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

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.
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 photoidentification method was tested and results
indicate it as a suitable tool for capturerecapture 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

Correia, M. (2014). Trends in seahorse abundance in the Ria Formosa, South Portugal: recent scenario and future prospects. Doctoral dissertation, Universidade do Algarve, Portugal.