3.0 How Marine Mammals Fit Into the Ecosystem
3.1.1 Arctic Killer Whale Information from Traditional Ecological Knowledge and Sighting Networks
Killer whale sightings in the eastern Canadian Arctic have increased in recent years, especially in Western Hudson Bay, and have shown a recent advance in distribution with climate change. However, little else is known about their distribution and ecology.
The Orcas of the Canadian Arctic (OCA) project was initiated in 2005 as a collaboration with universities, provincial, territorial and federal government, industry, consulting companies and Inuit organizations. A study on killer whales in the Canadian Arctic was initiated in 2006, and has continued through 2007 and 2008, to monitor killer whales in this region.
Inuit traditional knowledge of killer whales has been collected since August 2007; initial efforts focused on the Hudson Bay region. To date, interviews have been conducted in five communities: Repulse Bay, Igloolik, Hall Beach, Rankin Inlet, and Arviat. Additional interviews are planned for 2008 through 2010. Interviews thus far have provided a wealth of local knowledge on Arctic killer whales, including distribution, migration and movement patterns, seasonality, and predation impacts on other marine mammal species.
At the same time, a sightings database has been created and has been evaluated for general sighting reliability, with a combined focus on species identification, observer type, and spatial and temporal accuracy. A photographic database is also in development, which will allow for the identification of individual killer whales.
Traditional Ecological Knowledge (TEK) can be defined as the knowledge claims of persons who have a lifetime of observation and experience, but are untutored in the conventional science paradigm. The use of TEK and its integration with science is prompted by many because it reflects the resource use and long observational experience of local people, and because it can provide a longer historical record than scientific data in remote areas. In contrast, western science relies on an experimental approach and hypothesis testing to obtain information on natural processes.
TEK and science differ in the ecological information they provide, both in observational intensity and geographic coverage. These differences may lead to separate conclusions about the seasonal distribution and aggregation of beluga populations and thus influence management decisions. To date, few studies have been attempted to analytically compare and contrast the two data sets. The collation of TEK into a structured format for review to understand the strengths and weaknesses remains a challenge for both scientists and the holders of TEK alike.
In this study, information on movements and aggregation of beluga whales were obtained from TEK interviews with 427 hunters resulting in 3,253 records maintained in a database by Makivik Corporation, Kuujjuaq, QC. Satellite transmitters were deployed on 30 beluga whales in eastern Hudson Bay, Canada. Seasonal distribution and movements were compared using GIS approaches that allowed common formatting of the datasets.
Estuarine centres of aggregation in the summer were evident in both datasets (Fig. 4). The telemetry data showed that beluga made extensive use of offshore waters where 76 percent of the locations were greater than 15 km from mainland Quebec. However, these offshore movements were not evident in the TEK data, where 83 percent of the records indicated that beluga were limited to inshore regions. The telemetry data also showed that beluga whales remained within the Hudson Bay arc region throughout the summer, but left this area to overwinter off the coast of Labrador. The TEK data reported the presence of beluga all around the coast of Nunavik throughout the summer and also the presence of beluga whales in the Hudson Bay arc region during winter.
Results from the two approaches underline the importance in understanding how the data are collected. Both approaches represent different sampling methods that have both strengths and weaknesses. The satellite telemetry provides independent information on whale movements, particularly of whales of known origin; in this case of whales from the eastern Hudson Bay (EHB) population. However, the deployments are relatively few (n=30) and on average, the transmitters are lost from the animals after about three-four months. The TEK observations cover a longer period of time and a larger area, but observations are largely coastal because it is dangerous to hunt offshore in small boats. The TEK data also failed to detect the overwintering off the Labrador coast because of the difficulties in winter travel, the short days, and the large area that beluga whales moved through.
However, TEK detected that some whales overwintered in the EHB area. Where do these whales come from? Do some whales from the EHB population actually overwinter in this area as well? Or do the overwintering whales belong to another population that summers elsewhere-from James Bay for example? The TEK data also indicated that some beluga are found all along the Nunavik coast during summer. Are these whales that are observed in the Hudson Strait area in summer just stragglers from the migrating populations that exit Hudson Strait in summer? Or, do they represent some other remnant population that summers in this area?
From this study, it is evident that both data sets can provide complementary information and when this information is in agreement, conclusions are likely to be more solid. However, when the different approaches lead to different conclusions, it is important to understand why these differences exist. In most cases this involves a clear understanding of the different sampling methods that are employed, their strengths and their weaknesses.
3.2.1 Diving Characteristics and Sightability Estimates of Eastern Arctic Bowhead Whales Based on Satellite-Linked Telemetry
Bowhead whales in Canada consist of two populations, one that summers in the Beaufort Sea in the western Arctic, and one that resides in waters of the eastern Canadian Arctic and west Greenland. The eastern Arctic population was once considered to be two unique populations. However, recent evidence strongly suggests that they belong to a single population, likely segregated seasonally by age and reproductive class. The strongest evidence for this conclusion is the result of satellite-linked tracking.
The tracking and monitoring of whale behaviour with satellite-linked and data-archiving tags has provided an important window of opportunity to document what was previously unattainable for such an elusive and remote species. The tracking of long-range movements of bowhead whales has demonstrated that these whales, once thought to maintain two discrete ranges, actually share both summering and wintering areas. Analysis of movements and identification of seasonal residency have provided new information on migration routes, new insight into life history, and the potential importance of particular habitat.
Using the latest satellite-linked telemetry technology, tagged animals have also provided information on dive behaviour, demonstrating that bowhead whales can dive to depths of 400 meters. Among the valuable information provided by examination of diving behaviour, the proportion of time spent out of sight is important when estimating the abundance of bowhead. It turns out that bowhead whales spend close to 75 percent of their time at depths greater than four meters.
Ringed seals contribute to the bulk of the Inuit subsistence harvest of marine mammals and are the main food resource for polar bears. The evolutionary adaptations of ringed seals to exploit the land-fast ice habitat for reproduction and survival could expose this species to critical challenges with predicted global warming. Concerns have arisen over possible declines in ringed seal numbers in western Hudson Bay, as indicated by hunter knowledge, reduced pregnancy rate, reduced pup survival, older age structure, and reduced growth and number of polar bears. Four aerial surveys that took place from 1995-2000 estimated the population size as declining from 70,000 to 45,000 seals.
An aerial survey was conducted in May 2007 and capture and tagging of ringed seals was conducted in September 2007. The goal of the field work was to collect samples and tag seals, in order to better understand ringed seal movements and foraging ecology in Hudson Bay. Satellite tagged seals will provide information on how often seals are not visible from planes and thereby allow correcting the aerial survey estimates. Basic biological information, such as condition and morphology, as well as samples from seals, both live and dead, were also collected.
3.2.3 Development of PAM Methodology to Non-Intrusively Monitor Whales in their Environment with Examples from the St. Lawrence Seaway and Arctic
Marine mammals intensively use underwater acoustics to communicate, navigate, and detect prey and predators. Like birds, many species and sub-groups can be identified by their specific calls. Recording these signature calls then reveals the presence of the species in the monitored area. As sound propagates very efficiently in water, the detection area can be quite large, exceeding 100 kilometres in favourable conditions for low-frequency vocalizations. This surpasses visual detection ranges by a large factor. Thus, marine mammal scientists from various disciplines have worked for several decades to harness this acoustic potential to non-intrusively detect and monitor whales in their environment. With the rapid development of knowledge and technology in this field a new methodology, named PAM (Passive Acoustic Monitoring), is emerging (Fig. 5).
Various PAM systems have been used throughout the world, ranging from simple recording systems and a single hydrophone thrown overboard to large-scale shore-cabled military systems to hear entire ocean basins. In collaboration with the University of Quebec at Rimouski, a research program was launched in 2002 to develop and explore PAM systems to monitor whales in the St. Lawrence Estuary.
After developing an autonomous hydrophone, called AURAL, to record audio files, a fleet of them was deployed in the Saguenay-St. Lawrence Marine Park during the summers of 2003-2005 to explore the possibility of different configurations to detect and localize blue and fin whale calls. Several localization methods were explored to find the position of the whales in the 75-km long basin. Despite the particular difficulty of the study area, propagation modeling was combined with noise characteristics to show that the technology can detect and localize blue and fin whales from their calls, with a high efficiency. A study is ongoing to monitor the local beluga population.
The plan for the next generation of these systems is to operate in real-time and have artificial intelligence to automatically solve the detection and localization of the calls. Additionally, a three-year research project to develop a network of cooperating intelligent buoys has been conducted. This system will be tested in the field in coming years. In the meantime, AURALs were deployed at several locations in the Arctic and Hudson Bay in 2004 to monitor the changes in the timing of seasonal occupation of bowheads, beluga, and bearded seals. Results are starting to reveal clear relations with changes in ice conditions.
Many animal species segregate by sex. Such segregation may be social in nature, or ecological, or both. Grey seals, like many large mammals, as sexually size dimorphic. To investigate sexual segregation of habitat in grey seals, satellite tracks from 95 adults breeding at Sable Island were collected from 1995-2005 (Fig. 6). Differences were most pronounced just before (October-December) and immediately after (February-March) breeding. During both periods, males primarily used areas along the continental shelf break, while females mainly used mid-shelf regions. These differences may serve to maximize fitness by reducing intersexual competition during key foraging periods.
Season and sex explains most of the observed variation in adult diets. Estimates of diet were derived from the analysis of fatty acids (the building blocks of fat) in blubber biopsies taken from free-ranging individuals using a method called quantitative fatty acid signature analysis. The differences were most evident during the post-breeding foraging period when energy acquisition is important to female recovery of nutrient stores needed to support pregnancy. Females selected fewer and higher quality prey species in spring than males. There were no sex differences in the diets of juveniles.
The frequency of feeding in seals has traditionally been inferred from the state of digestion of prey in stomach contents. These estimates are rather imprecise and may not be representative of entire foraging trips to sea. In a new study, small radio transmitters were placed in the stomachs of adult grey seals. These transmitters continuously recorded changes in stomach temperature associated with feeding. The results showed that the number of feeding events is typically greater in males than females, as is the time associated with feeding per day. Seals, on average, fed on 57.8 percent of days, and had an average of 1.7 meals per day. Grey seals tended to have many single feeding events with long periods separating each event, as was expected for a large carnivore. These results provide new insight into the basis of sex differences in diving and diet.
Harp and hooded seals are two of the most abundant marine mammals in the North Atlantic. As such, both species play important roles in structuring this ecosystem. Considerable research has been carried out over the past decade to determine their ecological role and the potential impact of seal predation on the population dynamics of their prey. However, these are extremely complex problems and definitive answers are not easy to determine.
The development of satellite telemetry has provided scientists with the opportunity to improve their understanding of the movements and habitat use of free ranging seals. Both harp and hooded seals are pelagic species that spend much of their time in the open ocean. Harp seals are found mainly along the continental shelves where they dive to relatively shallow depths (100-200m) although they have been observed diving as deep as 800m. In contrast, hooded seals inhabit the edges of the continental shelves and deep water slopes. Hoods regularly dive to depths greater than 300m and, occasionally, deeper than 1500m.
Both species feed on a variety of fish and invertebrates. The exact diet varies with age, sex, location, season and year. In general, harp seals feed upon a variety of small forage fish such as capelin, Arctic cod (or Polar cod in Europe, Boreogadus saida), herring and sand lance. They also prey upon invertebrates such as amphipods and shrimp. Although hooded seals feed on many of the same species, they tend to take larger amounts of deep water species such as Greenland halibut and redfish which are found along the shelf edges.
Traditionally, diets have been determined using hard parts found in the stomachs. However, every method has potential biases that may reduce the accuracy of diet estimates. New techniques such as fatty acid signatures and DNA analysis of stomach contents are providing new information on the diets over longer temporal and spatial scales. They are also providing new insights into the importance of individual prey species consumed by harp and hooded seals, and provide an opportunity to determine the extent of biases associated with each method.
Consumption of important prey species by seals in Atlantic Canada has been estimated using bioenergetics models. Harp seals are important predators off the east coast of Newfoundland and in the northern Gulf of St. Lawrence while hooded seals feed primarily off Newfoundland and around the Flemish Cap.
A number of studies have attempted to determine the impact of seals on fish stocks in the northwest Atlantic, particularly the impact of harp and/or grey seals on Atlantic cod. In general, these studies have indicated that although seals consume substantial amounts of commercial fish species and important forage species, the impact of these removals on the current fish stocks is difficult to determine. Seals are important predators of both large and small cod and could be playing a role in the non-recovery of cod stocks, but seal predation can not account for a large component of mortality in most areas and therefore, the total impact of seal predation cannot be determined. Often, estimates of age specific cod consumption by seals are inconsistent with the high mortality observed among older age groups. Little is known about the functional response of seals to changes in abundance of prey, other sources of mortality, or possible ecosystem effects such as competition for forage fish and positive feedback through seal predation on piscivorous fish.
3.4.1 Report from National Workshop on the Impacts of Seals on Fish Populations in Eastern Canada
A five-day day meeting was held in November, 2008, in Halifax, Nova Scotia, on topics related to the potential impacts of seals on fish stocks in Eastern Canada. Seals are hypothesized to have five kinds of negative natural effects on prey populations:
- transmission of parasites causing increased mortality of fishes,
- disruption of spawning, causing reduced reproductive success, and
- other indirect effects on fish behaviour caused by risk of seal predation. This was the second of two workshops and presentations focussed on new analyses and model results arising from research identified at the first workshop. The second meeting was attended by 30 invited participants from Canada, Norway, and the United States of America. Members of the fishing industry, graduate students from Dalhousie University, and interested scientists from the Bedford Institute of Oceanography also attended parts of the meeting. The principle objective of these workshops was to review what is known, identify gaps in our understanding, and determine what could be concluded about the impacts of seals on fish stocks in eastern Canada. A report of the meeting will be completed early in 2009.
The continental shelf ecosystem of the Eastern Scotian Shelf has experienced drastic changes. Once-common top predators are now a small fraction of their historical abundance, and much of the current community structure is now dominated by pelagic fishes and invertebrates. Within this food web, Atlantic cod and grey seal populations have exhibited opposite trends. Since 1984, cod populations have decreased exponentially at a rate averaging 17 percent per year. At the same time, grey seals have continued to increase exponentially, at a rate averaging 12 percent.
The impact of grey seals on Atlantic cod dynamics was re-examined using more than 30 years of data on the population trends of cod and grey seals, and incorporating new information on seal diet and seasonal distribution. The closure of the cod fishery over ten years ago allowed for a better estimation of natural mortality rates.
Between 1993 and 2000, cod comprised, on average, approximately two percent of a grey seal's diet. Since the closure of the fishery, grey seals have imposed a significant level of instantaneous mortality, 21 percent of the total mortality on average. However, most of the natural mortality on cod during this period was from unknown sources. Nevertheless, given that cod abundance declined during the study, grey seal consumption of cod did contribute to the failure of the cod stock to recover.
Killer whales – the most important aquatic predator of most marine mammal species – have recently been hypothesized to have driven the collapse of populations of seals, sea lions, and sea otters in the North Pacific. This hypothesis maintains that industrial whaling of large, baleen whales in the 20th century eliminated this source of preferred prey of mammal-hunting killer whales, which forced these predators to switch to alternative prey species. This idea has not been widely accepted for various reasons, including the lack of evidence that killer whales have ever been a major predator of large, baleen whales. However, this point raises an interesting question – if killer whales do not prey routinely on the great whales, why not?
To better understand the tactics and strategies that baleen whales use to avoid these predators, published reports and unpublished observations of predator-prey interactions involving killer whales and baleen whales were compiled and examined. This study revealed that baleen whales have two distinct and contrasting antipredator strategies when harassed or attacked by killer whales: the fight strategy, where whales group together and physically defend themselves, and the flight strategy, where whales flee at high speed and, if overtaken and physically attacked, make no attempt to defend themselves.
Fight species include bowhead whales, right whales, humpback whales and grey whales, all of which have relatively robust, stocky body shapes and are relatively slow but maneuverable swimmers. These whales typically escape from killer whale attacks by rolling and splashing at the water's surface, and striking at the predators with their tail flukes. In contrast, the flight species – minke whales, sei whales, fin whales and blue whales – all share the same highly streamlined body shape and are fast swimmers. These whales usually escape successfully from pursuing killer whales by fleeing at high speeds of 20-40 km/hr.
Many aspects of the life history, behaviour, and morphology of baleen whales are consistent with their antipredator strategy, and it appears that evolution of these traits has been shaped by selection for reduced predation. For example, fight species often congregate for breeding in shallow nearshore waters, where they are better able to defend themselves and their calves from killer whales. Most flight species seldom enter confined waters that might inhibit an escape sprint. Fight species also tend to have encrustations on their bodies (callosities and barnacles) that may serve as armour or weapons for enhanced defense.
Overall, it seems that these divergent antipredator strategies of baleen whales are highly effective for avoiding predation by killer whales, particularly for adults. As a result, killer whales seldom attack these large whales and focus instead on smaller, easier to catch and more profitable mammalian prey. It seems doubtful that depletion of the great whales by whaling had a significant influence on killer whale dietary habits.
The harbour seal is one of seven pinniped species found in the northwest Atlantic. Although this species is well studied throughout much of its distribution, relatively little is known about the ecology and population status of these seals in Newfoundland and Labrador waters. The objectives of this project were to determine baseline contaminant profiles for harbour seals and their major prey species, collect the necessary ecological data to evaluate if the species might be a good indicator of ecosystem health, and begin considering how to use data for monitoring longer-term cumulative effects of future coastal developments related to offshore oil production.
Based on a limited number of reconnaissance boat surveys, opportunistic shore-based haul-out counts, and interviews with fishermen, harbour seals are relatively common but patchily distributed in low densities. The exceptions are the northwest and northeast coasts of Newfoundland and Labrador where there have been few observations. It appears that local abundance of seals at known haul-out sites along the south and southwest coasts have increased; abundance at more northern sights has remained low.
Generally, harbour seals are known to consume a wide variety of prey and their diet varies geographically, seasonally, and is age dependent. Analyses of stomach samples collected from 1985-2003 indicated consumption of approximately 30 different species of fish and invertebrate prey, but ten species accounted for almost 95 percent. Winter flounder, Arctic cod, short-horned sculpin, and Atlantic cod were the most important.
From 2001-2003, a total of 66 tissue samples were collected throughout the province, and analyzed for total mercury, trace elements, and total persistent organic pollutants (POP) levels. The concentrations of trace elements and total mercury were generally comparable to other northern seal species. Total mercury, selenium, and cadmium showed the greatest variability within, and among, sampling sites; seals sampled along the south and east coasts of the province had the highest concentrations of renal cadmium. The source of cadmium is unknown at this time.
Based on the suite of POPs examined, harbour seals sampled from Newfoundland waters were less contaminated than those from the St. Lawrence Estuary population, and generally similar to those from the southern Gulf of St. Lawrence. Mirex and PCB concentrations were five-ten times higher in the Estuary population while DDT and chlordane were two-five times higher than in Newfoundland seals. Mature males had higher POP levels than females, but there were no differences between the sexes of juveniles.
These new data on harbour seals will provide a basis for future ecological studies, population assessments, development of ecosystem indicators for coastal areas, and research addressing how contaminants accumulate in coastal food chains.
Sea otter aggregations, called rafts, can include over 100 individuals resting as a group in the water. Males and females segregate and occupy spatially distinct raft areas. Rafts form consistently in the same area over periods of years. From field observations, there appear to be differences in the characteristics of rafting locations used by males and females, as well as differences in the daily formation of rafts. Female rafts are more numerous than male rafts.
In this study, the characteristics of 49 raft sites were measured in a GIS environment. Physical characteristics included average depth, distance from large and small land masses, exposure, and bottom complexity. Male rafts were, on average, more protected from the southeast than were female rafts. Several male rafts were located in inlets, whereas no female rafts were. Male rafts were closer to large land masses, and female rafts were closer to small land masses. Rafts of both genders occurred in depths less than 20 meters.
Gender differences in raft site characteristics likely reflect differences in the needs and constraints confronting adult male and female sea otters. Adult females give birth to a single pup each year. Females care for their pups for six to eight months. Large land masses are a potential source of terrestrial predators and bald eagles, which are known to prey on pups. Land masses less than 50 meters were typically associated with rock reefs and variable shallow habitat, features providing a variety of shallow foraging habitat. To obtain the benthic invertebrates upon which sea otters feed, females must leave their pups unattended at the surface while they dive. Shallow depths may be preferred by females with pups.
3.5.3 From Physics to Whales: The Example of the Baleen Whale Feeding Ground of the St. Lawrence Estuary
Baleen whales from the northwest Atlantic migrate annually to the Gulf of St. Lawrence during the ice-free season to refill their fat reserves by foraging on their preferred prey. This intensive annual feeding is fundamental to the health of the individuals, their reproduction, and growth of the population. The question is what makes and controls these aggregations.
Using an ecosystem approach, this question was addressed by a systematic research plan, in order to understand the oceanographic processes determining the prey aggregations at a traditional whale feeding ground of the St. Lawrence system. Located at the upstream end of their annual migration route, it has been regularly visited by several species, notably blue, fin and minke whales. This high perennial recurrence is indicative of the presence of a persistent process that steadily maintains the aggregated preys at the head of the channel.
The research program showed that this area is the site of the richest krill aggregation in the northwest Atlantic. The mechanism (Fig. 7) responsible is the pumping of the adult krill from a large part of the Gulf of St. Lawrence. Tidal interactions of currents and krill with the channel slopes concentrate adult krill in up-welling slope currents. This current feeds the Marine Park krill aggregation. Pelagic forage fish such as capelin also aggregate in this area. Further research on their aggregation dynamics at the channel head and the Saguenay fjord entrance is underway.
Intensive hydro-acoustic measurements of krill distribution of the whole Gulf of St. Lawrence were required to get the full picture of an exceptional perennial baleen whale feeding ground in Eastern Canada. Modeling also pointed out areas of other krill aggregations in the St. Lawrence system, which matches the published data from a survey over the Gulf.