Declining seahorse populations linked to loss of essential marine habitats

Abstract

Habitat loss is a key driver in the decline of terrestrial and marine species worldwide. In the marine environment, habitat loss is considered to be a major threat to seahorses Hippocampus spp. This study assessed changes in the population abundance of White's seahorse Hippocampus whitei from mark-resight surveys undertaken between 2006 and 2015 in Port Stephens (New South Wales, Australia). The mark-resight closed population estimates found that the population at the Seahorse Gardens site had declined by 97% between 2009 and 2015, and similarly, the population at the Pipeline site was found to decrease by 83% over the same time period. Comparison of habitat composition data found a significant decline in available marine habitats between 2009 and 2015 at both sites. SIMPER analysis indicated that 5 habitat types (soft coral Dendronephthya australis, sand, seagrass Halophila ovalis, sponges and algae) contributed 76% of the dissimilarity between the sampling periods. The preferred habitats of H. whitei had significantly declined at both sites, with D. australis declining at the Seahorse Gardens site by ∼96% from 2009 to 2015 and by ∼73% at the Pipeline site, whilst sponge habitat was also found to decline by ∼49% at the Seahorse Gardens site and ∼25% at the Pipeline site. The significant decline of H. whitei abundance is concerning, as there is no evidence to suggest that the populations are recovering, and given the large decline of habitat availability at both sites, it would be difficult for seahorses to recolonise these areas without some form of habitat remediation.