Population dynamics in drying times: How does seasonality affect apparent survival of resident versus transient mesoconsumer fishes in an estuarine environment?

Background/Question/Methods

In seasonally-pulsing environments, natural disturbance regimes are key drivers regulating population dynamics. Anthropogenic influences can alter these patterns, thereby modifying their effects. This is particularly true for aquatic communities where hydrological alterations have changed natural flow. In the seasonally-inundated Everglades, marsh flooding regimes have been highly modified, resulting in decreased hydroperiods and increased magnitude and duration of marsh drying. Fishes are known to move into areas of deep water that function as refuge habitats during periods of drying. However, little is known about how this aquatic habitat compression affects the survival of fishes. To examine this, we estimated apparent survival of largemouth bass (Micropterus salmoides floridanus), a transient freshwater species, and common snook (Centropomus undecimalis), a resident estuarine species. We expected that dry season survival would differ between species with decreased survival for bass and consistent survival for snook. Over a four-year period in the Shark River Estuary, a coastal estuarine habitat, apparent survival rates were estimated using a Cormack-Jolly-Seber model with PIT-tag mark-recapture data from electrofishing. Additionally, an angler citizen-science program was implemented to increase recaptures and estimate fishing-pressure. We used an information theoretic approach to test alternative models and quantify rates of apparent survival during periods of drying.

Results/Conclusions

Apparent survival of largemouth bass in the upper estuary varied as a function of upstream marsh hydroperiod, with marked decreases in survival rates occurring during periods of marsh drying. While, apparent survival for common snook did not decrease as a function of marsh hydroperiod. Furthermore, natural inter-annual variation in rainfall patterns allowed us to examine the strength of seasonal dry-down on survival rates. Despite variation in the magnitude and duration of dry-down across years, there was a comparable rate of decrease in survival for bass across years. The results suggest that although the Shark River Estuary functions as a refuge habitat, there are substantial amounts of disturbance-induced mortality occurring consistently on a yearly basis resulting from the loss of floodplain habitat. Under current hydrological management, large-bodied freshwater fish populations may be mediated predominantly by disturbance related to the higher frequency of marsh drying in the region. Ongoing watershed restoration efforts are expected to reduce the effects of habitat loss on the mortality of fishes by increasing watershed discharge duration and magnitude. Understanding how populations respond to both natural and altered disturbances is important for conservation and management, particularly as anthropogenic impacts including climate change may continue to alter natural systems.