Do harbour seals in fact target juvenile salmon of conservation concern? Biased diet sampling has provided an illusory perspective of seal impacts

Harbour seals forage throughout coastal waters and typically exhibit restricted foraging ranges. A small fraction of the population resides in estuaries of salmon-bearing rivers. Seals are known to consume salmon, but are also thought to target salmon species of conservation concern and deemed responsible for stock declines. DNA metabarcoding and hard part analysis were used to estimate diet from scat collections ( n = 2705) undertaken at multiple haul out sites in the Strait of Georgia between 2015 and 2019, including both estuary and nonestuary sites. Proportions varied by salmon species with a range of 0.5%–5.1% and were on average 4.5 times higher in estuary sites. These results are contrasted with previous collections made between 2012 and 2014 ( n = 1435), which predominantly focused on estuary sites for which mean salmon proportions were 2–7 times higher. These contrasting results underscore the variation in seal diets between sampling locales and years, and the subsequent propagation of bias to estimates of total consumption of salmon when extrapolating spatially and temporally restricted mean diet estimates to the seal population as a whole.