Project 1.2.4

Grey Seals (Halichoreus grypus) as Bioprobes: Spatial and Temporal Interactions With Prey and Physical Oceanography

PIs: S.J. Iverson, W.D. Bowen, I. Jonsen

Co-PIs/Collaborators: L. Comeau, K. Fennel, M. Hammil, J. Sheng, D. Swain, K. Thompson

HQP: D.C. Lidgard, S. Lang

Overview. We plan two studies to better understand the ecological function of large marine carnivores in continental-shelf ecosystems, using grey seals as our model species and novel acoustic technology developed for OTN: 1) spatial and temporal patterns of prey encounters by a mobile, large marine predator, and 2) predator movements and foraging distribution in relation to fine- and meso-scale seasonal oceanography in eastern Canada.

The first study will contribute to science advice on impacts of pinniped predation on the dynamics of prey populations of commercial or conservation importance. The second study seeks to understand the oceanographic features that grey seals may use to condition the way they search the environment for food and in turn predict how climate variability and long-term change may affect upper-trophic level predators and alter their impact on continental shelf ecosystems.

Project (A). Spatial and temporal patterns of prey encounters by a large, marine predator: using acoustic Vemco Mobile Transceiver tags on grey seals.

Scientific Rationale. Most predators must search their environment for prey. Predator-prey interactions have important population and community level consequences that depend on the frequency of predator and prey encounters. How pinnipeds and other large marine predators exploit the patchy distribution and abundance of prey is poorly understood because it is difficult to measure the relevant variables by direct observation.

We are using grey seals equipped with newly developed two-way, coded acoustic tags (Vemco Mobile Transceivers, VMT) and GPS/satellite-linked location telemetry, to study the temporal and spatial patterns of prey encounter rates. We are also estimating the diets of our study animals using quantitative fatty acid signature analysis (QFASA). Understanding the scales of foraging success and diet in wide-ranging generalist predators, such as the grey seal, is essential with respect to modeling the impact of seal predation mortality on commercial fish stocks and species at risk. The study will focus on two depleted populations, Atlantic salmon (Salmo salar) and Atlantic cod (Gadus morhua), in continental shelf waters of Atlantic Canada. It has been suggested that grey seal predation may be impeding the recovery of both species, despite the fact that current data from stomach and fecal contents as well as QFASA indicate that cod comprise a small fraction of the diet. It is possible that our understanding of grey seal diet is biased and thus, the use of VMTs on grey seals and coded acoustic tags on prey provide a novel approach to answering these critical management issues. We will also for the first time be able to use novel acoustic telemetry and GPS to investigate whether associations among seals occur at sea, and if so whether they occur due to the sharing of a common resource (i.e., by chance) or exhibit some level of social structure. In addition to assessing species trophic interactions, we will use these data to help validate behavioural state-space switching models, and will collect fine- (<500m ) and meso-scale information on the distribution and movements of prey species that have been tagged with coded acoustic tags.

Project (B). Grey seal movements and foraging distribution in relation to fine- and meso- scale seasonal oceanography on the Scotian Shelf.

Scientific Rationale. Understanding how marine animals use the ocean is crucial for the development of conservation and management strategies both for individual species and continental shelf ecosystems threatened by climate change. Through use of satellite tags it is now possible to identify migration corridors and hot spots frequented by animals. These habitat use patterns may increase the heterogeneity in predation mortality, the probability of incidentally catching endangered species, and may also help locate areas of biological importance. Locating migration corridors and hot spots and understanding how animals use these critical areas will be needed to effectively develop and apply management strategies for sustainable exploitation of commercial species while minimizing by-catch of endangered species.

Several oceanographic properties might condition the way animals use the ocean. In some cases, the oceanographic influence may directly affect the way the animal moves whereas in other cases the oceanographic features may serve to concentrate prey making foraging more profitable for the predator. Pinnipeds are large homeotherms and as such their movements and habitat use ought to be strongly influenced by prey availability. The ocean observing system proposed in Theme 1 will allow us to investigate the extent to which seals respond to prey availability at scales that are relevant to the response of prey to the physical and biochemical environment.

Here we will study the influence of the physical and biochemical environment on the movement and habitat use of grey seals. This will only be possible due to the synthesis of physical and biochemical measurements collected from OTN lines and autonomous gliders to generate a dynamically consistent, time-varying, three dimensional view of the ocean.

Summary of work to date

The objectives of this study are: to examine the hypothesis that grey seals are responsible for the high levels of natural mortality among adults in two declining and depleted Atlantic cod stocks, southern Gulf of St. Lawrence and Eastern Scotian Shelf, and to provide data on grey seal foraging locations to help validate behavioural state-space switching models.

To date, 35 grey seals from Sable Island have been deployed with telemetry instruments (Table 1).

Table 1. Deployment and recovery of telemetry instruments on grey seals, Sable Island, 2009 to 2011
Trip Dates Deployment Recovery
October 13-30, 2009 7 males, 8 females
December 9, 2009 to February 3, 2010 5 males, 8 females
September 7-21, 2010 6 males, 14 females
December 9, 2010 to January 26, 2011 6 males, 14 females

Each seal had the following instruments deployed: GPS-ARGOS satellite tag (Wildlife Computers), VHF Temple tag (Advanced Telemetry Systems), and BCT (Business Card Tag) tag (Vemco).

  • GPS-ARGOS satellite tag: these tags collect geographical locational, depth, and light data. These data are important for determining where interactions between individual seals and seals and cod occur.
  • VHF Temple tag: used for relocating seals when they return to Sable Island.
  • BCT: This tag alternates between acting like a 69kHz coded transmitter, emitting a unique ID, and acting like a monitoring receiver, collected unique IDs from other seals and fish. The BCT is designed for use up to 1000m depth, making it suitable for deep diving animals such as seals. When an ID is received, the date and time of the received tag ID is recorded. Thus, with the known locations from the GPS-ARGOS tags we are able to determine where interactions between seals and between seals and fish occur.

For each capture, the animals were weighed, immobilised with Telazol (tiletamine and zolazepam), and body measurements taken. The summed weight of tags was 0.002% of deployment body weight. Of the seals deployed, data have been obtained from 33 of the 35 seals; two seals failed to return to Sable Island and thus data were lost. Of the two that were not recovered, one remained close to Brown’s Bank and never returned to Sable Is., and the GPS-ARGOS instruments on the second animal failed and thus the animal could not be relocated.

Two hundred Atlantic cod have been caught and deployed with VEMCO acoustic transmitters in the southern Gulf of St. Lawrence (September 2009 and 2010), and 100 have been deployed on cod in NAFO Division 4W on Canso Bank (November 2010). GPS and satellite data from the 33 seals have been processed allowing us to examine their movements in the Atlantic (Figure 1). Data from the Vemco BCT tags deployed on the 33 seals did not reveal any encounters with tagged cod, although there were approximately 1900 encounters recorded between individual seals in 2009 and 1170 in 2010. The absence of encounters with cod is likely due to the low number of cod tagged at the time of deployment of the 33 seals.

Figure 1. Movements of male (left) and female (right) grey seals tagged on Sable Island.

Grey Seal Tracks