Water Level Management

Seasonal changes in water levels and long-term hydrologic regimes directly influence snail survival and recruitment. Drying events have obvious negative impacts on both snail survival and egg cluster production.  High water events, where depths exceed 2-3 feet in emergent marsh habitats, primarily impact egg cluster production (especially in the dry season).  At this time, published data related to water level management impacts on snails are largely restricted to the impacts of dry downs.  More detailed information on high water impacts will be provided following peer review of available data.

For decades, water management plans to support apple snails (ultimately to support snail kites) emphasized the importance of avoiding dry downs (i.e., water table below ground level) (see Historical Perspectives). There is no question that dry downs have direct deleterious effects on snails, but in recent years snails have been found to be well adapted to dry down conditions (e.g., see Darby et al. 2008).  Dry downs have a beneficial indirect effect on snails in terms of habitat; periodic drying events promote and maintain wet prairie and sawgrass habitats that support snails, snail kite foraging, and egg cluster production, respectively (also see Snail Kite).  However, the duration and timing of these drying events needs to be considered to avoid a dramatic snail population decline.  The most extensive study of snail dry down tolerance was published by Darby et al. (2008) (also see Dry Down Impacts).  Based on these findings, we conclude that dry down conditions lasting less than approximately 8 weeks have little to no effect on adult snail survival.  Since adult snails stop mating and laying eggs in dry conditions, we recommend avoiding drying events as much as possible during the peak breeding season (April-June).  Juvenile (i.e., smaller) snails have lower dry down survival rates, so avoiding drying events in the April-June period also allows more growing time such that the young-of-the-year snails obtain larger sizes and consequently survive longer in dry down conditions.  Managed dry downs that result in dry conditions encompassing the peak breeding season result in a poor recruitment year and ultimately a population crash (Darby et al. 2004).  Darby et al. (2008) concluded that drying events that last < 8 weeks and avoid dry season egg production could occur every 2-3 years with minimal long-term impacts on snail densities.

Timing peak egg cluster production in the mid to late dry season suggests a benefit is derived from laying eggs when water levels tend to recede (or conversely, tend not to rise) (see Darby 1998).  Turner (1998) reported that flooding apple snail eggs resulted in lower embryonic survival, and of course chances of this happening during the dry season are much lower compared to the wet season (June-November).  In addition, Darby et al. (pers. comm.) found that actual water depths (not just changes in water stage) affect snail egg production on a per female basis.  The best available data (and only data, as far as we know) suggests that water depths between approximately 15 and 40 cm (6 and 16 inches) results in significantly higher per capita egg production than when water depths exceed 60 cm (~2 feet).  Consequently, to support apple snails, we recommend that water levels recede from January through May (or possibly into June), and that water depths in emergent marsh habitats that support egg production (e.g., wet prairie and sawgrass ecotones) fall between 15 to 40 cm in March-June.

These recommendations are based on a limited number of studies and have not been corroborated by long-term field studies under fluctuating water conditions. These recommendations will be updated periodically based on newly available information. Dr. Darby is also working with the Joint Ecosystem Modelling team (D. DeAngelis et al.) to create a mathematical model to perform 'what if' scenarios of hydrologic impacts on an apple snail population.  Management actions designed to promote apple snail production (or at least to minimize negative impacts) should be based on a thorough examination of available peer-reviewed manuscripts and reports. Contact us for additional information.

Literature Cited:

• Darby, P.C.  1998.  Florida apple snail (Pomacea paludosa Say) life history in the context of a hydrologically fluctuating environment.  Ph.D. Dissertation.  University of Florida, Gainesville, FL.

• Darby, P.C., P.L. Valentine-Darby, H. Franklin Percival, and W.M. Kitchens.  2004.  Florida apple snail (Pomacea paludosa SAY) responses to lake habitat restoration activity.  Archives of Hydrobiologia 161 (4):561-575.

• Darby, P.C., R.E. Bennetts, and H.F. Percival.  2008.  Dry down impacts on apple snail demography:  Implications for wetlands water management.  Wetlands 28(1):204-214.

• Turner, R. L. 1998. Effects of submergence on embryonic survival and developmental rate of the Florida applesnail, Pomacea paludosa: Implications for egg predation and marsh management. Florida Scientist 61:118-129.