Movement of summer flounder (Paralichthys dentatus) in relation to hypoxia in an estuarine tributary
Date
2010
Authors
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Publisher
University of Delaware
Abstract
Estuarine waters generally provide favorable physiochemical and biological conditions for early growth and survival of ecologically and economically important fishes. However, given their close proximity to expanding urban and rural coastal populations, estuaries are especially susceptible to the threat of increased nutrient loading and may contain areas of low dissolved oxygen (DO), or hypoxia. The present study looked at the effects of hypoxia on movement of estuary-dependent juvenile and adult summer flounder (Paralicthys dentatus). Summer flounder were captured from Indian River Bay, Delaware, surgically implanted with Vemco V7-4L acoustic tags, and released into Pepper Creek, a tributary of Indian River Bay that experiences diel-cycling hypoxia. Overall, 17 juveniles and 8 adult summer flounder were released into Pepper Creek during September in 2007 and 2008, and July in 2009. Vemco VR2 acoustic receivers and YSI 600XLM multi-parameter sondes were deployed along the longitudinal axis of Pepper Creek to capture the spatial and temporal dynamics of summer flounder movement in relation to hypoxia. Individual fish tracks were plotted on spatiotemporal DO contour plots using MATLAB software and analyzed with respect to the following environmental variables: DO, temperature, spatial and temporal DO and temperature gradients, insolation, and tide. Fish detections were labeled as either "movement" or "stay put" detections, with "movement" detections further coded as "upstream" or "downstream." The results show that summer flounder remained fairly sedentary in Pepper Creek during healthy DO concentrations, but responded actively to growth reducing and lethal DO concentrations. Some flounder avoided unhealthy DO (! 4.8 mg O2/l) while others initiated movement only when DO fell into the severely hypoxic range (! 2.3 mg O2/l), with activity driven by the specific nature of the DO environment. Rapid drops in DO to unhealthy levels triggered active movement responses, as did absolute DO values ! 2.3 mg O2/l, whereas slowly declining and/or spatially extensive DO > 2.3 mg O2/l triggered a passive behavior (no movement). Binary logistic regression was used to determine which environmental variables predict overall activity and specifically directional movement. With respect to the general summer flounder population, the temperature variables significantly discriminated between movement and stay put detections. For fish that experienced severe hypoxia, DO and the spatial temperature gradient predicted overall movement activity. Summer flounder also relied primarily on the spatial DO gradient and tidal flow for directional guidance within Pepper Creek. Upstream movements were generally done when the spatial DO gradient was negative and tide was flooding, while downstream movements were made during positive spatial DO gradients and ebbing tides. Additionally, tidal flow may have occasionally inhibited escape from hypoxic areas for smaller summer flounder. Residency time in the creek was shorter for fish released into unhealthy DO concentrations as opposed to fish released in DO > 4.8 mg O2/l, as was the exposure duration to unhealthy DO levels, with diel-cycling hypoxia compressing optimal summer flounder habitats. Offshore emigration from the bay also appeared to be delayed for a few summer flounder. Given the relatively low DO concentrations that elicit movements, summer flounder in Pepper Creek are experiencing DO levels that may stunt growth or result in mortality. Overall, these findings suggest that many flounder will tolerate progressively declining or steady unhealthy DO concentrations (! 4.8 mg O2/l), perhaps reaping the benefits of the productive nursery grounds to counteract growth-reducing DO conditions, but once exposed to severe hypoxia (! 2.3 mg O2/l), will abandon this potentially risky passive behavior and move permanently out of the creek.