Update on "Seals and Cod"

...cod rally at the eleventh hour to save the seals...?

by Debbie MacKenzie, November 26, 2010

Above, DFO graphs of seal numbers in the Scotia-Fundy region. The rest of the 300,000 grey seals live in the Gulf of St. Lawrence. The zig-zag pattern reflects seasonal migration.

Recent signs of groundfish recovery?

Below is DFO's latest published research data (2) on Scotian Shelf fish stocks, updated to summer 2009. "4VW" refers to the Eastern Scotian Shelf, where grey seals are currently concentrated, and a major seal cull demanded by the fishing industry. "4VWX" refers to the entire Scotian Shelf and Bay of Fundy, while "4X West" means just the Bay of Fundy. Click on graphs below to enlarge.

In July, 2009, 4VW cod biomass shot up to a level not seen for 20 years, following a major slump. Grey seals eat cod.

A massive increase in redfish was found in 2008, and again in 2009. Grey seals eat redfish.

Haddock increased to a level not seen since the 1970's. Grey seals do not seem to eat haddock, but haddock will share the benefit from any seal-induced productivity increase.

Witch flounder increased to a level not seen since the 1970's. Grey seals eat flatfish.

It is unclear if grey seals eat turbot,

or halibut.

Grey seals eat a few lobsters.

Herring abundance is not accurately determined with a bottom trawl survey, which is designed to sample groundfish. In fact, the success of the commercial herring fishery has tended to vary inversely with the number of herring in the bottom trawl. The recent relative lack of herring in the bottom trawl as shown in this graph coincided with a tenfold increase in the commercial catch of herring on the offshore Scotian Shelf in 2009. Grey seals mainly eat herring and other similar, small "pelagic" schooling fish.

The pink dots show the Eastern Scotian Shelf (4VW) total fish biomass (as determined by DFO's bottom trawl survey), which has recently risen to a level higher than it was in 1970, and a level unmatched at any time since the late 1980s. An increase in overall productivity caused by a high density of seals would be reasonably expected to raise the total biomass index.



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.

1. How many seals are there?
2. How many fish are there?
3. How could cod rebuild their numbers while interacting with peak seal numbers?
4. Will Canada cull the grey seal herd?
5. Who can prevent a misguided seal cull?


1. 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.

2. How many fish are there?

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.

3. How could cod rebuild their numbers while interacting with peak seal numbers?

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."

4. Will Canada cull the grey seal herd?

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.

5. Who can prevent a misguided seal cull?

The Honourable Gail Shea
House of Commons

Minister of Fisheries and Oceans
Parliament Buildings, Wellington Street
Ottawa, Ontario
Canada K1A 0A6

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

2. 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":



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