Trends in seahorse abundance in the Ria Formosa, South Portugal: recent scenario and future prospects

Publication type: Thesis (Doctor of Philosophy)
Publisher: Universidade do Algarve
Date:  2014
Author:  Miguel Correia

Abstract
In the last decade, seahorse population census in the Ria Formosa identified stable populations of Hippocampus guttulatus and H. hippocampus at the highest densities described for these species worldwide. However, recent studies in the Ria reported a significant decrease in seahorse numbers of 94% for H. guttulatus and 73% for H. hippocampus.
Therefore, this study aimed to identify aspects responsible for the variation in seahorse abundance, first by looking at the current trend of seahorse population by comparing it with previous surveys. Secondly, new techniques were tested to improve the survey efficiency of seahorse population and new tools were developed to promote seahorse population recovery, with potential application in seahorse population management programs.
In this study, it was observed that both seahorse species abundance increased since the previous survey (2008/2009) although at lower numbers than in the 2001/2002 surveys and a significant correlation between the percentage coverage and H. guttulatus density was found. The new underwater visual census tested, proved to be more effective and accurate in a low seahorse density scenario. In this study, seasonal variations in seahorse density were observed in all sites and were probably related to human activities, natural events, and holdfast availability. In addition, a new photo-identification method was tested and results indicate it as a suitable tool for capture-recapture studies and less invasive that other commonly used methods. To cope with habitat degradation, artificial holdfast units (AHU) were designed and first tested in laboratory to assess seahorse preference. The preferred AHU was then deployed in 4 locations in the Ria Formosa, that differed in depth, water current and seahorse abundance in the vicinities of the selected site. Results showed seahorse settlement at high densities and validated the usefulness of the AHU as a habitat recovery tool.

DOI: 10.13140/RG.2.1.4740.1128