Defining winter trophic habitat of juvenile Gulf Sturgeon in the Suwannee and Apalachicola rivermouth estuaries, acoustic telemetry investigations
Three automated listening post-telemetry studies were undertaken in the Suwannee and Apalachicola estuaries to gain knowledge of habitats use by juvenile Gulf Sturgeons ( Acipenser oxyrinchus desotoi ) on winter feeding grounds. A simple and reliable method for external attachment of small acoustic tags to the dorsal fin base was developed using shrink-tubing. Suspending receivers on masts below anchored buoys improved reception and facilitated downloading; a detection range of 500–2500 m was realized. In the Apalachicola estuary, juvenile GS stayed in shallow water (< 2 m) within the estuarine transition zone all winter in the vicinity of the Apalachicola River mouth. Juvenile GS high-use areas did not coincide with high density benthic macrofauna areas from the most recent (1999) benthos survey. In the Suwannee estuary, juveniles ranged widely and individually throughout oligohaline to mesohaline subareas of the estuary, preferentially using mesohaline subareas seaward of Suwannee Reef (52% of acoustic detections). The river mouth subarea was important only in early and late winter, during the times of adult Gulf Sturgeon migrations (41% of detections). Preferred winter feeding subareas coincided spatially with known areas of dense macrofaunal benthos concentrations. Following a dramatic drop in air and water temperatures, juvenile GS left the river mouth and estuary, subsequently being detected 8 km offshore in polyhaline open Gulf of Mexico waters, before returning to the estuary. Cold-event offshore excursions demonstrate that they can tolerate full-salinity polyhaline waters in the open Gulf of Mexico, for at least several days at a time. For juvenile sturgeons, the stress and metabolic cost of enduring high salinity (Jarvis et al., 2001; McKenzie et al., 2001; Singer and Ballantyne, 2002) for short periods in deep offshore waters seems adaptively advantageous relative to the risk of cold-event mortality in shallow inshore waters of lower salinity. Thus, while juveniles can tolerate high salinities for days to weeks to escape cold events, they appear to make only infrequent use of open polyhaline waters. Throughout the winter foraging period, juvenile GS stayed primarily within the core area of Suwannee River mouth influence, extending about 12 km north and south of the river mouth, and somewhat seaward of Suwannee Reef (< 5 km offshore). None were detected departing the core area past either of the northern or southern acoustic gates, located 66 and 52 km distant from the river mouth, respectively.