The relationships between acoustic target strength and body length for Altantic sturgeon

This technical note presents the results of field measurements of acoustic target strength (TS) for Atlantic sturgeon (Acipenser oxyrinchus oxyrinchus) of known total length (TL).An established relationship between TS (dB) and TL (cm) is a fundamental requirement of fishery hydroacoustic investigations.Equations describing relationships for Atlantic sturgeon of selected size ranges were derived for two ultrasonic frequencies (200 and 420 kHz) using a digital split-beam sonar directed vertically at tethered sturgeon.Species-specific TS-TL equations are necessary to provide accurate density or abundance estimates, fish length frequency distributions, and to potentially differentiate sturgeon from other fish species during fisheries hydroacoustic surveys. BACKGROUND:The range of the Atlantic sturgeon extends from Labrador, Canada, to the St. John's River, Florida.They are found in rivers, estuaries, and coastal waters.Atlantic sturgeon commonly attain lengths of up to 79 in.(200 cm), although the largest historical record was for an adult that weighed 811 lb and was 14 ft long.Life expectancy ranges from 50 to 75 years.General accounts of sturgeon feeding portray them as opportunistic benthivores, feeding primarily on mollusks, polychaete worms, amphipods, isopods, shrimp and small bottom-dwelling fishes and insect larvae (Gilbert 1989;Smith 1985).The Atlantic sturgeon is anadromous, entering freshwater rivers to spawn at water temperatures ranging from 13.2 to 23 C.This occurs in spring through early summer; typically February-March in the southern portion of its range and May-July in Canada (Smith 1985;Bain 1997;Smith and Clugston 1997).Substrate is a key spawning habitat parameter for Atlantic sturgeon, as hard bottom (small rubble, gravel, hard clay, and limestone) is required for successful egg attachment and incubation, while also protecting larvae from predators (Smith and Clugston 1997).Atlantic sturgeon may remain in fresh or brackish waters until they reach 2.5 to 3 ft in length.Juveniles can remain in riverine and estuarine systems for periods of 1 to 6 years before migrating to the coast and onto the continental shelf where they grow to maturity.Since juveniles are known to congregate at fresh and saltwater interfaces, these areas may serve as juvenile nursery habitat.Important habitats for Atlantic sturgeon include spawning areas, estuarine nursery areas, inlets that act as migration corridors to and from freshwater spawning habitat, and nearshore wintering grounds for adult and older juveniles.McCord (2003) reported that Atlantic sturgeon overwinter in deep channels and holes within coastal sounds and bays.Thus a variety of coastal, estuarine, and riverine habitats are used by Atlantic sturgeon during various life stages and are necessary for species survival.Knowledge gaps identified by the Atlantic State Marine Fisheries Commission include: identifying and mapping spawning locations; identifying wintering habitat of sub-adults; habitat usage during non-spawning seasons; determining methods to quantify population abundance and habitat requirements; and potential negative impacts associated with dredging.Detailed reviews on the status, life history, and ecology of the Atlantic sturgeon can be found in Smith (1985);

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