MEPS 224:77-85 (2001)  -  doi:10.3354/meps224077

ABSTRACT: Chronographic tethering devices were used to investigate the effects of habitat type, prey type, tidal state and time of day on predation in the Damariscotta River, Maine (43°57¹N, 69° 35¹W) and in St. Joseph Bay, Florida (30°00¹N, 85°30¹W). Traditional tethering data (presence/absence) revealed that there were significant differences in the number of predation events between sites (p = 0.037) with more predation events occurring at the St. Joseph Bay site. In addition, there were significant differences in predation between prey types (p < 0.001). Regardless of site, significantly more Œsoft¹ bodied prey types (shrimp) were removed than Œhard¹ bodied prey types (brachyuran crabs). Separating the data by site revealed that there were significant differences between habitat types (seagrass interior, seagrass edge and unvegetated substrate) at the Damariscotta River site (p = 0.027), but not at the St. Joseph Bay site (p = 0.943). Using the time to predation recorded on the chronographic tethering devices, survival times were calculated. Based on prey survival time, comparisons were made between predation intensity in monotypic stands of the seagrasses Zostera marina L. (Damariscotta River) and Thalassia testudinum (St. Joseph Bay), and bare substrate and the grass edge at both study sites. There were no significant differences in survival time between habitats at the Damariscotta River site (p = 0874). However, the average prey survival time was significantly less for the seagrass bed edge at the St. Joseph Bay site than for the T. testudinum bed interior and unvegetated habitats (p = 0.019). In addition, the time to predation allowed the effect of tidal state and light versus dark on predation to be assessed. While neither tidal state nor light versus dark had any effect on predation at the Damariscotta River site (p = 0.096 and p = 0.481 respectively), at the St. Joseph Bay site, there were significantly more predation events during the day than at night (p < 0.001), regardless of tidal state (p = 0.664). The use of the chronographic tethering devices revealed that while there was no significant difference in the number of predation events or rate of predation at the St. Joseph Bay site, the interface (i.e. Œedge¹) between bare or unvegetated substrates and seagrass assemblages was associated with the highest predation risks.

KEY WORDS: Chronographic tethering · Edge effects · Predation · Thalassia testudinum · Zostera marina