Indirect Effects of Grey Seals on Cod Productivity
Description
The graph illustrates some findings of the first year of research (2011) into the indirect effects of Grey Seals on cod productivity. The higher the number is on the cod condition index, the better the mean condition of cod in a given study area. The red bars represent the condition of cod in the study area with more seals and the blue bars represent cod in the location with fewer seals, indicating that cod were in substantially better condition in the area with fewer seals.
Credit: DFO, Doug Swain
The Atlantic Cod stock in the southern Gulf of St. Lawrence cannot sustain a commercial fishery since high natural mortality among adult cod and reduced individual growth rates have led to unusually low productivity. An important component of the high mortality rate is thought to be due to predation by Grey Seals, although the indirect effects of predators may be greater than the direct effect of prey consumption. For example, increased risk of predation can affect the foraging behavior of some fish species. Greater understanding of cod-seal interactions will help to determine the causes of low productivity, providing valuable information to Fisheries Management concerning why the stock has not yet recovered from overfishing in the past.
This research aims to identify the potential indirect effects of Grey Seal predation on cod productivity, including how foraging success and seal worm parasite loads affect cod condition. Comparisons will be made between cod samples gathered from two locations: an inshore area that is heavily used by grey seals (preliminary findings indicate that cod there are in poorer condition), and a deeper-water area farther offshore where seals are less common. Other data to be gathered at the two sites include parasite loads, and seal sightings and the prevalence of seal predation on longline catches to determine the difference in seal abundance. Ultimately, the project will deliver a report and scientific paper on the variation in cod condition and parasite prevalence at a fine spatial scale, comparing areas of high and low seal densities.
Program Name
Fisheries Science Collaborative Program (FSCP)
Year(s)
2010 - 2015
Ecoregion(s)
Atlantic: Gulf of St. Lawrence, St. Lawrence Estuary
Principal Investigator(s)
Doug Swain
Fisheries and Oceans Canada
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