Update on "Seals and Cod"
by Debbie MacKenzie, November 26, 2010
A high density of grey seals on the Eastern Scotian Shelf now seems to be kick-starting a cod recovery. Seals concentrate nitrogen in the surface water, raising overall productivity and triggering faster growth of fish food.(1) Nothing else explains the recent resurgence of cod and other groundfish in the area.* Regardless, Canada's Department of Fisheries and Oceans and federal Fisheries Minister Gail Shea are preparing to authorize a large, misguided grey seal cull on Sable Island, Nova Scotia, which risks causing substantial harm to cod and other wildlife. No public consultation on this decision is planned.
How many seals are there?
Between 1970 and 2010, grey seal numbers in Eastern Canada grew tenfold, expanding from 30,000 to 300,000. Recently, the seal herd seems to have leveled out at "carrying capacity". Two thirds of Canada's grey seals live on the Eastern Scotian Shelf and congregate near Sable Island.
Cod on the Eastern Scotian Shelf long supported a large commercial fishery before declining dramatically ("collapsing"), leading to a fishery closure in 1993. No sign of cod "rebuilding" was seen until very recently, after seal numbers in the area rebounded. See the series of graphs at right.
Seals have never been shown to "harm" fish populations, including fish they eat directly. A growing body of scientific information supports instead that seals actually benefit fish populations. Compelling relevant observations have recently been published in PLoS ONE by Joe Roman and James J. McCarthy, (1) who conclude that "marine mammals provide an important ecosystem service by sustaining productivity in regions where they occur in high densities", and who warn that "An unintended effect of bounty programs and culls could be...decreased overall productivity."
The fishing industry's urgent demand for a massive cull of Atlantic Canadian grey seals to "mitigate" their "impact on fish" has led to DFO's closed review of the (presumed only to be negative) "impact of seal predation on fish populations" to generate "science advice" to support culling over 200,000 grey seals to "aid cod recovery". The Canadian Broadcasting Corporation covered this story in May, 2010, (3) ten months after DFO found (but did not publicize in the media) a dramatic increase in groundfish (2) in the area where the seals are living. DFO's "science advice" to support the "science based" cull is expected before Christmas, just in time for this winter's grey seal pupping season on Sable Island.
The Honourable Gail Shea
email Gail Shea: Min@dfo-mpo.gc.ca
(Or email Debbie MacKenzie at Codmother@bellaliant.net )
1. Roman J, McCarthy JJ (2010) The Whale Pump: Marine Mammals Enhance Primary Productivity in a Coastal Basin. PloS ONE 5(10): e13255. doi:10.1371/journal.pone.0013255 http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0013255
Clark, D., J. Emberley, C. Clark, and B. Peppard.
2010. Update of the 2009 Summer Scotian Shelf and Bay of Fundy Research Vessel
Survey. DFO Can. Sci. Advis. Sec. Res. Doc. 2010/008. vi + 72 p.
3. "Sable Island seal cull studied by DFO" online story: http://www.cbc.ca/canada/nova-scotia/story/2010/05/27/ns-grey-seals-sable-island.html
And CBC television aired "Seal Cull Considered":
plus "Department of Fisheries and Oceans looks at two ways to reduce the population of grey seals":