Abstract
Physicochemical variability in estuarine systems plays an important role in estuarine processes and in the lifecycles of estuarine organisms. In particular, seasonality of freshwater inflow to estuaries may be important in various aspects of fish lifecycles. This study aimed to further understand these relationships by studying the movements of a top-level estuarine predator in response to physicochemical variability in a large, temperate south-east Australian estuary (Shoalhaven River). Mulloway (Argyrosomus japonicus, 47–89 cm total length) were surgically implanted with acoustic transmitters, and their movements and migrations monitored over two years via fixed-position VR2W acoustic receivers configured in a linear array along the length of the estuary. The study period included a high degree of abiotic variability, with multiple pulses (exponentially high flows over a short period of time) in fresh water to the estuary, as well as broader seasonal variation in flow, temperature and conductivity. The relative deviation of fish from their modal location in the estuary was affected primarily by changes in conductivity, and smaller fish (n = 4) tended to deviate much further downstream from their modal position in the estuary than larger fish (n = 8). High-flow events which coincided with warmer temperatures tended to drive mature fish down the estuary and potentially provided a spawning signal to stimulate aggregation of adults near the estuary mouth; however, this relationship requires further investigation. These findings indicate that pulse and press effects of freshwater inflow and associated physicochemical variability play a role in the movements of mulloway, and that seasonality of large freshwater flows may be important in spawning. The possible implications of river regulation and the extraction of freshwater for consumptive uses on estuarine fishes are discussed.
Highlights
Estuaries represent some of the most variable aquatic ecosystems on earth (e.g. [1,2,3]), but the responses of estuarine species to such variability is often poorly understood [4,5]
It is thought that variability in temperature and freshwater inflow are responsible for much of the temporal variability observed in estuary dynamics and species interactions [12]
Abiotic Variability During the study period, fish were exposed to a high degree of environmental variability, including several moderate to high flow events (Fig. 3a)
Summary
Estuaries represent some of the most variable aquatic ecosystems on earth (e.g. [1,2,3]), but the responses of estuarine species to such variability is often poorly understood [4,5]. Natural stressors in the estuarine environment can be induced by rainfall and concomitant changes to river flow, which can alter the physicochemical habitats in an estuary (e.g. through salinity stratification [9]) and increased nutrient inputs into estuaries [10]. The effects of such changes often cascade throughout the trophic chain, either through altered productivity regimes or altered habitat availability Natural sources of variability in estuarine systems can lead to changes at the scale of hours to years, and are most often associated with variations in freshwater inflow, temperature, tides, wind, and exchange with adjacent coastal waters [12]. It is thought that variability in temperature and freshwater inflow are responsible for much of the temporal variability observed in estuary dynamics and species interactions [12]
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