Presence of harbour seals ( Phoca vitulina) may increase exploitable fish biomass in the Strait of Georgia

Abstract

We tested what degree harbour seal ( Phoca vitulina) populations compete with fisheries for commercially harvested species, and to what degree seal populations depend on exploited species as prey. In the Strait of Georgia (SoG), harbour seals mainly feed on fisheries target species, Pacific herring ( Clupea pallasii) and Pacific hake ( Merluccius productus), while herring is also a main prey of hake. Using an Ecopath model constructed based on 2005 conditions, we ran three scenarios: altering herring fishing mortality, removing seal populations and sensitivity analyses of the herring vulnerability parameter. Our results show that with more herring available, the seal population will increase greatly, but with less herring available, the seal populations in the SoG decreases gradually. Our model suggests that the total biomass of commercial fish populations in the SoG may decrease substantially with seals absent. A cull of harbour seals may not increase total fisheries catch in the SoG. Herring benefit from seal predation on herring’s largest predator, hake, so that herring may decline when seals are removed. However, this result is highly dependent on model parameterization. When juvenile herring are considered less vulnerable to hake predation (i.e., when we assume there are many refuges in which to hide), the herring population is less negatively impacted by seal removals. This indicates that survival during this crucial life-stage is important to herring abundance. The model also suggests that, with seals removed, the ecosystem would be dominated by hake.