Summary 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.

Gulf sturgeon were tagged with telemetry tags and were tracked and relocated in fall and early winter of 1996 and 1998 to determine migration patterns and winter feeding habitats after they emigrated from the Suwannee River, Florida, into the Gulf of Mexico. We hypothesized that their migration would generally follow the drowned Suwannee River channel across the West Florida shelf. Fish left the river in late Oct. or early Nov., about the time river water temperatures fell below 20 C. Tracked and relocated fish moved slowly and remained offshore of Suwannee Sound in nearby shallow (<6 m) marine-estuarine habitats until at least mid or late Dec. The relatively small area (~115lrm 2) within which fish were consistently relocated in 1998 probably is a critically important feeding habitat because adult Gulf sturgeon, which do not feed while in the river, occupy it for up to half their short (4-5 mo) marine residency. The fish left the area in late Dec. or early Jan., most lil

233 234 W. H. Neill et al.Ecophys.Fish is a quantitatively explicit interpretation of concepts originally formalized by F.E.J. Fry, almost 60 years ago. Fry's "physiological classification of environment" and his concept of "metabolic scope for activity" were coupled with conventional bioenergetics to provide the model's theoretical basis. The model's inputs are initial size of fish, and time series of temperature, pH, dissolved-oxygen concentration (DO), salinity, and food availability and its energy content. Outputs are food consumption, oxygen consumption, waste production, energy content of fish biomass, and growth. Indirectly, the output is a measure of relative fitness of the fish-environment system to support fish growth.Two variants of the model represent the euryhaline red drum (Sciaenops ocellatus) and the freshwater bluegill (Lepomis macrochirus). Ecophys.Fish had its beginnings in laboratory experiments with juvenile red drum. These experiments enabled definition of functions and their parameterization, leading to a working model that effectively simulated growth of red drum in various pond and estuary trials with caged fish. Subsequently, Ecophys.Fish was converted to simulate growth rates of caged bluegill involved in stream ecoassays. The latter work confirmed the model's generality and the utility of automated routine respirometry for empirically estimating a key model parameter.Ecophys.Fish comprises an effective tool for resolving sources of variation in fish growth, even in natural systems with high levels of environmental variability. Moreover, the model has utility for probing biological and ecological mechanisms underlying fish growth and production. Finally, Ecophys.Fish is capable of producing rich hypotheses, e.g., 1) the optimum temperature for growth decreases whenever DO, food availability, or energy density of available food is limiting; 2) with unlimited DO and food availability, the optimum temperature for growth increases with increasing fish size but only when energy density of food is limiting; and, 3) when neither availability nor energy density of food is limiting, growth can be much faster under diel-cycling regimes of temperature and DO than under the optimum constant temperature/DO regime. Under Ecophys.Fish, environmental regimes that are best for survival are not necessarily those that are best for growth.

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