My thesis examined the implications of life history and ecology for population-level responses of the European long-snouted seahorse ( Hippocampus guttulatus Cuvier 1829) to habitat alteration, exploitation and a recommended minimum size limit for seahorse management. The research employed a mark-recapture study, underwater visual censuses (UVCs) and catch data from an unrelated experimental sampling program in the Ria Formosa Lagoon (southern Portugal). These small-bodied fish are characterized by rapid growth rate, early age at maturity, high natural mortality, short generation time, short life span and multiple spawnings per year, traits that are usually associated with resilience to exploitation. However specialized parental care, complex social interactions, small adult home ranges and benthic habit confer risk to H. guttulatus. Population-level responses to experimental reductions in non-selective fishing effort differed in magnitude and direction between H. guttulatus and its smaller congener, Hippocampus hippocampus: the abundance of H. guttulatus increased significantly while its congener decreased in abundance. Hippocampus guttulatus preferred more complex, vegetated habitats, while H. hippocampus preferred more open, sparsely vegetated habitats. Thus it seems probable that H. guttulatus fared better in the more complex habitats that developed when seining stopped, whereas H. hippocampus fared better in the less complex habitat arising from repeated seining. I used an age-structured stochastic simulation to evaluate the biological implications of a recommended generic strategy (10 cm minimum size limit) for managing the exploitation and international trade of seahorses, using H. guttulatus as a model. The smallest size limit that was robust (using the magnitude of population decline and probability of quasi-extinction as criteria) to a range of fishing rates, maximum population growth rates (rmax) and models of density-dependence

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