Seahorse population fluctuations in the Ria Formosa Lagoon, South Portugal

Identifying sources of marine population variability is a central goal in marine ecology and conservation, which requires long-term, repeated monitoring to assess whether there is cause for concern, particularly for species with vulnerable life history traits. Between 2001 and 2009 there were substantial declines in populations of two seahorse species (<i>Hippocampus guttulatus</i> and <i>H. hippocampus</i>) throughout the Ria Formosa lagoon (94% and 73%, respectively). During 2010-2013, the lagoon was re-surveyed using 16 of the sites previously monitored. This new data set was then compared with the information obtained from those same locations in two earlier studies (2001-2002 and 2008-2009). <i>H. guttulatus</i> populations increased significantly between 2008-2009 surveys and in the most recent 2010-2013 surveys but there were no significant differences between 2001-2002 and 2010-2013 surveys, suggesting that these populations are fluctuating rather than consistently decreasing. In contrast, there were no significant differences in <i>H. hippocampus</i> densities among the 16 sites surveyed throughout the three sampling periods, although the ability to detect any change was hampered by the overall low densities of this species in all time periods. Fluctuations in <i>H. guttulatus</i> densities were positively correlated with the percentage of holdfast coverage but with none of the other environmental variables tested. These results highlight the importance of holdfast availability in maintaining stable seahorse populations. While population fluctuations are certainly more promising than a consistent downward decline, such extreme fluctuations observed for seahorses in the Ria Formosa lagoon could still leave these two species vulnerable to any additional stressors, particularly during low density periods.

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