A scientific study on mass mortality of Indian mackerel (Rastrelliger kanagurta (Cuvier, 1816)) along the lower stretches of river Godavari

Abstract

A mass mortality event of Indian mackerel (Rastrelliger kanagurta (Cuvier, 1816)) was observed on 5th July 2024 along the lower stretch of the Godavari River estuary, where the species is known to enter periodically. Water samples collected from Masakapalli, Brahmapuri, and Yanam showed markedly low salinity levels of 1–6 psu, pH values of 8.07–8.23 and ammonia concentration ranging 0.01–0.13 mg/L. The substantially reduced salinity likely imposed osmotic stress on this predominantly marine species. While the relatively high pH, in combination with the measured ammonia concentrations may have increased the toxicity of un-ionised ammonia (NH₃), compounding physiological stress in the fish and potentially contributing to the observed mortality. Oceanic parameters in nearby coastal waters, including sea surface temperature (28.7 °C), salinity (33.2 psu), chlorophyll-a concentration (1.8 mg/m³), and sea level anomaly (3.3 cm), remained within normal ranges, indicating no significant anomalies in adjacent coastal waters. These observations suggest that the mortality event was most likely linked with the conditions of pronounced freshwater dilution, associated osmotic stress, and enhanced ammonia toxicity prevailed along the lower stretch of the Godavari River estuary.