Alasdair McIntyre: Life in the World's Oceans
14.1 POST Technology and History
Acoustic tags have been in use for 50 years (Johnson 1960), but in 2001 a fisheries biologist, David Welch, and his colleagues proposed to the Alfred P. Sloan and Gordon and Betty Moore Foundations to design and build a very large network of listening lines to track salmon in the ocean. They reasoned that satellite tags were too big to use on salmon, archival tags were too unlikely to be recovered (and their light-based geo-location estimates were too inaccurate at the time), and radio tags, although useful for tracking salmon in rivers, were useless in the ocean because of rapid attenuation of electromagnetic signals in seawater.
POST was built around acoustic tags and receivers manufactured by a Canadian company, Vemco (www.vemco.com). Vemco's tags could be implanted in small fish, detected at relatively long distances, programmed to have relatively long tag lifespans and, most importantly, the system generated few false-positive signals. Several studies have assessed the effects of the tags on the survival and behavior of fish that carry them (Lacroix et al. 2004; Zale et al. 2005; Welch et al. 2007; Chittenden et al. 2009a; Rechisky & Welch 2009), and helped to define fish size limits for tagging. Early on, the number of available unique tag identification numbers was small so they were re-used, which quickly became very confusing on the large scale of the POST array. POST helped to motivate the development of a system with many identification numbers that are unique worldwide. Early Vemco receivers had short battery lives and could not be used in deep water, but by the time POST was scaling up, several thousand second-generation VR-2 receivers had been sold on the West Coast. These receivers were tough, reliable, had batteries that lasted one year, and, with a maximum depth of about 500 m, could be deployed almost anywhere along the continental shelf. Most importantly, all of the tags and receivers were compatible. However, the early receivers had to be physically retrieved to download the data. Most of the original POST network has now been replaced with a newer generation of VR-3 receivers equipped with long-lived batteries (four to seven years) and acoustic modems by which a boat can download data from the surface without physically recovering the receiver. This has generated significant cost savings over the life of the array and made it easier to keep receivers in position full-time, year-round. Welch's research and development company, Kintama Research Corporation, tackled the problems of deploying large-scale arrays and developed the architecture and tag programming for the original demonstration array forming the core of POST. They designed specialized protective flotation collars and anchors (Fig. 14.3), improved moorings to reduce losses to trawling and storms, and built portable surgery stations and data-recording systems for large-scale tagging. They are currently modeling optimal array geometries for specific research projects, which depend on a host of factors including the research objectives, noise level in the area of the line, behavior of the tagged animal, tag parameters (loudness and programming), and position of the receiver relative to features such as the surface, the bottom, thermoclines, and haloclines. Where measurable, POST lines have obtained high enough detection efficiencies to produce useful survival estimates for juvenile salmon (Melnychuk 2009). With support from US and Canadian government agencies and foundations, the POST array is maturing into a network of highly engineered, long listening lines that now spans 3,000 km from California to Alaska and is maintained year-round for use by any researcher. POST shares data with independent researchers who maintain their own, smaller receiver networks (some in grids or other geometries). We have begun the process of integrating POST data into large-scale ocean-observing systems including OBIS (see Chapter 17), the Ocean Tracking Network, and the Global Ocean Observing System (GOOS) system.
