Center of Expertise in Marine Mammology: Scientific Research Report 2012-2014

Predator-Borne Acoustic Transceivers and GPS Tracking Reveal Encounters with Acoustically Tagged Free-ranging Marine Fish

W. Don Bowen, Lidgard, D.C., Jonsen, I., and Iverson, S. J.

There is increasing evidence that upper trophic level predators, such as seals, play an important role in shaping and stabilising ecosystem structure and functioning. Although technological advances in telemetry have led to a greater understanding of movement, foraging behaviour and habitat use of upper trophic level marine predators, still little is known about how marine mammals interact with their prey or competitors. The innovative use of location telemetry coupled with coded acoustic transmitters and transceivers, through a collaborative research program called the Ocean Tracking Network (OTN), promises to radically improve our understanding of predator behaviour and predator-prey interactions in the ocean.

The Ocean Tracking Network (OTN) is a $35 million Canada Foundation for Innovation (CFI) - International Joint Ventures Fund global research and technology development platform headquartered at Dalhousie University, Halifax, Nova Scotia, Canada, that aims to revolutionize the way oceans are viewed and understood. Through the CFI funding in partnership with NSERC and DFO, OTN is implementing a robust, open-access global infrastructure in Canada’s three oceans to establish a new and unique ocean observation system, one that is centered on scientifically documenting animal movements, habitat use, and survival in relation to changing physical, chemical and biological ocean conditions. Starting in 2008, OTN began deploying Canadian state-of-the-art acoustic receivers and oceanographic monitoring equipment in key ocean locations. “OTN Canada” is the seven-year Canada-wide integrative research network program that was funded beginning in January 2010 by the Natural Sciences and Engineering Research Council (NSERC) of Canada.

One of the Atlantic components of OTN is the use of grey seals (Halichoerus grypus) as “bioprobes” to collect both biological and physical data from the continental shelf off eastern Canada. Predator–prey interactions have important population and community level consequences that depend on the frequency of predator and prey encounters. Nevertheless, little is known about the spatial and temporal patterns of encounters between predators and prey in the ocean. Through the use of acoustic and satellite telemetry, a team of researchers from the Bedford Institute of Oceanography (BIO) and Dalhousie University have begun to collect data on the spatial and temporal pattern of grey seal encounters with Atlantic cod (Gadus morhua) and Atlantic salmon (Salmo salar). These fish species are of conservation concern and commercial interest and grey seal predation is hypothesized as one factor influencing recovery. Interactions among acoustically tagged grey seals and between grey seals and acoustically tagged tuna, a presumed competitor for prey, are also being revealed for the first time.

Figure 4. Adult male grey seal equipped with an Argos/GPS tag (head) and VMT (middle back) heading to sea at Sable Island, June 2014. (Photo: W. D. Bowen)

Figure 4. Adult male grey seal equipped with an Argos/GPS tag (head) and VMT (middle back) heading to sea at Sable Island, June 2014. (Photo: W. D. Bowen)

During the first four years of study (beginning in 2009), the VMT on 16 of 64 adult grey seals recorded detections from 17 adult Atlantic cod and 7 Atlantic salmon that had been implanted with coded acoustic tags (Figure 5). An examination of the temporal and spatial pattern of these seal−fish interactions suggested that one salmon and two cod might have been eaten. However, in addition to acoustic detection of prey, most of the acoustically tagged grey seals also detected one another. Over the first four years about 7,000 detections were recorded among tagged seals. Detailed analysis of the 2009 data revealed that these detections were not randomly distributed in space and time but preferentially occurred at offshore banks where feeding is presumed to mainly take place (Figure 6).

This research has two components. The first is to equip grey seals with an Argos satellite-linked and GPS location and behaviour tag and a Vemco mobile transceiver (VMT) that both receive and transmit coded acoustic messages (Figure 4). The second is to tag prey, such as Atlantic cod, with Vemco coded acoustic tags that identify individual fish through the transmission of unique acoustic messages. The VMT was programmed to transmit on an irregular schedule, every 60 to 180 s (to avoid VMTs transmitting at the same time and causing code collisions and false detections) and to remain in listening mode for the remainder of the time. Over the same period, a total of 623 Atlantic cod were tagged with a Vemco V13 acoustic transmitter in the southern Gulf of St. Lawrence and the Eastern Scotian Shelf, as were about 300 Atlantic salmon.

Figure 5. Distribution of fish deployments (Δ), grey seal−fish encounters (o) and movement tracks (light blue) for 16 grey seals on the Eastern Scotian Shelf and in the southern Gulf of St. Lawrence from 2010 to 2012. Atlantic cod are represented by orange symbols, Atlantic salmon smolts by red and Atlantic salmon kelts by yellow. The white arrows indicate the location of a possible predation event.

Figure 5. Distribution of fish deployments (Δ), grey seal−fish encounters (o) and movement tracks (light blue) for 16 grey seals on the Eastern Scotian Shelf and in the southern Gulf of St. Lawrence from 2010 to 2012. Atlantic cod are represented by orange symbols, Atlantic salmon smolts by red and Atlantic salmon kelts by yellow. The white arrows indicate the location of a possible predation event.

Figure 6. Location of associations between grey seals for A) all seals (n = 14), B) seal 98430 and C) seal 98428. Figures B) and C) are examples to illustrate the correlation between location of associations, bathymetry and seal behaviour. Pr(ARS behaviour) is a continuous measure (0 to 1) of the probability of exhibiting area-restrictive search behaviour (an indication of foraging) according to the hidden Markov model.

Figure 6. Location of associations between grey seals for A) all seals (n = 14), B) seal 98430 and C) seal 98428. Figures B) and C) are examples to illustrate the correlation between location of associations, bathymetry and seal behaviour. Pr(ARS behaviour) is a continuous measure (0 to 1) of the probability of exhibiting area-restrictive search behaviour (an indication of foraging) according to the hidden Markov model.

The need to recover the VMT for data retrieval has limited deployments to locations where confidence in instrument recovery is high, such as Sable Island, and has thus restricted both species and areas where this tag could be used. To overcome these limitations, a Bluetooth link was integrated into the VMT and GPS-satellite linked transmitter through a partnership with SMRU Limited, St. Andrews, Scotland and Vemco, Canada. The two-unit design allows data collected by the VMT to be transmitted via Bluetooth to the satellite transmitter, which relays the data to the ARGOS satellite system for retrieval. To evaluate in-situ performance, units were deployed on two adult female grey seals on Sable Island, NS in October 2012 and recovered during the subsequent breeding season. Data archived by the VMT were compared with data uploaded via ARGOS and found to be for all intentions identical. In July 2013, eight Bluetooth VMTs were deployed in the Gulf of St. Lawrence and another five were to be deployed in 2014.

Researchers have demonstrated that a novel combination of acoustic transceivers and ARGOS/GPS technology can be used to determine the spatial and temporal pattern of interactions between grey seals and fish species in two large marine ecosystems, the Eastern Scotian Shelf and the southern Gulf of St. Lawrence, Canada.

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