Predation by the pile perch, Rhacochilus vacca, on aggregations of the gastropod Littorina sitkana

The pile perch, Rhacochilus vacca (Embiotocidae), is abundant on the Pacific Coast of North America and is known to crush hard-shelled prey with its heavy pharyngeal teeth. We investigated whether pile perch predation has the potential to limit or regulate populations of Littorina sitkana, a direct-developing gastropod found on wave-sheltered shores near Bamfield, British Columbia, Canada. Laboratory experiments showed that pile perch required an average of only 19.2 sec (SE = 5.61; n = 20) to crush and swallow a large L. sitkana, which resulted in consumption rates averaging 33.3 (SE = 6.27; n = 4) large snails per day. The snails had no size refuge from predation by adult fish because even small fish (fork length 21 cm) could crush the largest L. sitkana present on the shore (>11 mm shell width). Indeed, some fish showed a significant preference for large snails (shell width >6.3 mm) over small snails (3.35 mm < shell width < 4.0 mm). Our 1998 observations with SCUBA during daytime high tides showed that the density of pile perch foraging in the intertidal averaged 0.119 (SE = 0.0205; n = 20) individuals per square meter (estimated fork lengths 5-40 cm). However, the intertidal distribution of pile perch that were actually consuming prey was highly aggregated. In the field, we investigated whether predation by this fish on snails deployed onto boulders was density dependent. The fish swam parallel to the shore and located and consumed 40% of the patches of L. sitkana that we deployed (n = 70) within 50 min. This foraging behavior resulted in density-dependent predation on the deployed snails in only one of the three tidal cycles (or 2 of the 12 days) of our experiment. We offer several proximate reasons for the low frequency of density-dependent predation found in this study and conclude that the pile perch may not be an important regulating factor for L. sitkana populations at this site at the present time. However, the very high predation rates we observed suggest that this predatory fish is an important limiting factor at this site.