An examination of the population dynamics of syngnathid fishes within Tampa Bay, Florida, USA

Abstract

Seagrass ecosystems worldwide have been declining, leading to a decrease in associated fish populations, especially those with low mobility such as syngnathids (pipefish and seahorses). This two-year pilot study investigated seasonal patterns in density, growth, site fidelity, and population dynamics of Tampa Bay (FL) syngnathid fishes at a site adjacent to two marinas under construction. Using a modified mark-recapture technique, fish were collected periodically from three closely located sites that varied in seagrass species (Thalassia spp., Syringodium spp., and mixed-grass sites) and their distance from open water, but had consistent physical/chemical environmental characteristics. Fish were marked, photographed for body size and gender measurements, and released the same day at the capture site. Of the 5695 individuals surveyed, 49 individuals were recaptured, indicating a large, flexible population. Population density peaks were observed in July of both years, with low densities in late winter and late summer. Spatially, syngnathid densities were highest closest to the mouth of the bay and lowest near the shoreline. Seven species of syngnathid fishes were observed, and species-specific patterns of seagrass use emerged during the study. However, only two species, Syngnathus scovelli and Hippocampus zosterae, were observed at high frequencies. For these two species, body size decreased across the study period, but while S. scovelli's population density decreased, H. zosterae's increased. Across six of the seven species, population size declined over the course of this preliminary study; however, seasonal shifts were impossible to distinguish from potential anthropogenic effects of construction.

Seasonal and spatial variation in the reproductive biology of the dwarf seahorse Hippocampus zosterae

“…Previous research suggested that low recapture rates of H. zosterae resulted from large spatial sampling distribution and long‐time intervals between collections (Masonjones et al . ). In the present study, the aim was to reduce re‐sampling intervals between collections and focus sampling more intensively in areas where individuals were previously collected and released back after marking, to increase recapture rates.…”

“…Together, the data in the present survey was analysed along with the fish numbers from the syngnathid population survey conducted 2005–2007, to investigate seasonal reproduction trends, such as peak breeding, seasonal sex ratios and juvenile recruitment, which were not reported in the density and habitat preference study by Masonjones et al . ().…”

Seahorse (Hippocampinae) population fluctuations in the Ria Formosa Lagoon, south Portugal

“…Extreme fluctuations have been documented in populations of at least six seahorse species, but the causes of such fluctuations have been more difficult to discern: Hippocampus abdominalis Lesson 1827 (Martin‐Smith & Vincent, ), Hippocampus reidi Ginsburg 1933 (Freret‐Meurer & Andreata, ), Hippocampus zosterae Jordan & Gilbert 1882 (Masonjones et al , ), Hippocampus hippocampus (L. 1758) (Caldwell & Vincent, ), Hippocampus guttulatus Cuvier 1829 (Caldwell & Vincent, ) and Hippocampus whitei Bleeker 1855 (Harasti et al , ). Among the variety of hypothesized causes for the fluctuations in these seahorse species were human‐related activities, including direct effects (coastal construction and exploitation) and indirect effects (changes in water quality) (Masonjones et al , ), natural fluctuations due to recruitment failure (Martin‐Smith & Vincent, ), changes in predator abundance (Harasti et al , ) and changes in holdfast availability (Rosa et al , ). With the exception of H. whitei , in which fluctuations were associated with predator abundances in New South Wales, Australia (Harasti et al , ), none of the other documented population declines were linked with any obvious cause (Martin‐Smith & Vincent, ; Freret‐Meurer & Andreata, ; Masonjones et al , ; Caldwell & Vincent, )…”

“…Among the variety of hypothesized causes for the fluctuations in these seahorse species were human‐related activities, including direct effects (coastal construction and exploitation) and indirect effects (changes in water quality) (Masonjones et al , ), natural fluctuations due to recruitment failure (Martin‐Smith & Vincent, ), changes in predator abundance (Harasti et al , ) and changes in holdfast availability (Rosa et al , ). With the exception of H. whitei , in which fluctuations were associated with predator abundances in New South Wales, Australia (Harasti et al , ), none of the other documented population declines were linked with any obvious cause (Martin‐Smith & Vincent, ; Freret‐Meurer & Andreata, ; Masonjones et al , ; Caldwell & Vincent, )…”

Monthly population density and structure patterns of an endangered seahorse Hippocampus capensis: a comparison between natural and artificial habitats

“…H. zosterae and. Scovelli populations in Tampa Bay, FL, U.S.A., showed some evidence of migrating out of shallow environments during the dry season [Mason‐Jones et al ., ) and a H. whitei population in Port Jackson, Australia, also migrated to deeper water after the breeding season (Vincent et al ., )] and this might be another explanation for the variability in H. capensis population densities across months. In addition, an increase in predators were found to play a role in decreasing population numbers of H. whitei in a marine protected area in Australia (Harasti et al ., ) and the decrease in H. capensis numbers during the summer may also have resulted from a possible increase in predators.…”