Seasonal movements of crayfish in a fluctuating wetland: implications for restoring wading bird populations

Summary Among aquatic fauna of fluctuating wetlands, seasonal interhabitat movement is a widespread phenomenon that can strongly influence predator–prey interactions and population dynamics of vertebrate predators. For example, movements increase the availability of small fishes during seasonal drying, which is critical to the reproductive success of many wading bird (Ciconiiformes) species. However, few studies have examined movements of aquatic fauna in relation to changes in water depth, especially for those taxa that can physiologically tolerate periods of drying and whose responses to hydrology are difficult to predict (e.g. crayfish). Here, we experimentally explore the movement responses of a key prey species in the Florida Everglades – the slough crayfish Procambarus fallax – to a seasonal decline in water depth. The study was conducted in four replicate wetland impoundments (wetlands) which comprise habitats characteristic of the Florida Everglades. In two wetlands, we induced a 60-cm drop in water level and examined crayfish movements between higher-elevation ridge habitats and lower-elevation slough habitats when water levels were (i) relatively deep on ridges; (ii) relatively shallow on ridges; and (iii) below ground on ridges but above ground in sloughs. In two control wetlands, we followed the identical sampling protocol but maintained the deep stage on the ridges. Crayfish movements were quantified by changes in abundance and mark–release–recapture (MRR). Crayfish abundance changed little within habitats while water remained on the ridges, but it decreased on the ridges and considerably increased in the sloughs after the ridges dried. These changes were clearly due to loss of water on the ridges as abundances in control wetlands remained constant throughout sampling. Elevated slough abundances were not explained by differences in trapping efficiency or episodic releases of recently hatched crayfish, suggesting they were due largely to movements from ridges to sloughs. This was supported by the MRR data. Our results show that seasonal water-level recessions can induce crayfish to move from higher to lower-elevation habitats where they become concentrated and potentially available to foraging birds. We discuss the potential implications of these movements for restoring hydrologic conditions for wading birds in large fluctuating wetlands.