Center of Expertise in Marine Mammalogy: Scientific Research Report 2009-2011

2.0 Role of Marine Mammals in the Ecosystem

2.1 Summary of the Zonal Advisory Process on the Impacts of Grey Seals on Fish Populations in Eastern Canada (Don Bowen)

Possible negative impacts of seal predation on fish populations of commercial and conservation interest (e.g. Atlantic cod) continue to be debated. Contributing to this debate is the observed growth in grey seal populations in eastern Canadian waters over the past four decades and the large declines in several fish populations to the point where fishing has been stopped. Natural mortality of adult fish also has been estimated to be unusually high in these collapsed and non-recovering fish populations. Grey 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.

The Department of Fisheries and Oceans (DFO) held a 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). However, neither workshop was designed to provide advice in response to question from fisheries managers.

In October 2010, the Department of Fisheries and Oceans convened a 5-day Zonal Advisory meeting of national and international scientists, fishermen, and fisheries managers to provide scientific advice on 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, and 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?

To attempt to answers these questions participants discussed the result of 32 scientific analyses covering the following topics (DFO Proceedings 2008/021):

  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.

In attempting to answer these questions, science is faced with considerable sources of uncertainty. This is because none of the processing affecting seal population dynamics, their consumption of prey, including Atlantic cod, the dynamics of cod and the ecosystem supporting seals, cod, other predators and their prey can be measured without error. Measurement uncertainty can be magnified as scientists attempt to estimate mortality of cod caused by seals because there is uncertainly in estimating the total population of seals, the proportion of the diet containing cod, the size of the cod population, to name just a few.

It is difficult to summarize the results of 5-days of discussion in just a few paragraphs and distilling those analyses and discussions into advice with respect to the management questions was both difficult and accompanied with much debate. Nevertheless, the conclusions of the meeting are summarized in the Science Advisory Report (2010/71). There was broad agreement on a number of conclusions. Grey seals inhabit three linked Atlantic marine ecosystems south of the Laurentian Channel: the southern Gulf of St. Lawrence (NAFO subarea 4T) which freezes in winter causing many fish populations to migrate and overwinter in the warm deeper waters off Cape Breton (NAFO subarea 4Vn) and two Scotian Shelf ecosystems (NAFO subareas 4VsW and 4X). There are distinct cod stocks in each of the three ecosystems. All stocks have shown declines of at least 80% in abundance and all remain low today. Overfishing reduced the stocks in 4T, 4Vn, and 4VsW to low abundance by the early 1990s. Overfishing also contributed to the lesser decline in the 4X stock up to the mid 1990s. Despite the severely reduced fishing mortality, survival of adult cod in 4T has remained at a low level over this period, and the stock has continued to decline. The 4VsW cod stock fell rapidly in the late 1980s leading to collapse, followed by fishery closure in 1993. Stock biomass remained low for over a decade but has recently shown an increase and improved survivorship. The 4X cod stock also experienced high stock mortality and continued to decline after the mid 1990s when fisheries were restricted.

There have been dramatic changes in these ecosystems over the past several decades. Groundfish stocks and fisheries in the southern Gulf have been replaced by small-bodied demersal fishes and invertebrate fisheries. Similarly, the eastern Scotian Shelf ecosystem groundfish fisheries have now been replaced by fisheries on invertebrate species such as shrimp and crab. On the western Scotia Shelf invertebrate fisheries have also increased to unprecedented levels, but some fishing for groundfish continues.

For management purposes, the grey seal population is divided into three herds based upon pupping sites. The largest herd numbering some 260,000 to 320,000 seals, depending on population model assumptions, occurs on Sable Island. The rate of increase of this herd has slowed from 12.8% during the 1980s to approximately 4% in the last 5 years. The southern Gulf of St. Lawrence (Gulf) herd numbers around 55,000-71,000 animals. The coastal Nova Scotia herd is the smallest of the three, numbering around 20,000-22,000 animals. Seals from each of these herds range widely throughout the year while foraging and may contribute to colonization of new breeding sites. Although little is known about historical abundance, current population size is the largest measured in the past several hundred years.

Determining the diet of grey seals relies on indirect methods because there are limited opportunities to directly observe what they eat. The methods used are based on recovery of hard parts such as fish ear bones from stomach contents, intestines, and feces, and the analysis of blubber chemistry in seals and their prey. Each of these methods has strengths and weaknesses. It is also difficult to obtain a representative sample of the diet from grey seals because they range widely and their diet varies by sex, season, area and other factors. Analyses of the above data sources indicate a wide range of values for the percentage of cod in the diet of grey seals; an overall average of 2-7% in 4VsW, and in 4T, from 1% for females in summer to 24% for the only sample of males in winter.

Models of food consumption indicate that grey seal consumption of cod in recent years lies within the range of 4,500 to 20,000 tonnes per year for 4T, and between 3,000 to 11,000 tonnes per year for 4VsW. These estimates themselves have high variance and their wide ranges reflect uncertainty attributable to the assumptions made to address gaps in sampling in 4T and the treatment of the diet data in 4VsW.

Culling is widely practiced as a means to limit predation on livestock and wildlife and can be effective at reducing predator abundance. Culling also has been used to reduce seal species. Although widely practiced, the extent of seal population reduction and the response of targeted prey populations to culls have rarely been evaluated. Results from other predator control programs indicate that unintended consequences in food webs, that will be difficult to predict, are nonetheless commonly observed. Thus, any intervention in the southern Gulf would first require a thorough investigation of the likely multi-species impacts of a cod-seal interaction in this ecosystem, and second would require a carefully designed program that would include clearly-stated objectives and rigorous monitoring of the seal and cod populations and the ecosystem to evaluate the consequences.

Adult male grey seal (brand M720, 24 years old) fitted with a Vemco mobile transceiver (back) and an Argos satellite GPS tag (head).

Adult male grey seal (brand M720, 24 years old) fitted with a Vemco mobile transceiver (back) and an Argos satellite GPS tag (head).
Photo: W.D. Bowen

Although there was broad agreement on the conclusions above, there was debate concerning the consequences of grey seal consumption of cod on recent and forecasted dynamics of cod, particularly in the 4T area. This is partly because of the added uncertainly assciated with estimating predation mortality on cod rather than just consumption of cod. In 4T, grey seals were considered by some to be significant source of mortality for large cod (>35cm) and other adult, bottom-dwelling fish. Satellite tracking indicates that some grey seals, in particular males, forage where large aggregations of adult cod occur. Digestive tract samples from seals foraging on overwintering aggregations of cod contain a relatively high proportion of cod (about 24% in males and 10% in females, based on intestine samples), and a high proportion (58%) of these cod were greater than 35cm in length. Others were less convinced that available information provided convincing suport for the impact of grey seals on this cod stock.

For 4VsW cod, the magnitude of grey seal predation compared to other sources of mortality varied widely with assumptions of several predation models. Most models left a large portion of mortality unaccounted for and attribute only a small (less than 17%) portion of total cod mortality to seal predation. Comparable information is not available for the mortality inflicted by grey seals on cod in 4X and 4Vn.

Despited the considerable amount and diversity of information brought to bear on this difficult issue, there is still much uncertainty regarding the impact of grey seal predation on the dynamics of cod stocks in Altantic Canada.

2.2 At-sea associations in grey seals: insights from a new data-logger (Don Bowen)

Figure 6: Movements of adult male (orange) and adult female (red) grey seals in the fall of 2009 with the location of at-sea encounters between instrumented seals indicated by white filled circles.

Figure 6: Movements of adult male (orange) and adult female (red) grey seals in the fall of 2009 with the location of at-sea encounters between instrumented seals indicated by white filled circles.

A great deal is known about the behaviour of pinnipeds during the breeding season. By contrast, little is know about the nature and extent of social interactions among pinnipeds while at sea foraging because their behaviour cannot be observed. Investigators at the Bedford Institute of Oceanography and Dalhousie University (Damian C. Lidgard, Ian D. Jonsen, and Sara J. Iverson) are using a novel acoustic technology to examine the nature of spatial and temporal associations among grey seal seals while at sea. Fifteen adult grey seals (Halichoerus grypus) from Sable Island, Canada were fitted with VEMCO Mobile Transceivers (VMTs) and Argos Satellite-GPS transmitters during October 2009. VMTs both transmit the equivalent of an acoustic fingerprint, which identifies each individual seal, and receives these "fingerprints" from other seals also fitted with a VMT. An encounter between two individuals included a cluster of detections that occurred in less than 30 minutes. Argos satellite tags transmitted GPS locations of seals so that the location of interactions could be determined. Two behavioural states (slow and fast movement) were assigned to GPS locations using a state-space model. Tags transmitted for an average of 73 days with an average of 95 locations per day. Twelve of the thirteen VMTs recovered showed seals interacted with other seals while at sea. More that 1,800 detections were recorded in about 200 encounters (Fig. 6). The median duration of an encounter was about 20 minutes and the median number of encounters per seal was 20. The spatial distribution of slow and fast behavioural states indicated that seals exhibited slow movement (thought to represent foraging) while on offshore banks where frequently consumed prey is known to occur, and fast movement (travelling) between these locations. Females were more likely to encounter other tagged seals when engaged in slow movement, while males showed no tendency with respect to either behavioural state. These data (the first of a multi-year study) suggest the occurrence of short-term associations at foraging grounds and provide new insights into the foraging ecology of this marine carnivore. Tagging prey, such as Atlantic cod (Gadus morhua) with coded acoustic tags may also provide a means of investigating predator-prey interactions.

2.3 Killer whale foraging specialization on Chinook salmon (John K.B. Ford)

Killer whales are the oceans' apex predators and one of the most widely distributed mammals in the world. As a species, killer whales can be considered a generalist predator, with a diet that includes a diverse array of different types of prey, including seals, dolphins and large whales, all types of fishes, from small schooling herring to whale sharks, many types of invertebrates, such as squids and octopus, and even reptiles, such as the leatherback turtle. Overall, almost 150 species of marine organisms have been documented as killer whale prey.

Despite this catholic diet of killer whales generally, field studies in several global regions have revealed that local populations of killer whales can have remarkably specialized diets, and may forage selectively for only a very small subset of the prey species that the predator is capable of consuming. These ecologically-specialized populations, or 'ecotypes', may have distinct patterns of seasonal distribution, social structure, behaviour and vocalizations that are strongly influenced by their particular predatory lifestyle. Different ecotypes of killer whales often have overlapping ranges in the same waters, yet they do not mix and are thus reproductively isolated. Such long-term reproductive isolation is thought to have resulted in such genetic divergence among some ecotypes that they are thought to potentially represent distinct species.

Long-term studies of killer whales off Canada's west coast by scientists with DFO's Pacific Biological Station have revealed the existence of three distinct ecotypes in the region, known as 'residents', 'transients', and 'offshores'. Residents feed on a variety of fishes and some squid, but their diet is dominated by salmon. Transients do not appear to feed on any fishes, but instead target marine mammal prey almost exclusively, including seals, sea lions, porpoises, dolphins, and small whales. The diet of offshore killer whales is poorly known but appears to include a high proportion of sharks, which may account for the unusually severe tooth wear seen in this ecotype (the skin of sharks is highly abrasive). These three ecotypes, which are each composed of populations of a few hundred whales, all share coastal waters of British Columbia but they have never been seen to travel together or mix in any way.

Pod of killer whales

Pod of killer whales
Photo: Brian Gisborne

Resident killer whales are the best known of the three ecotypes. These whales move seasonally according to the migration patterns of their main prey, Pacific salmon. They've long been known to congregate in good salmon fishing areas along the coast during the summer peak of the salmon migration to spawning rivers, and it was assumed that they likely fed on the five species of Pacific salmon roughly in proportion to each species' availability. However, dedicated field studies on the foraging behaviour of these whales revealed that is was not the case. Using scales and tissue recovered from the water at the site of salmon kills for species identification, it was discovered that resident killer whales feed preferentially on Chinook salmon, one of the rarest of the salmon in the area. Abundant species such as sockeye and pink salmon, which outnumber Chinook by 1000 or more to 1 during their summer migration, are surprisingly not significant in the whales' diet.

The whales' preference for Chinook salmon is understandable – it is by far the largest of Pacific salmon and tends to have the highest oil or fat content, making each fish a higher energy density than any of the other salmonids. Many stocks of Chinook salmon spend their entire life cycle in the whales' coastal habitat, making them available for year round feeding. Why the whales do not prey on the abundant sockeye and pink salmon is more difficult to explain. These salmon species are relatively small in body size and are only available to the whales for a brief time in the summer when transiting coastal waters while en route from the high seas to their spawning rivers. During this period, Chinook salmon are also migrating and are readily available as well. It is likely that the whales have a predatory focus on Chinook salmon throughout the year, and this focus continues even during the summer when other salmonids are more abundant. The whales' foraging tactics are likely finely tuned for efficient predation on Chinook salmon, and the smaller species may be more difficult to catch and are likely not as profitable.

Resident killer whale with Chinook salmon

Resident killer whale with Chinook salmon
Photo: Brian Gisborne

Chinook salmon appear to be so important to resident killer whales that the availability of this single prey species may be critical to their survival. After two decades of slow but steady population growth, the two separate populations of resident killer whales along the west coast, the northern and southern residents, went into a sharp decline in numbers during the late 1990s. Demographic analysis revealed that this decline was driven primarily by a dramatic increase in mortalities during this period, and secondarily by reduced calving rates. Although both the northern and southern populations stabilized and even began increasing slightly in the early 2000s, they were listed Threatened and Endangered, respectively, under Canada's Species at Risk Act in 2003. An analysis of the coast-wide abundance of Chinook salmon over a 25-year period revealed a very strong correlation between whale survival and Chinook salmon abundance. Most striking was that resident killer whale mortalities spiked to levels 2 to 3 times higher than expected during the late 1990s, when Chinook abundance dropped by almost half the long term average for several consecutive years.

As resident killer whale recovery may depend on sufficient availability of Chinook salmon, it is important that we have the best understanding possible of seasonal and spatial patterns of predation by these whales and the potential effects that human fisheries may have on Chinook abundance. Our research is currently focused on improving our understanding of the Chinook salmon stocks that are important to killer whales through genetic stock identification from prey fragments, and estimating the numbers of Chinook salmon that are needed to sustain current whale abundance and provide for future population growth. We are also working closely and collaboratively with whale and salmon scientists and managers both within DFO and with the National Ocean and Atmospheric Administration (NOAA) in the United States to determine whether existing fisheries are potentially having an effect on the recovery of resident killer whales.

2.4 Blue whale feeding ecology in the St Lawrence River estuary (Véronique Lesage, Thomas Doniol-Valcroze)

Feeding is central to an animal's life history and ecology. Large predators do not feed continuously but rather in bouts of intense activity separated by periods of searching, resting or socializing. Moreover, feeding does not occur randomly in space, as animals select precise areas with characteristics of prey density, accessibility and predictability that maximize their chances of meeting their energy requirements. Every summer, blue whales from the endangered North Atlantic population come to the St Lawrence River estuary to feed on dense aggregations of euphausiids. Documenting the timing and location of foraging success is therefore of utmost importance to assess and monitor habitat quality on this feeding ground.

In marine systems, however, feeding happens mostly under the surface and is rarely observable directly. In this study, we have used data-loggers to record, at every second, the depth and swimming speed of 10 blue whales during their dives in the St Lawrence estuary. By detecting the rapid speed changes that are characteristic of lunging behaviour and mouth opening, we have been able to pinpoint the exact moment, depth and location of each feeding attempt. With this information, we have shown that blue whales feed at all times of the diurnal cycle and intensify their feeding activity at night when prey are accessible at shallow depths. This is in contrast to previous assumptions in the literature that blue whales did not feed at night. Using radio-telemetry, we have also been able to describe the habitats where blue whales concentrated their feeding effort, and how different habitats were used at different phases of the tidal cycle (e.g., feeding at the shelf edge when flood tidal currents were concentrating euphausiids against the steep slopes).

Blue whale

Blue whale
Photo: Thomas Doniol-Valcroze

Moreover, we have shown that St Lawrence blue whales used optimal strategies to adapt their dive times and feeding effort to the depth of their prey. In particular, feeding rates were consistently higher when blue whales performed short feeding dives at shallow depths. These results suggest that diving predators may judge habitat quality in terms of prey accessibility at shallow depths rather than selecting habitat solely based on prey density or abundance. Taken together, these strategies may allow blue whales to optimize a short seasonal window of feeding opportunity and maximize resource acquisition. Indeed, feeding rates diminished over the summer feeding season, and were negatively correlated with the time each animal spent in a social pair, suggesting a trade-off between feeding and socializing with the approach of the breeding season. Better understanding of the behaviour and feeding ecology of large whales can help predict their responses to environmental changes and anthropogenic pressures.

This project was conducted in collaboration with Robert Michaud and Janie Giard from the Group for Research and Education on Marine Mammals in Tadoussac, Quebec.

2.5 Baleen whale feeding ground oceanography: the oceanographic trap (Yvan Simard)

Figure 7: Example of the functioning of the 'oceanographic trap' from the Saguenay–St. Lawrence Marine Park baleen whale feeding ground of the St. Lawrence estuary.

Figure 7: Example of the functioning of the "oceanographic trap" from the Saguenay–St. Lawrence Marine Park baleen whale feeding ground of the St. Lawrence estuary.

A group of killer whales in the Churchill estuary area taken on the same day predation by killer whales on beluga was observed 20 km to the west in Button Bay, 27 August 2011.

A group of killer whales in the Churchill estuary area taken on the same day predation by killer whales on beluga was observed 20 km to the west in Button Bay, 27 August 2011.
Photo: Pete Ewins

What makes particular ocean areas especially attractive for feeding baleen whales? Several such ecosystem hot spots are "oceanographic traps" for their preferred zooplankton food. An example of how these systems are working is presented in Figure 7, for the baleen whale feeding ground of the Saguenay–St. Lawrence Marine Park.

This traditional feeding ground already existed when the first European whalers arrived 450 years ago. Today, this area is one of the most intensive whale-watching sites of the world. What are the fundamental processes responsible for the making and long persistence of this food-rich area, regularly visited by Northwest Atlantic whales? This is the question a multi-disciplinary team addressed in the ecosystemic and oceanographic research program briefly summarised here.

Using multi-frequency hydroacoustics, oceanographic measurements, plankton sampling, and high-resolution 3D circulation modelling coupled with ground-truthed krill behaviour models, the merged results clearly evidenced how ocean processes combine to trap krill in predictable locations, notably channel and canyon heads, where special habitat properties converge. This functioning mode 1, called "the oceanographic trap", involves here: 1) the underwater topography; 2) the strong and persistent 2-layer estuarine circulation that sorts krill by size and pumps adult krill towards the channel head; 3) the negative phototactism of krill to avoid visual predators, which drives their diel vertical migrations, and which forces them to concentrate under upwelling conditions occurring along slopes every tidal cycle, very strongly and intensively at the head of the Laurentian channel; and finally 4) the advection at semi-diurnal and fortnight tidal frequencies as at lower frequencies, which modulates the whole process by imprinting its variability in the aggregation and dispersion of krill. The krill aggregation process depicted by this research is most likely general and applicable to other whale feeding grounds with this food aggregation mode 1. Details can be found in Simard (2009) and the references listed below.

Present ecosystemic research is addressing the transport coupling between the adult krill source region, in the Gulf, and the aggregation at the channel head in the Marine Park, using a series of ocean observatories that are monitoring the currents and krill biomass at several locations along the transport route and continue over the annual cycle. Eventually, indicators of baleen whale ecosystem state could be developed by merging the observatories results with coupled circulation and krill modelling.

2.6 Arctic killer whale predation (Steve Ferguson)

Killer whales (Orcinus orca) have a global distribution, but many high-latitude populations are not well studied. Anecdotal evidence, sighting reports, Inuit traditional knowledge, and photographic identification indicate that killer whale occurrence in Hudson Bay is increasing. Killer whales were not known to be present in the region prior to the mid-1900s but have since shown an exponential increase in sightings. Killer whales have been observed preying on a number of marine mammal species in Hudson Bay. Of particular concern is predation on bowhead whales (Balaena mysticetus) in Foxe Basin, narwhal (Monodon monoceros) in northwest Hudson Bay, and beluga (Delphinapterus leucas) in southwest Hudson Bay. The impact of killer whale predation on marine mammal species is unknown without long-term studies and direct observation of killer whale hunting behaviour. We conducted a semi-directed interview survey of Traditional Ecological Knowledge to provide additional information on the feeding ecology of killer whales. Local resource users are knowledgeable observers of their environment, and Inuit hunters and community elders have extensive knowledge about killer whales. Using this information and defining killer whale energetic requirements and considering population demography of prey, we can begin to assess the basic requirements of predator–prey dynamics in Hudson Bay marine ecosystem. To estimate predation impact we used a simple mass-balanced marine mammal model that included age structure, population size, and predation rate inputs. For the Hudson Bay region, model results indicated that killer whales do not show strong prey specialization and instead alternatively feed on narwhal and beluga whales early and late in the ice-free season. Evidence does support the conjecture that during the peak of the open water season, killer whale predation can focus on bowhead whale prey. The mixed foraging strategy used by killer whales included seasonal predator specialization and has management and conservation significance since killer whale predation may not be constrained by a regulatory functional response.

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