Seasonal Refugia and Trophic Dormancy in Gulf Sturgeon: Test and Refutation of the Thermal Barrier Hypothesis
Kenneth J. Sulak, R. Allen Brooks, and Michael T. Randall
Abstract.—During summer, anadromous Gulf sturgeon Acipenser oxyrinchus desotoi in the Suwannee River congregate in several localized areas, termed holding areas. While they are residing in these areas, their movement and feeding activity appear to be limited. Holding areas have been hypothesized to be areas under the cooling influence of Floridan Aquifer spring water, which forms cool thermal refugia relative to the main-stem river and the Gulf of Mexico. According to the thermal barrier hypothesis, (1) Gulf sturgeon in the Suwannee River become restricted to local coolwater refugia; (2) within these refugia, benthic prey rapidly become depleted, such that Gulf sturgeon cannot feed and lose weight as summer progresses; and (3) confined to the thermal refugia, sturgeon cannot gain access to macrofaunal prey in other (warmer) reaches of the river. No aspect of the thermal barrier-spring water refuge hypothesis has ever been tested, and scant supporting evidence has been presented. Given that important Gulf sturgeon population management and critical habitat protection decisions depend on an accurate knowledge of sturgeon life history in relation to critical habitats, we tested this hypothesis. Two major Suwannee River Gulf sturgeon holding areas were compared with nonholding areas in terms of three parameters: water temperature, macrofaunal benthos prey availability, and within-year and within-summer movement patterns. The results for all three parameters refute the thermal barrier hypothesis. Furthermore, the literature on the world’s sturgeon species establishes that prolonged periods of trophic dormancy and residence in local holding areas within rivers are common life history attributes of many sturgeon species, occurring under greatly varying temperature conditions. The common attribute of holding areas is that they provide refuge from high-velocity currents within deep holes, depressions, or back eddies. Sturgeon seek out low-velocity microhabitats within high-velocity, big-river macrohabitats. Residence in a refuge and limitation of swimming activity are probably important to energy conservation during normal periods of trophic dormancy and/or when environmental conditions are physiologically limiting. In the Suwannee River, a typical Gulf sturgeon holding area consists of a 500–2,000-m-long, 3– 4-m-deep, sand-bottom run lying just below a 4–7-m-deep scour hole that is limited downstream by a 1–2-m-deep sand shoal.