By-catch-associated demographics of two threatened seahorses from the south-east coast of India

Abstract

Seahorses comprise a charismatic, high-value and high conservation-concern group of fishes, whose demographics, including the dynamics of populations, are poorly studied. In this paper, we fill this key knowledge gap by determining the population dynamics of two threatened species, Hippocampus kuda and H. trimaculatus, from the south-east coast of India using samples encountered in trawl by-catch. Hippocampus kuda showed a comparatively greater asymptotic length, whereas H. trimaculatus had a greater growth coefficient and growth performance index. Further, H. trimaculatus showed two recruitment peaks (March/April and December) in a year, against a single peak (February) for H. kuda. An assessment of mortality rates (instantaneous total, natural and fishing) indicated a high vulnerability of the local population of both species to fishing pressure. Hippocampus kuda had a greater mean length at first capture than H. trimaculatus. Biomass-per-recruit analysis showed that the exploitation of H. kuda is closer to the exploitation rates under which stock will be reduced to half the unexploited biomass, suggesting their extreme vulnerability to overexploitation, even as ‘by-catch’. Demographic parameters of the two threatened seahorses indicated that even as ‘incidental catch’, these species are vulnerable to overfishing, and species-specific conservation guidelines need to be developed and their on-ground implementation and enforcement ensured.