Experimental Attempts to Reduce Predation by Harbor Seals on Out-Migrating Juvenile Salmonids

—During spring, harbor seals Phoca vitulina feed at night under two bridges spanning the Puntledge River in Courtenay, British Columbia, Canada. Positioned parallel to one another, ventral side up, the seals form a feeding line across the river to intercept thousands of out-migrating salmonid smolts. During a 4week observation period in the spring of 1996, we attempted to disrupt the seals’ feeding patterns by (a) deploying a mechanical feeding barrier (cork line), (b) altering the lighting conditions (lights on a bridge were turned off), and (c) installing an acoustic harassment device. We found acoustic harassment to be the most effective feeding deterrent. Of the other two deterrents, turning off the bridge lights was more effective than deploying a cork line, which had little effect. Acoustic harassment devices appear to be the most effective, nonlethal means for protecting juvenile salmonids from harbor seal predation in portions of the Puntledge River. Natural predators that prey upon both out-migrating and returning anadromous fish can detrimentally affect the survival of depressed fish populations (Bigg et al. 1990; Fraker 1994; Olesiuk et al. 1995). In the northeast Pacific, seals and sea lions are commonly observed feeding on returning adult Pacific salmon Oncorhynchus spp. in rivers and estuaries during summer and fall (Spalding 1964; Olesiuk et al. 1990). Seals also intercept outmigrating smolts in spring and early summer (Olesiuk et al. 1995). Among the better-studied seal– salmon interactions are those in the Puntledge River on Vancouver Island, British Columbia (Bigg et al. 1990; Olesiuk et al. 1995; Trites et al. 1996; Figure 1). Harbor seals Phoca vitulina in the Puntledge River regularly position themselves side by side, ventral side up, in the upstream shadow of two bridges near the light–shadow boundary. The seals * Corresponding author: [email protected] Received November 29, 1999; accepted June 5, 2000 swim against the river current and hold their position in the water. Minimal movements of their hind flippers cause no apparent disturbance to the surface waters. This feeding strategy allows the seals to form an almost continuous barrier so they can intercept smolts that drift downstream near the surface. Apparently, the seals are assisted in their feeding efforts by the bridge lights that illuminate