The ontogenetic ecology and conservation of exploited tropical seahorses

7 December 2007Morgan, S.K.

This research investigated the life history and demographics of exploited, tropical seahorses. My thesis was designed to use the unusual morphology of seahorses (Hippocampus comes and H. spinosissimus in the central Philippines), to test the life history structure and strategies of tropical reef fishes. I then made direct use of ecological findings in order to evaluate risk associated with the capture of tropical seahorses for active and growing international markets.

Hippocampus comes has a broadly bipartite life history comprising three biologically distinct phases; planktonic newborns, settled juveniles and adults. Most vital rates that govern the population turnover of H. comes confer low susceptibility to risk. Simultaneously, aspects of abundance (range and distribution), developed parental care, behaviour and susceptibility to fishing, increase the species’ inherent vulnerability. Stage-structured matrix models examined the outcomes of temporal closures and size-based harvesting for managing artisanal seahorse fisheries. Simulations showed that when fishing occurred throughout the year at relevant intensities, slot sizes provided better protection for populations than minimum size limits, as well as greater cumulative catches over 10 year time horizons.

The ontogenetic ecology of tropical seahorses is comparable to most warm-water reef fishes, sharing: 1) a broadly bipartite life history, 2) dispersal likely to confer demographic connectivity at the scale of 10s-100s of km, 3) young that advect passively early in the pelagic phase, and 4) ontogenetic habitat associations that segregate benthic individuals into multiple ecologically distinct stages.

Unusually, seahorses have: 1) a pelagic phase inferred to last between 5-10 days, among the shortest noted in reef fishes, 2) the apparent ability to orient to solar cues and 3) juveniles that associate with macroalgal beds, seldom reported as nursery grounds in tropical reef ecosystems. For research and conservation, long-term trends in annual population abundance should be used to estimate population growth rates (λ) and to assess whether cyclic dynamics exist in seahorse populations. Approaches in this thesis are relevant to the management of data-depauperate small-scale fisheries.