Does tethering of mobile prey measure relative predation potential? An empirical test using mummichogs and grass shrimp

Tethering prey organisms continues to be a popular method of evaluating spa tial and temporal variation of predator effects on population or community dynamics in marine environments. However, this technique relies on an untes ted fundamental assumption that there is a simple, reliable relationship be tween losses of tethered prey and site-specific risk of mortality due to pr edation. To test this, we placed tethered and free grass shrimp Palaemonete s pugio together in mesocosms (1.3 m diameter) and varied the level of pred ation potential by exposing the prey to different densities of the mummicho g Fundulus heteroclitus, a common predatory fish in salt marshes of the eas tern US. There were 10 replicate experimental runs, each involving 6 mesoco sms that included 3 tethered and 27 free shrimp and either 0, 1, 2, 4, 8, o r 16 predators. Survival of tethered shrimp was assessed after 15, 30, 45, 60, 90, 120 and 180 min, but survival of free shrimp was determined only at the end of each run (180 min). In the absence of predators, average surviv al of tethered (93.3 %) and free (99.3 %) shrimp was similar. At the highes t predator density, none of the tethered shrimp survived to 180 min, while survival in the free group was 85.1%. A significant interaction (ANOVA, F-5 ,F-108 = 11.88, p < 0.0001) between tether treatment and predator density i ndicated a difference in the slopes of the relationships describing surviva l of free and tethered prey across the experimental gradient of predation p otential. Chronological analysis of tethered prey losses across the experim ental predation gradient also detected significant interactions (repeated-m easures ANOVA, F-30,F-324 = 3.24, p < 0.001) between the effects of predato r density and exposure time. Tethering grass shrimp as a means of assessing even relative effects of predation by mummichogs on free prey is of questi onable value because the relationship between relative loss rates of tether ed and free shrimp changed across a gradient of predation potential and var ied with exposure time.