Because of the size of the ocean it has been impossible to track the movements of most animals in the sea until recently. Seawater is electrically conductive and therefore prevents radio signals from being sent underwater. Detection of radio-tagged animals is therefore not possible, and other technologies which allow detection of individually identifiable tagged animals (PIT tags) have too small a detection distance to be used in the ocean. Existing technologies allowing detection of animals in the ocean (e.g. sonar) do not permit identifying individual fish and capture of such tagged animals in the ocean almost inevitably results in their death. This precludes multiple recaptures and therefore establishing the movement pattern of animals in the ocean (direction, rate of movement, and areas of residence). These technical problems made it impossible to establish a meaningful understanding of the movements of marine animals in the ocean.

This situation has now changed. The development of two new types of electronic tags permits tagging and tracking of individual animals over the vast distances of the Pacific Ocean. Archival (data storage) tags record and archive in a non-volatile memory information on the depth, temperature, and ambient light level experienced by the animal. Because the time these measurements are recorded is precisely known because of an accurate clock, it is possible to use the recorded light data to estimate the daily position of a tagged salmon to within ca. ±0.5º latitude or longitude (Welch and Eveson 1999 and In Press). This is equivalent to determining the daily position of the salmon to within ca. 50 kms. Such accuracy is sufficient for basin-scale studies of animal movements, such as are undertaken by large salmon in their second or later years of life in the sea.

Unfortunately, resolution of movements to such accuracy is still insufficient for use in studying movement in near-coastal regions, which is a very different ecosystem. As the shelf zone is only 20-30 kms wide for most of the region from California to the Aleutians, light-based estimates of position are not of sufficient resolution to identify movements along this narrow coastal strip.

However, because the continental shelf is very narrow (less than 20 kms wide in most places), the narrowness of the shelf lends itself to a broad-scale monitoring program using sonic tags for animals such as juvenile salmon that remain on the shelf. Recently developed technology allows detecting uniquely identifiable sonic tags using low-cost passive receivers ($1K US per receiver). These receivers can detect sonic tags within an ca. 1 km diameter circle centered on the receiver, recording the date and time that individual tags are detected for up to 1 year, with a maximum recording capacity per receiver of 300,000 detections (about 800 per day, on average). The sonic tags are small enough to be surgically implanted in animals as small as 15-16 cm body length (e.g. salmon smolts). They have a lifetime of ca. 4 months for the smallest size tags.

• A chain of 20-30 receivers submerged on the bottom of the continental shelf should therefore be capable of detecting all individual animals crossing the monitoring line, recording the exact time each animal was detected, the identity of the individual, and fix the position of the animal relative to the shore to within 600 m. A series of listening lines would make it possible to establish migration speed and direction of travel for each tagged individual, and either define the areas of coastal residence or confirm which animals leave the continental shelf at the start of the Aleutians, and identify whether they enter the Bering Sea or remain in the North Pacific Ocean.

Many marine animals remain confined to the continental shelf ecosystem for much or all of their life history. For example, after entering the ocean from freshwater, Pacific salmon smolts generally move up and around the West Coast of North America following the continental shelf (Fig. 1). Earlier work had speculated that juvenile salmon began to move off the shelf by early fall and move directly into the Gulf of Alaska (e.g. Hartt and Dell 1986). However, an extensive sampling period, all juveniles were found to remain over the continental shelf until the start of the Aleutians, at the end of the Alaskan Peninsula (at the head of the final arrow in Fig. 1; Welch et al 1998 and In Prep.). Because the continental shelf, shown in light blue, is very narrow along the West Coast of North America the migration corridor for the juveniles restricts them to a long thin region which can be monitored at many locations at relatively low cost.

Marked juveniles captured during these surveys indicate that most salmon swim rapidly northwards along the continental shelf (Fig. 2). However, some stocks of coho and chinook smolts remain as year-round residents of the coastal zone, while others migrate at least as far as the Aleutian Islands before moving offshore. There are some populations of Pacific salmon which also move south along the continental shelf, opposite to the general pattern of movement (e.g. Weitkamp et al 1995); at present, it is uncertain why or which groups do so. Identifying which groups do so is an important management issue, because this may partially determine which groups experience poor marine survival.

The basic principle underlying the proposed tracking program for the continental shelf is newly developed acoustic technology. The miniature pingers are 24 mm long and have an operational life under continuous operation of ca. 4 months. Field tests of the acoustic receivers indicate that the receiver can detect these pingers from distances of perhaps 0.6-1.0 kms. As the shelf on the West Coast is usually less than 20 kms wide, a string of 20-30 receivers laid across the shelf and down the slope region to a depth of roughly 500 m should be capable of detecting all tagged animals crossing its path. The approximate cost of a single acoustic monitoring line would be on the order of $50K, so for roughly $2M a network of 20 or so acoustic listening lines could be deployed that would stretch from California to the Aleutian islands, which would be capable of detecting individual animals as they crossed the monitoring lines.

Such an acoustic network would provide the basis for reconstructing the movements of any animal present on the continental shelf that was tagged with a uniquely identifiable sonic tag. If initial testing and design phase indicate that a credible program can be developed, the scientific objectives that a full-scale research program could possibly address include:

• Determine the ocean migration pathways of multiple species of animals and their rates of migration along the continental shelf;
• Establish which stocks of salmon move to the offshore open Pacific or remain as coastal residents.
• Establish the shelf feeding grounds for shelf -resident marine animals;
• Determine the period of time animals remain as coastal residents in various sections of the coast expected to be significantly affected by climate change and global warming.
• Establish movement patterns for immature and maturing salmon by tagging these animals in the ocean one or more years prior to return to their natal rivers. This would also allow tracking their movements up-river during the return migration by placing receivers at various points in rivers.

This page is maintained and was last updated 14-Nov-00 by Pip Sumsion (sumsionp@dfo-mpo.gc.ca).