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 tethered prey and quantify predation pressure in aquatic habitats. The system incorporates a small digital clock with a pressure-sensitive triggering mechanism. Survival time is a continuous variable that can be analyzed with parametric statistical tests, and in comparison with presence/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 triggering mechanism was set to ensure that shrimp escape behavior would not trigger the clock. In laboratory trials with pinfish Lagodon rhomboides as predators, triggering efficiency for predation events was measured at 65%. A field experiment was also conducted in 3 estuarine habitats located in the Galveston Bay system of Texas, USA. On unstructured sand bottom the efficiency of the triggering mechanism was similar to that measured in the laboratory. Structure in seagrass and salt marsh habitats, however, appeared to affect triggering efficiency and reduced the number of usable observations in these habitats. Despite this complication, significant differences in survival time were detected. Predation pressure appeared to be lower in the seagrass and salt marsh habitats examined compared with nonvegetated sand bottom