Terms of Reference

Zonal Advisory Process on the Impacts of Grey Seals on Fish Populations in Eastern Canada

October 4-8, 2010

Halifax, Nova Scotia, Canada

Chairperson: Mike Chadwick


There is ongoing debate about the possible negative impacts of seal predation on fish populations of commercial and conservation interest (e.g. Atlantic cod). One factor contributing to this debate is the growth in grey seal populations in eastern Canadian waters over the past several decades and the concurrent decline or in some cases collapse of several fish populations to the point where fishing has been stopped. Natural mortality of adult fish has been estimated to be unusually high in these collapsed and non-recovering fish populations.

Seals are hypothesized to have five possible kinds of negative effects on prey populations: 1) predation, 2) competition for food, 3) transmission of parasites causing increased mortality of fish, 4) disruption of spawning causing reduced reproductive success, and 5) other indirect effects on prey productivity caused by changes in fish behaviour in order to reduce the risk of seal predation.

An external review commissioned by the Canadian Department of Fisheries and Oceans (Report of the Eminent Panel on Seal Management 2001) dealt with predation effects in great detail, but other types of hypothesized effects were not considered within the terms of reference of the review. The results of the Eminent Panel stated that the interaction between seals, groundfish and other species is complex and variable. They concluded that there is little evidence that seal predation is having a major impact on most commercial fish stocks. Since the Eminent Panel submitted their findings, considerable new research has been conducted on seal population size, changes in the natural mortality of fish and the impacts of predation.

The Canadian Department of Fisheries and Oceans (DFO) hosted a recent two-part workshop to review the impacts of seals on Atlantic cod stocks in eastern Canadian waters. The first workshop focused on the nature and quality of available data, and identified data analyses and modeling studies that could be carried out with existing data to more fully address the issue of seal impacts on recovery of commercial fisheries (DFO Proceedings 2008/021). The second workshop reviewed these new analyses (DFO Proceedings 2009/020). The overall objectives of the two workshops were: 1) to review research on the trophic interactions (e.g. impacts of predation) between seals and Atlantic cod stocks in eastern Canada, with a focus on grey and harp seals, 2) to review similar research conducted elsewhere that may provide insight into the effects of grey and harp seal predation on Atlantic cod stocks, 3) to review research on the effects of seal transmitted parasites as a source of fish mortality, 4) to review research on the possible negative non-trophic indirect effects of seals on spawning success and feeding behaviour of fish, 5) to review available information on the possible impacts of reductions in seal population size on fish population size and exploitable biomass, and the economics of seal management, and 6) to consider the design of experimental or other research that would clarify the impact of seals on the dynamics of cod stocks.

Several hypotheses to explain the population declines and high levels of natural mortality of Atlantic cod stocks in eastern Canada were examined at the second workshop, including unreported catches (i.e., the mortality is due to fishing, not natural mortality), disease, contaminants, starvation (i.e., poor condition), life-history change, impacts of increased seal abundance (predation, parasites, other impacts) and increased predation by other predators. Results of the workshop concluded that the weight of indirect evidence suggests that grey seal predation could account for much of the high natural mortality of southern Gulf cod. The most recent southern Gulf cod advice also states that predation by grey seals is considered to be a significant component of cod natural mortality in the southern Gulf, and at current rates of natural mortality, stock growth is not likely unless productivity increases well above levels observed in the past decade. For 2009-10 and beyond, the Minister announced a targeted removal program for up to 15,000 grey seals that would be expected to be preying on concentrations of the southern Gulf cod stock.


There is no formal advisory statement dealing with the interaction of grey seals with cod stocks other than in the southern Gulf. The research reviewed at the workshops mentioned above indicates that the available evidence was less conclusive in other areas such as the Eastern Scotian Shelf.  For those other areas, there were alternate hypotheses (other than grey seal predation) that had not been ruled out and the concordance between grey seal population trends and those of the status of cod was not as obvious. Work is continuing on these issues. In the context of the current meeting, the focus will be on Atlantic cod, but may include examination of other relevant prey species.

Objectives of the 2010 Advisory Meeting

In the event that there will be a targeted removal of grey seals, Fisheries and Aquaculture Management (FAM) has asked Science to answer the following questions:

How many grey seals would have to be removed over five years to measurably lower natural mortality on southern Gulf cod and other cod stocks that are experiencing high natural mortality? What might be the ecosystem responses (e.g. abundance of other predators and prey) to grey seal targeted removal, particularly as it may impact cod recovery?

A number of working papers will be considered at this science advisory process covering the following themes:

  1. Direct evidence for cod consumption by grey seals;
  2. Indirect evidence for cod consumption by grey seals;
  3. Minimum decrease in natural mortality to restore cod populations to reference levels;
  4. Changes in grey seal abundance, distribution and ecology;
  5. Grey seals reduction scenarios to restore cod populations;
  6. Examples of control of large marine predators in other parts of the world; and
  7. Design of a controlled experiment to test impact of grey seal control on mortality of southern Gulf cod.

The working papers will address the following topics/questions:

  1. Direct evidence for cod consumption by grey seals
  2. Provide stock/area-specific estimates of grey seal diet, prey consumption and mortality:

    1. What are the estimated diets of grey seals
    2. What are the estimates of the proportions and the size distributions of cod eaten by grey seals
    3. What are possible sources of bias and uncertainty in estimates of grey seal diets
    4. What is the temporal and spatial overlap of grey seals and cod and what are possible sources of bias and uncertainty in estimates of the seasonal distribution of the grey seal and cod populations, and
    5. What are the estimated rates and trends in grey seal predation mortality on cod and to what extent can these estimates explain size-specific natural mortality of cod?
    6. What is the sensitivity of the results in topic 1.e to the possible biases and uncertainties identified in 1.c and 1.d?

In addressing these questions, participants will review the different methods of estimating diet and the different modeling approaches to estimating both consumption and predation mortality. The latter is expected to include two-species predation models, multi-species models and ecosystem models.

  1. Indirect evidence for cod consumption by grey seals
    1. What are the trends in natural mortality in cod and other relevant fish species in each ecosystem?
    2. What are the alternative hypotheses for causes of elevated natural mortality for the species for which it is observed (cod and others)?
    3. Based on a weight-of-evidence approach, which of these hypotheses are most likely to contribute to the observed increases in natural mortality?

The minimum list of alternative hypotheses that should be examined for all ecosystems and affected species is:

    1. Unreported catch
    2. Disease
    3. Contaminants
    4. Parasites
    5. Poor fish condition
    6. Life-history change
      • increased mortality cost to reproduction due to maturation at an earlier age or smaller size
      • evolution of early senescence
    7. Predator pit
      • due to predation by grey seals, harp seals and other predators

It is acknowledged that in many cases the available information may be very limited for some hypotheses. Nonetheless, there should be a detailed evaluation of the available data and literature.

  1. Minimum decrease in natural mortality to restore cod populations to reference levels

What is the minimum decrease in cod natural mortality necessary to have a high (90%) likelihood of an increase in cod biomass (above the limit reference point of 80K tonnes) over 10 and 15 years, assuming current productivity parameters (growth, maturation rate, etc)?  

  1. Changes in grey seal abundance, distribution and ecology
  1. What factors may have played a role in any increased impact of grey seals on fish prey in general and/or cod in particular?  Include a historical review of changes in the ecology, distribution and natural predators of grey seals.
  2. Are any of these factors amenable to management actions, including population control actions?
  1. Grey seals reduction scenarios to restore cod populations
    1. Considering if grey seal predation is a factor in M, what would be the minimum reduction in grey seals necessary to be 50% certain of attaining a detectable increase in productivity to the level that the population will increase in the absence of fishing (if this were the only management measure taken)?
    2.  Given the data and different assumptions about the diet of grey seals, what might be the consequences of 10%, 20% and 40% reductions of the grey seal population on direct consumption of cod and other fish prey and how would they translate into changes in natural mortality?
    3. What might be the consequences of taking no action?
  1. Examples of control of large marine predators in other parts of the world
    1. What has been the experience of other attempts to control predators (particularly, but not necessarily exclusively, marine predators) in order to achieve specific outcomes in terms of targeted prey? 
    2. Based on that literature experience, what level of understanding is be needed to guide a decision on implementation of predator control programs in other ecosystems?
  1. Design of a controlled experiment to test impact of grey seal targeted removal on mortality of southern Gulf cod

Using the results of the above what would be the design of an experiment to test the impact of grey seal control on cod mortality? What stock, location, methodology, time frame and degree of control would be required to observe an increase in cod productivity? In addition, for any given level of grey seal population removal/reduction, what would be the nature (i.e. scope, scale and duration) of how a removal/reduction could be applied in order to maximize the likelihood and size of a potential decrease in cod natural mortality?


The findings and recommendations of this Zonal Advisory Process (ZAP) will be documented via a Proceeding and Science Advisory Report of the Canadian Science Advisory Secretariat (CSAS). Science advice will inform FAM on the extent to which management decisions regarding grey seal population control are likely to achieve measurable increases in cod productivity and biomass.

Outputs from the meeting will include a CSAS Science Advisory Report, CSAS Research Documents based on the working papers presented and CSAS Proceedings to document the discussions of the meeting.


This meeting is intended to provide an open and transparent peer-review of the existing scientific information which will provide an opportunity for knowledgeable experts to contribute to achieving the objectives as outlined in the Terms of Reference (not yet posted).  As such, attendees are expected to participate fully in the discussion and offer objective, informative and constructive input that will help to inform the scientific process.  It is not intended that participants attend the meeting merely to be informed on this issue, nor to advance a particular position without scientific foundation.  Participants have been selected to participate in this advisory process if they have been identified to have specific knowledge and/or expertise on this topic. Participation is on an individual basis, owing to knowledge and expertise of the topics listed above, and not as a representative of a particular group. This process will follow the standard CSAS guidelines for participation to DFO science advisory processes. It will include experts from DFO Science and other sectors of the Department, as well as a broad range of invited external participants with relevant expertise on the issues of interest.