Wildlife and Fisheries Management
Like all submersed aquatic macrophytes, hydrilla can provide habitat, food resources, and refugia for fish and wildlife. For fishes, a number of studies have evaluated the value of hydrilla as habitat. Intermediate coverage of hydrilla has been associated with quality largemouth bass (Micropterus salmoides) populations, causing high production of young fish (Colle and Shireman 1980; Maceina et al. 1995; Hoyer and Canfield 1996; Miranda and Pugh 1997; Tate et al. 2003), and quality fisheries (Moxely and Langford 1985; Maceina and Reeves 1996). However, high hydrilla coverage (> 85%) can reduce fishing effort due to difficulties in angler access (Colle et al. 1987), may cause fish growth rates to decline (Colle and Shireman 1980), and represents a risk to fish populations due to low oxygen and potential fish kills.
In the 1980s, FDEP funded a multi-year research project to address relations between aquatic plants and the limnology (i.e., water quality and fisheries) of Florida lakes (Canfield and Hoyer 1992). This research demonstrated that fish populations only had a chance of being negatively impacted when aquatic plant coverage was less than 15% or more than 85%. These findings suggested aquatic plant control programs have a broad window of plant coverage that they could work within, without causing major negative changes in water quality and/or fisheries (Canfield and Hoyer 1992). An important issue when controlling submersed aquatic plants, however, is that that the significance of hydrilla for quality largemouth bass fisheries may be related to lake size (Hoyer and Canfield 1996). Small lakes have relatively high littoral area to surface area ratios, possibly allowing adequate habitat for sport fish even in absence of submersed macrophytes. In large lakes, submersed plants such as hydrilla may substantially increase the available habitat over that which is provided by littoral areas (Hoyer and Canfield 1996). Thus, hydrilla management strategies for fishery benefits should consider lake size and the other available habitat present at each lake.
Considering wildlife, hydrilla provides a food resource for some aquatic birds including waterfowl. Johnson and Montalbano (1984) found 12 species of waterfowl consuming, and preferring, hydrilla while over-wintering in Florida. Similarly, Montalbano et al. (1979) found hydrilla to be the most common plant food found in the esophagi of ducks and coots in two Florida lakes. Esler (1990) reported that total bird use in experimental plots in a Texas reservoir was substantially greater with the presence of hydrilla than without the plant. Examining aquatic bird data on 46 Florida lakes, Hoyer and Canfield (1994) showed that bird abundance and species richness remain relatively stable as macrophyte abundance increases, but birds that prefer open water habitats (e.g., double-crested cormorant, Phalacrocorax auritus) are replaced by species that use macrophyte communities (e.g., ring-necked duck, Athya collaris). Additionally, some species require certain types of emergent vegetation to be present regardless of the total lake-wide coverage of aquatic plants (e.g., least bittern, Ixobrychus exilis). Thus, for aquatic plant management some judgments have to be made regarding the positive and negative impacts on habitat preferences for aquatic bird species.
Some strategies for managing hydrilla with Fluridone have included temporary deviations of lake level regulation schedules and these associated changes in water level could influence habitat availability for several bird and fish species. Participants in the Hydrilla Issues Workshop noted that manipulated low water used for hydrilla control should be considered in the context of available basin-wide habitat for birds rather than lake-specific habitat considerations (Appendix II). Wading bird densities in Florida generally decline with increases in water levels (Breininger and Smith 1990; David 1994), and thus, temporary declines in water levels for hydrilla control could increase wading bird habitat.
There is no evidence that hydrilla directly influences habitat quality for the endangered snail kite Rostrhamus sociabilis. However, low water levels have been associated with poor nest success (Beissinger and Snyder 2002) and lower juvenile survival (Dreitz et al. 2004) for snail kites. Changes to lake regulation schedules therefore may influence habitat quality for snail kites and should be considered in the plans. Dreitz et al. (2001) contended that relatively few adult snail kites might exhibit low nest success during low water due to their ability to select alternative locations when water levels decline. Nevertheless, hydrilla management strategies that include changing water levels should obviously consider habitat for wading birds and the snail kite in the context of regional habitat availability.
Recommendations
Recommendation 11: Hydrilla management actions should aim to keep non-target impacts to a minimum because non-target impacts of hydrilla control measures on native plant abundance could greatly reduce available fish and wildlife habitat. Where control of resistant hydrilla is limited because of budgetary considerations and/or insufficient selective management tools and where hydrilla coverage is not impacting the designated uses of a lake, FDEP should consider allowing some hydrilla to persist. Where water level manipulations are needed to improve the efficiency of hydrilla control with Fluridone, aquatic plant management plans should consider the impacts of water level changes on fish and bird populations.
Justification: Research has found no evidence that a wide range of hydrilla coverage (15% to 85% coverage) represents a threat to wildlife and fisheries, and in most cases, hydrilla even provides beneficial habitat. However, high hydrilla coverage (> 85%) can cause problems for fisheries and hydrilla coverage greater than 40% to 50% generally cause problems with recreational activities. Water-level manipulations in lakes have been shown to significantly influence bird and fish populations.