Habitat preferences and the effects of seagrass density on population demographics and feeding ecology of pipefish in the Indian River Lagoon, FL

Seagrass density in the Indian River Lagoon (IRL), Florida, has declined over the past 20 years, especially in the central lagoon. Loss or degradation of seagrass may negatively impact seahorses and pipefish (family Syngnathidae) that have strong associations with these habitats. Results from mesocosm studies showed that juvenile Syngnathus louisianae preferred the seagrass, Halodule wrightii, in 60 +/- 6% of the observations relative to two macroalgae species, Gracilaria sp. and Caulerpa prolifera, or the water column or sand. Male and female S. scovelli had a more varied pattern for habitat preference in that 33 +/- 4% of the observations for females occurred in H. wrightii followed by 25 +/- 4% in sand. Male S. scovelli had essentially the same habitat preferences as females; however, the males showed a diurnal migration from the sand during the day to vegetative habitats at night. In the presence of a perceived predator in the mesocosm, male S. scovelli did not prefer any particular habitat, suggesting that in the presence of a predator any habitat will do. Results from additional mesocosm studies showed that S. louisianae had a preference for 100% H. wrightii in 56 +/- 9% of the observations relative to lower seagrass densities of 50% and 25%. Such strong preference for a seagrass density of 100% suggests this species may be adversely impacted by the decline in seagrass density in the IRL. Female and male S. scovelli had equal preferences for sand and 100% and 50% density seagrass; however, they did not prefer 25% density seagrass. Therefore, S. scovelli may be less impacted by decreases in seagrass density. Results from field studies in the IRL showed that S. scovelli collected from 50% density seagrass beds were significantly smaller than pipefish in the 100% density seagrass. Despite a direct relationship between size and seagrass density, no differences in fecundity were found between seagrass densities. Additional field studies showed a change from a benthic to pelagic diet for mid-sized pipefish. Pipefish from 5-7.9 cm consumed a larger volume of harpacticoids in 100% seagrass when compared with the same fish size class in 50% density seagrass. Furthermore, pipefish in the 50% seagrass density consumed calanoid copepods, not harpacticoids. Although S. scovelli demonstrated a preference for 50% density seagrass in laboratory mesocosms, field observations of population demographics and feeding ecology suggest that 50% density seagrass may not be an ideal habitat for this species.