Subject: Case Study: Seals and Cod

Michael Williamson (whe_william)
Mon, 28 Mar 1995 09:58:23

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Date: Tue, 28 Mar 1995 09:59:09 -0500 (EST)
From: Michael Williamson <WHE_WILLIAM@FLO.ORG>
Subject: Case Study: Seals and Cod
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From:	SMTP%"MARMAM@UVVM.BITNET" 27-MAR-1995 19:31:32.67
Subj:	seals & the cod fishery
Date:         Mon, 27 Mar 1995 18:30:09 -0500
Reply-To:     Marine Mammals Research and Conservation Discussion
Sender:       Marine Mammals Research and Conservation Discussion
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From:         Mike Mascia <mbm4@ACPUB.DUKE.EDU>
Subject:      seals & the cod fishery
To:           Multiple recipients of list MARMAM <MARMAM@UVVM.BITNET>
In-Reply-To:  <>
        I would be interested in comments on the following, PURELY
SPECULATIVE, hypothesis concerning the relationship between the seal
population and the recovery of the cod fishery.
        In terrestrial ecosystems, it has been demonstrated that the
presence of a healthy population of top predators is essential to
maintain balanced populations of others species lower in the food chain.
When non-preferential top predators, (which eat whatever they find, find
whatever is most common, and therefore eat the most common species most
often) are removed from an ecosystem, middle level predators explode in
abundance (a process termed "mesopredator release") and drive species low
on the food chain to local extinction or extreme rarity.
        Extropolating this scenario to the ocean, doesn't this
suggest that a healthy seal population is essential to the
recovery of the cod fishery?  If seals are in fact non-preferential top
predators, and juvenile cod are at the low end of the food chain, seals
may serve a vital role in enhancing cod recruitment by limiting the
populations of mesopredators.  Without seals, mesopredator release would
occur, dramatically increasing the pressures on juvenile cod and further
delaying or eliminating the rebound of the cod fishery.
        I apologize for my ignorance of the specific predator/prey relationship
between seals and cod, which may invalidate this hypothesis.  I recognize
that I am making assumptions for which I have no scientific basis.
However, my intent is simply to ask those more knowledgeable than I if
current scientific data support or discredit the scenario that I have
Michael Mascia
Duke School of the Environment
On Sat, 25 Mar 1995, Stephen Best wrote:
> Dave R. made the following statements in one of his recent posts:
> >... seals
> >MAY BE one of the reasons for the slow (i.e. mostly non-existant)
> >rebuilding of the cod stocks...
> >Seals can and do
> >eat other species, as well, whish would explain their population
> >levels, and the reason the cod stocks are staying so low.
> >
> >Dave R.
> Until recently, Brian Tobin, the Canadian Minister of Fisheries and Oceans
> has maintained that a seal cull was not necessary, because there was no
> scientific evidence to support the premise that seal populations were
> adversely affecting fish stocks.  A few months ago his position changed
> under pressure from fishermen and sealers claiming that "common sense" and
> their "perception" was that there were too many seals and the populations
> needed to be controlled.
> To the best of my knowledge, there is still, as of yet, no scientific
> evidence, despite studies and research being conducted by the Canadian
> Department of Fisheries and Oceans, Memorial University and the
> International Marine Mammal Association to name just three groups, to
> suggest that seal populations are having either an adverse effect on fish
> populations or impeding the recovery of cod or any other species of fish.
> In fact, it may be worth taking a closer look at a study done of Cape fur
> seals that suggested that seal culling might actually have a negative impact
> on the South African fishery.  If there is some scientific work that has
> been done to suggest that harp or other seals are adversely impacting fish
> stocks, I would appreciate being made aware of it.
> ----
> Stephen Best, Vice President      | Tel:  1.519.925.3440
> International Wildlife Coalition  | Fax:  1.519.925.2003
> PO Box 988, Shelburne             | e-mail:
> Ontario, Canada, L0N 1S0          | CIS: 76407,526