1 Brief to the Senate Committee on Seal Populations in Canada W. Don Bowen, February 2023 Honourable Senator Manning (Chair) and members of the Committee, I am pleased to submit the following brief on the Committee’s study to “Examine and report on Canada’s seal populations and their effect on Canada’s fisheries”. I was a research scientist with the Department of Fisheries and Oceans from 1978 until my retirement in 2016. From 1978 to 1984, I worked at the Northwest Atlantic Fisheries Laboratory in St.

John’s, Newfoundland where I was responsible for research and assessment of the harp and hooded seal populations in the Northwest Atlantic. These programs had two main objectives. The first was to provide scientific advice on sustainable harvests and the second was to study the potential impact of seal predation on fish species of commercial importance. In 1984 I moved to the Bedford Institute of Oceanography as Division Manager of the Marine Fish Division (currently the Population Ecology Division).

This Division is responsible for providing advice on sustainable harvests of groundfish and seals and ecological analysis as input to ecosystem models in the Maritime Region. In 1989 I left management to develop a seal ecology program focussing on the population dynamics of grey seals and their role in marine ecosystems. I continue active involvement in this program as a research scientist emeritus.

From 1984 to the present, I have also been an adjunct professor in the Biology Department at Dalhousie University where I have mentored more than two dozen graduate students all of which have contributed to DFO’s grey seal program. Finally, from 2011-2014 I was chair of the Special Committee on Seals in the United Kingdom and from 2007-2015, I chaired DFO’s National Marine Mammal Peer Review Committee which provides science advice to the Minister of Fisheries and Oceans.

As I am sure the Committee is aware, the impacts of seal predation on fish and invertebrate species of commercial importance have been a topic of debate for decades dating back to at least the Malouf Commission (1984). Although there have been other assessments, the more recent comprehensive study of the impact of grey seals on fish populations took place over a decade ago. Nevertheless, many of the conclusions in the resulting international workshop proceedings (Bowen et al. 2008 and 2009) and research document (Anon. 2010) are still relevant today.

At one level, it is reasonable to imagine that providing an answer to the question of impact should be rather straight forward. After all it is a matter of fact that pinnipeds (seals, sea lions, and fur seals) eat fish and invertebrates. So why are we still struggling for answers? Although the question might seem straight forward, none of the components of the interaction between seals and fish can be observed directly and therefore indirect methods must be used to collect the information from which inferences are drawn about both seal and fish populations and the nature and extent of their interactions.

Furthermore, these interactions are nested within a changing ecosystem, adding uncertainty. This means that the attribution of the extent of seal

2 predation impact on a particular fish stock is characterized by irreducible uncertainties arising both from measurements of system components, and the complexity and our ignorance of their interactions. In other words, we are attempting to find answers to a wicked problem. This label does not solve anything, but it does underscore why it has been so difficult to provide advice to managers. Seals Forgetting about interactions for the moment, what do we need to know about seals in the context of predation?

We need to know - 1) how many seals there are, 2) how much food they consume, 3) what is the species composition of their diet, 4) how does the diet change in response to a change in prey availability, and 5) where that food is eaten. Each of these has a time dimension because seal numbers and their marine environment change over time and those changes can influence where seals feed and what they consume. Taking each of these in more detail, what do we know and what are some of the uncertainties.

Population size – the fundamental problem in estimating the size of a seal population is that there is no time or place during the year when all members of the population are available to be counted. Some fraction of the population is always at sea. However, for species like grey, harp, and hooded seals the number of pups born can be estimated by photographic aerial survey because they are born on land or ice over a short period of time.

From an estimate of the number of pups born, an estimate of total population size can be calculated using a population model and additional information on trends in birth rate, natural and hunting mortalities, and the sex ratio of males to females in the population. For grey seals, these additional sources of information come mainly from studies in the Gulf of St. Lawrence and from long-term studies (now 40 years) of individually marked seals at Sable Island conducted by DFO science. Population estimates are generally good and represent the most precise information we have about seal populations.

For example, grey seal numbers have increased substantially over the past 60 years. Thus, the observation by fishers that there are more grey seals in more places is in accordance with scientific evidence. Nevertheless, there is considerable regional variation in population trends and there has been a slowing of overall growth in numbers over the past decade or so. The last survey of pups on Sable Island revealed a decline in numbers suggesting that total population may be declining or may soon decline.

We know that juvenile survival has been low (less than 30%) for grey seals born since bout 2000 compared to those born in the 1980s (about 75%). Reduced juvenile survival is thought to be driving the decline in the number

3 of pups born. However, one survey does not make a trend so a conclusion about population status awaits another survey. Food requirements – This has been studied mainly in captive animals and we have a reasonably good understanding of food requirements as they are closely tied to differences in body size. This is not considered a significant gap in our knowledge with respect to estimating impacts of predation. At-sea distribution – After the breeding season, seals spend most of their time at sea where they are highly mobile and wide ranging.

Because many fish species (e.g., Atlantic cod and herring) have geographically distinct stocks, knowing where seals feed is critical to estimating stock-specific impacts. For the past several decades, DFO science and their partners have been using satellite-linked electronic tags to track the movements and diving behaviour of seals. To do this, seals must be live captured, sedated, tagged, and then released. This, coupled with the high cost of tags and satellite fees, limit the number of seals that can be tracked to a very small proportion of the population (much less than 1%).

This small sample is used to infer the seasonal and spatial distribution of the entire population – thus the uncertainty. Satellite-linked tags have been used to study the movements of harp, hooded, grey, and harbour seals on the east coast, but the most comprehensive information pertains to grey seals. Based on collaborative studies conducted by DFO science, Dalhousie University and the Ocean Tracking Network, current evidence shows that adults limit their movements mainly to continental shelf waters of up to 200 m and exhibit seasonal and sex differences in geographic distribution.

We have less information about the movements of juveniles. However, juveniles represent an important numerical part of the population, and they are likely to have a different distribution than that of adults because of their smaller size and lesser diving ability. Filling this gap will be important in providing credible estimates of the at-sea distribution of the entire population. A final, but critical point, is that knowing the spatial and temporal distribution of movements from tracking studies is not the same as knowing where seals feed.

Tracking data are necessary but not sufficient as they only tell us about the opportunity for interaction with fish, but they do not provide information on where feeding occurred. For that, movement models of tracking data provide indirect evidence for feeding by identifying restricted search behaviour suggesting that a seal is repeatedly diving in the same area and therefore may have found a patch of food. Other more conclusive methods have been used, but only on a small number of seals to date, largely due to the high cost of instruments.

Three types of instruments have been used along with satellite-linked tags to estimate where along a track feeding takes place. The first is stomach temperature telemetry where the seal is fed a small transmitter that records and transmits stomach temperature to a receiver on the seal’s back. A sudden drop in stomach temperature is used to infer the ingestion of prey. Accelerometry can also be used to infer attempted prey capture by the sudden and characteristic acceleration of the seal’s head.

4 Neither stomach temperature nor accelerometry can identify the type of prey. Finally, the gold standard, is animal-borne video, from a camera mounted between the seal’s shoulders. Recorded video can show not only attempted captures but capture success and more importantly the fish species eaten (e.g., Bowen et al. 2002 on harbour seals). This last approach shows great promise both to help validate estimates of diet and the location of feeding. Diet – Determining the diet of seals is the greatest challenge we face in attempting to understand the impact of seal predation on fish populations.

It also represents one of the greatest sources of uncertainty about seal impacts. There are several reasons for this. The first is that seals typically feed in remote areas while diving and therefore what is consumed cannot be directly observed. The second is that because they are wide ranging and feed in remote areas, obtaining a representative sample of what is eaten is difficult. Thirdly, each of the indirect methods available for estimating diet has known biases which complicate the

interpretation of the information collected. Two approaches have been used to estimate the species composition of the diet of pinnipeds, such as grey, harbour, and harp seals. The most common approach is the recovery of undigested prey parts (usually ear bones or otoliths) from stomach contents or in faeces of seals. Another is based on the composition of fat stored by seals in their blubber.

This approach is called quantitative fatty acid signature analysis (QFASA) and is based on the observation that the proportion of different fatty acids (the building blocks of fat) stored in blubber reflects the kind of prey eaten. This is because most species of fish and invertebrates have distinguishable fatty acid profiles (similar to fingerprints) and so by analysing the fatty acid profiles of seals one can estimate what must have been eaten to produce the observed fatty acid profile.

Stable isotopes of carbon and nitrogen can also be used to infer aspects of the diet, such as trophic level, i.e., the proportion of groundfish vs forage fish species, but cannot usually estimate the species composition of the diet, except where only a few types of prey are eaten. DNA profiling is a developing technology that can determine presence of species but reliable estimates of the proportion of species eaten is still a developing field. Strengths and weaknesses of these methods is given in Bowen and Iverson (2013).

Using grey seals as an example, it is important to understand that there is no such thing as ‘the’ diet. This is because the diets of adult males and females differ, diets change seasonally, they differ individually and regionally, and with age group (immatures vs adults). They also differ over time, most likely in response to changes in the distribution and abundance of prey species. Therefore, using some average diet in a model of impact can lead to erroneous conclusions.

Because seal diets change over time and space, and especially with rapidly changing ecosystems, reasonably current estimates of diet are needed to estimate the impact of seal predation on fish populations. In respect to grey seals, little research on diets has been conducted since the last seal-fish assessment some 15 years ago. Therefore, updating our information on diet should be a priority.

5 How diet changes in response to changes in prey – As the number of prey encountered by a seal changes due to changes in the abundance of the prey, the expected number eaten by seals also can change. This is known as the functional response of the predator. The shape of the functional response is a critical component of predation models as it determines the number of prey consumed. Despite its importance, the shape of the functional response of seals to changes in prey abundance is poorly understood.

This is because it is difficult to obtain the needed data given the difficulty in determining the seal diet in relation to the abundance of prey, such as Atlantic cod. Therefore, seal predation models often assign the shape of the functional response by analogy with other mammals for which there are data. This again introduces uncertainty in estimates of impact. Fish To evaluate the impact of seal predation on a fish population, an estimate of fish abundance, recruitment, and sources of mortality are needed.

Sources of mortality include the catch by fisheries, seal predation and other sources of natural mortality (such as disease, starvation, deaths from predators other than seals, i.e., other fish, whales, dolphins). The goal is to estimate the importance of mortality caused by seals relative to these other sources of mortality. There is also the need to understand if fish mortality caused by seals is additive to other sources of mortality or is compensatory (fish eaten by seals would have died anyway from other causes).

For example, disease or lack of food may make fish vulnerable to seals such that they would have likely died even if not eaten by seals. Population assessments of the fish and invertebrate species eaten by seals are often subject to large uncertainty. This, coupled with the uncertainty about the components of seal predation, makes it difficult to draw firm conclusions about the impact of seal predation. Impact of seal predation on fish stocks This issue has been under study by DFO since the Malouf commission in the 1980s.

Since then, DFO science has conducted numerous analyses to estimate the impact of seals on fish populations on the Scotian Shelf (Hammill and Mohn 1994; Mohn and Bowen 1996; Trzcinski et al. 2006, 2009, 2011; Harvey and Hammill 2010; Bowen et al. 2011; Anon 2010), in the Gulf of St. Lawrence (Hammill and Mohn 1994; Hammill et al. 1999; Chassot et al. 2007; Stenson et al. 2010; Hammill 2010; Anon 2010; Morissette et al. 2009; Morissette and Hammill 2011; Benoit et al. 2011a, b; Swain et al. 2011a,

b) and off Newfoundland (Stenson et al. 1995; Stenson and Hammill 2004; Hammill and Stenson 2004; Anon 2010). To date, no management action resulted from the findings of these analyses. The most recent comprehensive analysis and review of seal-fish predation interaction on the east coast was conducted in the form of two international workshops. The results of which were reported in Anon (2010). The models used in these previous assessments may be viewed as a serial process that builds on each previous step by adding some new information to form the next; the number of seal pups plus vital rates (e.g., birth rate) is used to estimate the number of grey seals, the number

6 of seals plus their food requirements and diet leads to estimates of cod consumption, a comparison of this consumption to the size of the cod stock leads to estimates of predation mortality, and finally the predation mortality plus a cod recruitment model leads to an estimate of longer term impact. In the initial stages of this process (for example, the grey seal population models), there are often sufficient data to formally examine the uncertainty of the results. In the later stages of modelling, uncertainty is dominated by model assumptions.

The use of alternative models can be used to evaluate some of the consequences of this uncertainty. Adaptive management “experiment” For reasons mentioned above, it is doubtful that additional studies on seals or fish will yield model results that constitute a eureka moment whereby all will become clear as to the impact of seals on fish stocks of interest. Nevertheless, to justify a large-scale and long-term reduction of seal populations there needs to be reasonable evidence that fewer seals would result in more fish.

Circumstantial evidence has been provided for several species of groundfish in the southern Gulf of St. Lawrence. However, even there the expectation that reduction of the grey seal population will result in recovery is based on limited information on the diet of grey seals. It has been argued that given the dire status of those fish populations in the Gulf of St. Lawrence, that management action in the form of a seal-removal experiment is warranted.

Such an experiment would be conducted over multiple years (probably 5 years minimum) with the goal of reducing the high mortality rate on adult fish, supposedly caused by seals, and thereby allow the fish stocks to grow. Experiments of this type were examined at international workshops (Anon. 2010) and more recently by Rossi et al. (2021). Reduction in the number of seals could be accomplished by commercial harvest or by a cull. No matter how this was done, it would be critical for the experiment that the number of seals killed was known and that the proposed reduction was in fact accomplished.

Neither of these can be taken for granted as thousands of seals would need to be removed in each year of the experiment. A reduction in the number of seals over several or more years is known as a press perturbation experiment. Because there is no control population where seals are not removed, the

interpretation of such an experiment would be ambiguous as changes in components of the system, other than the number of seals, might also have influenced the results. In addition to those mentioned above, there are several other essential requirements for the design of a perturbation experiment (UNEP 1999, Bowen and Lidgard 2013). First, a perturbation experiment must have clearly stated goals. Second, there must be performance measures that provide a quantitative

interpretation of the extent to which objectives have been met and the benefits realized within the expected timeframe that can be interpreted as being caused by the reduction in the number of seals. Third, there must be research programs to monitor the target fish population(

s) and seal populations to evaluate the long-term consequences of the experiment. Finally, there need to be estimates of the per capita seal consumption of the target population(

s) in relation to other sources of natural mortality. This information strengthens the

interpretation of the performance measures.

7 It is relatively easy to design a press perturbation experiment on paper. It will be considerably more difficult to carry out such a large-scale experiment in an enclosed sea (e.g., Gulf of St. Lawrence or the coast of British Columbia), or a continental shelf (e.g., Eastern Scotian Shelf). To my knowledge, a multi-year experiment, with clear objectives, performance measures, and quantitative evaluation of the response, on the scale required has not previously been conducted in the marine environment.

Therefore, before undertaking such an experiment several issues with respect to scientific capacity must be addressed. For example, is it possible to remove the proposed numbers of seals over the life of the experiment? Can the performance measure(

s) be measured with sufficient precision to determine if the expected effect can be detected and attributed to seal removals? Finally, the issue of unintended consequences must be considered. For instance, could the removal of seals result in a decrease in a target fish population by releasing predation pressure from another predatory species? Research priorities I believe DFO science is aware of the research needed to provide better advice on seal/fish interactions.

Research needs have been identified in the Malouf Royal Commission and numerous workshops and regular assessment meetings involving stakeholders, national, and international expertise. Although I use mainly grey seals for illustration below, the priorities for research are quite similar in the case of harp and hooded seals. There is a reasonably good understanding of seal population dynamics.

But given that the Sable Island grey seal population (the largest component) appears to be approaching or to have reached carrying capacity, we should expect vital rates, such age-specific birth rate and age- specific adult mortality rate, to change affecting our ability to assess population status. We have evidence that juvenile mortality is also changing. The lack of ice in the Southern Gulf of St. Lawrence is changing the distribution of grey seal breeding colonies and potentially the distribution of feeding locations.

Thus, continued research on seal breeding distribution, vital rates and population size is needed. Albeit based on a small sample relative to population size, the offshore distribution of adult grey seals is reasonably well known for the Sable Island population. Less is known about the at- sea distribution and feeding locations of juveniles from all colonies and the distribution of adults along the Nova Scotia coast and in the Gulf of St. Lawrence. These gaps in knowledge reduce confidence in estimates of impact but could readily be addressed with additional research funding.

Perhaps the biggest challenge is to estimate the diets of the seal populations. We know from earlier work that male and females have different diets, as do younger animals, and that that there are strong seasonal and geographic changes in the fish species consumed. Given large changes in the ecosystems involved and the size of the grey seal population over the past decade or more, providing advice on seal impacts will require a substantial increase in the

8 resources available to update seal diets and the geographic distribution of feeding areas. Multiple approaches to both diet composition and feeding distributions are recommended to provide robust conclusions. References Anon. 2010. Impacts of Grey Seals on Fish Populations in Eastern Canada. Canadain Sceince Advisory Secretariat Research Document, Science Advisory Report. 2010/071. Benoit, H. P., D. P. Swain, W. D. Bowen, G. A. Breed, M. O. Hammill, and V. Harvey. 2011a. Evaluating the potential for grey seal predation to explain elevated natural mortality in three fish species in the southern Gulf of St.

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Deep-Sea Research

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