mesh sized drift gill net (26–90 mm), cast nets with a (15-20 mm). Fish were gathered weekly from Munambam Fisheries Harbour (Lat. 10°1029653N, Long. 76°1022583E), Cochin Fisheries Harbour (Lat. 09°5623273N, Long. 76°1527643E) and Chellanam (Lat. 09°4729503N, Long. 76°1625513E). There are significance in all LWRs with r2 values those are ranged from 0.92 to 0.99 and b values are ranged from 2.61 to 3.37 (p < .001). The study provides new maximum length of E. thoracata.

Data from Cochin Estuary for five fin fish species were collected from January to December 2019. This study provides the first results on fishery dynamics of the species in this estuary and is aimed at estimating important population dynamic parameters and assessing the stock of this economically important fish species by the help of length frequency data collected. FiSAT II software was used for different analyses. The asymptotic length (L∞) and growth constant (K) of C.chanos were 421.05 mm and 0.71 year− 1, whereas for D.insidiator the asymptotic length (L∞) and growth constant (K) were 110.25 mm and 0.81 year− 1. The age at zero length (t0) was calculated for all examined species, ranged from − 0.07 to -0.19. The longevity (tmax) and growth performance index (ϕ) of P.planiceps were reported as 7.31 years and 4.81 whereas for M.cephalus were 2.14 and 4.6 respectively. The estimated total mortality (Z), natural mortality (M) and fishing mortality (F) of C.praesteus were 4.04, 1.37 and 2.67 year − 1, respectively, contrarily, for P.planiceps those values were 0.89, 0.49 and 0.4. The relative yield per recruit (Y/R) analysis revealed that the current exploration level is higher than the optimum exploitation rate, indicating an overexploitation of all examined fin fish species. Accordingly, needed management measures should include restrictions on fishing gear mesh sizes, reduced fishing effort and declaration of closed seasons and sanctuaries to protect these species from extinction.

A crucial aspect of sustainable resource management is understanding the trophic interactions amongst fish in the estuarine ecosystem. The goal of this study was to look into the food preferences, feeding strategies, trophic partitioning, as well as dietary overlap among four omnivorous species that live in the Cochin Estuary: Mugil cephalus (n = 73), Planiliza parsia (n = 35), Planiliza planiceps (n = 65) and Chanos chanos (n = 55) through the analyses of gut-content. Index of relative importance demonstrated that prey items in Mugil cephalus and Planiliza planiceps guts were dominated by Bacillariophyceae whereas in Planiliza parsia and Chanos chanos guts were dominated by Myxophyceae. The highest diet value (7.5) was recorded in Chanos chanos, while the lowest was recorded in Mugil cephalus (5.69). The highest niche breadth value (0.77) was recorded in Planiliza planiceps while the lowest value was recorded in Planiliza parsia (0.52). Pianka's overlap, evaluated with the help of null models structured by Ecosim 7.0, showed remarkable niche overlap between Mugil cephalus and Planiliza planiceps (0–0.92, P < 0.001), between Planiliza planiceps and Planiliza parsia ( 0–0.77, P < 0.05) and between Mugil cephalus and Planiliza parsia ( 0–0.7, P < 0.05). The fish's trophic niche width, along with prey-specific abundance confirmed that they are generalist feeders. The present results differed from the general hypothesis related to the omnivorous species and concluded that these four omnivorous species live in the same niche zones of the Cochin Estuary with no interspecific conflict.

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