CHRONOGRAPHIC TETHERING - A TECHNIQUE FOR MEASURING PREY SURVIVAL-TIME AND TESTING PREDATION PRESSURE IN AQUATIC HABITATS

A chronographic system was developed to measure survival time of tethe red prey and quantify predation pressure in aquatic habitats. The syst em incorporates a small digital clock with a pressure-sensitive trigge ring mechanism. Survival time is a continuous variable that can be ana lyzed with parametric statistical tests, and in comparison with presen ce/absence data normally obtained in tethering studies, this variable provides more information per tethered prey. The technique was tested using tethered brown shrimp Penaeus aztecus. The sensitivity of the tr iggering mechanism was set to ensure that shrimp escape behavior would not trigger the clock. In laboratory trials with pinfish Lagodon rhom boides as predators, triggering efficiency for predation events was me asured at 65%. A field experiment was also conducted in 3 estuarine ha bitats located in the Galveston Bay system of Texas, USA. On unstructu red sand bottom the efficiency of the triggering mechanism was similar to that measured in the laboratory. Structure in seagrass and salt ma rsh habitats, however, appeared to affect triggering efficiency and re duced the number of usable observations in these habitats. Despite thi s complication, significant differences in survival time were detected . Predation pressure appeared to be lower in the seagrass and salt mar sh habitats examined compared with nonvegetated sand bottom.