Salish Sea Marine Survival Project

64 2. PIT Tag Study 2015: Based on the timing of PIT tag detections, seal predation on smolts in estuaries occurs primarily at dusk and the following hour (Figure 5). During the 4 nights of peak outmigration, it appeared that seals consumed ~1kg Coho smolts per seal each night. This equalled about half of their daily energy needs, indicating that smolt consumption was only part of the daily foraging pattern. Extending the PIT tag detection data to the ~96 seals using the river, seals consumed ~6.19% of the outmigrating smolt population. Note that this estimate is substantially lower than the estimate provided above. The two estimates may be reconciled as follows: 6% of the juvenile salmon enter- ing the Strait (from hatcheries) may be eaten by estuary specialists (as in this study). Another 34% of mortality of smolts may occur from high numbers of seals feeding throughout the Strait of Georgia, giving a total of 40% overall seal-related mortality. 3. GPS/3D accelerometer study 2015: Reconstructing the high-resolution movements of the seals and quantifying feeding using counts of prey chasing events (PCEs) detected by accelerometry — revealed that the Big Qualicum estuary was a feeding hotspot for 47.0% of the seals, but was relatively small (accounting for 3% of prey chasing events) compared to the largest feeding area outside the estuary (26% of prey chasing events). Comparing the foraging behaviours of smolt specialists with non-specialist seals revealed 4 different foraging strategies. One consisted of seals (17.6%) that fed on Coho smolts and ignored Chinook in the river mouth, while a second group of seals (17.6%) appeared to target larger fish that preyed on Chinook smolts near the estuary. The two other seal groups did not feed in the estuary in association with the concentrated numbers of smolts, but either remained resident (52.9%) and fed near their main haulouts, or were transient (11.8%) and left the study area. These results suggest a high degree of individual foraging specialization — and show that a small number of seals were specialized in consuming Coho smolts, but did not respond to a later large pulse of smaller-bodied hatchery Chinook smolts during their outmigration. SIGNIFICANCE Preliminary results from estuary scat data have indicated that 20% of harbour seals in the Strait of Georgia may be eating up to half of the Chinook and Coho smolts that enter the Strait of Georgia. Ongoing work will evaluate the influence of sampling habitat (estuary vs non-estuary) as well as the annual variability in the proportions of salmon in diets. These evaluations may alter the current estimates of consumption. Harbour seals primarily consume adult salmon of lesser conservation concern in the fall (i.e., Chum and Pink salmon; August–November). But during the spring, seals prefer juvenile salmon of greater conservation concern (i.e., Coho, Chinook, Sockeye; April–July). Behaviour observations have included seal usage of log booms as haul-outs, and salmon use of log booms as refuges. Other SSMSP studies suggest the specialized foraging by some seals on smolts may be enabled by large pulses of large hatchery fish entering the Salish Sea each spring during a short window of time. NEXT STEPS DFO is continuing a harbor seal diet monitoring program to extend the Strait of Georgia time-series. Marine mammal scientists are refining aerial surveys and seal population numbers, and reviewing diet compositions. Further work also needs to be carried out on sea lion diets and movements to fully assess the impact of pinniped predation on salmon survival. SSMSP scientists are synthesizing the available information and working to design mitigation strategies. These may include changes to hatchery release strategies and size of smolts at release, as well as changes to log boom placement within estuaries. We also suggest that long--term monitoring is critical for assessing impacts of predation, particularly for species with large degrees of natural variability in production such as salmonids. Figure 5. Diurnal feeding intensity by seals on Coho Salmon smolts in the Big Qualicum River estuary. Graph provided by Hassen Allegue. Photo by Ryan Miller

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