High density, early maturing, and morphometrically unique Hippocampus erectus population makes a Bahamian pond a priority site for conservation

Abstract

Anchialine pond habitats are frequently associated with biota that can differ dramatically from nearby coastal systems. In this study, we investigated lined seahorses Hippocampus erectus by season, size, and sex across a tidal lake on the island of Eleuthera (The Bahamas). In total, 35 benthic transects of 30-60 m2 were established around the lake margin and assessed 4 times between 2014 and 2016. Seahorses were mapped along each transect and photographed for morphological analyses. Mean (±SE) landscape-level seahorse density was 0.14 ± 0.013 ind. m-2 (max. 0.66 ind. m-2), which was substantially higher than worldwide seahorse mean density. Local seahorse density differed both seasonally and spatially, 40% higher in the wet than dry season, and 48.8% lower in the south than in the north end of the lake. Sex-ratio was significantly male-biased in both the north end of the lake and during the wet months. Male mating effort varied seasonally and spatially, with a significantly higher frequency of gravid males in the dry season and the north end of the lake. Male seahorses were significantly larger than females, with additional morphological traits that indicate broader sexual dimorphism. Compared to other seahorse species, Sweetings Pond animals have a higher population density, smaller size at maturity, and a different shape than other H. erectus, indicating a population that is a discrete management unit. Given the CITES Appendix II listing of all seahorses and their decreasing numbers worldwide, this population represents a valuable biological resource that should be managed uniquely.