Jennifer S. Rehage^1,2, Kristine J. Dunker^1,2 and William F. Loftus^1
1U.S. Geological Survey, Florida Integrated Science Center, Homestead, FL, USA
²National Audubon Society, Tavernier Science Center, Tavernier, FL, USA

The role of abiotic factors in the organization of communities is a fundamental question in ecology. The structuring effect of abiotic conditions may be particularly important along transition zones or ecotones. In the Everglades ecosystem, mangrove-lined creeks link freshwater marshes to estuarine habitats. Previous studies have shown that these rivers are used by a diverse array of saltwater and estuarine fishes. The rivers may also represent critical habitat for freshwater-marsh fishes (including non-indigenous taxa) during seasonal dry periods. Historically, channels and pools at this ecotone served to concentrate fishes for avian predators. In this study, we examine seasonal and long-term dynamics of fishes in the oligohaline to mesohaline reaches of rivers within the southwestern region of Everglades National Park. In particular, we ask: (a) how does use by fishes of the upper-river habitat change over various time scales; (b) how do those changes relate to variation in abiotic conditions; and (c) how do changes in the fish community relate to anthropogenic activity (current management and future restoration)?

We sampled in two drainages: Rookery Branch, Squawk, and Otter creeks (RB/SOC), and North and Watson rivers (NR/WR). The drainages differ in the amount of freshwater inflows; RB/SOC drains marshes with longer hydroperiods than the NR/WR drainage. In both systems, we sampled the uppermost 600-m reach (accessible by motorboat) of six creeks. In each creek, we systematically sampled three 100-m-long sections by electrofishing. The uppermost 100-m section of each creek was sampled using two passive techniques: experimental gill nets that target large, mobile fishes, and minnow traps that target small fishes (standard length < 10 cm). We sampled three times per year: November (wet season), February (transition), and April (dry season).

Abundance and species composition differed between drainages and across seasons. Considering all sampling gears, fish catches were highest in February, and in the RB/SOC drainage. During February, electrofishing catches were more than 20 times greater in RB/SOC than in NR/WR. In RB/SOC from November to February, catches increased four-fold in electrofishing samples, eight-fold in gillnets, and nine-fold in minnow traps. In contrast, in NR/WR, electrofishing catch was highest in November, whereas in the minnow traps and gill nets, no seasonal variation was detected. Catch increases in RB/SOC reflected an influx of freshwater species into the creeks as marshes dried; salinities there reached 1-5 psu (practical salinity units). In RB/SOC, freshwater species such as gar, largemouth bass, mosquitofish, bluefin killifish, dollar sunfish, and other sunfishes increased in February and April samples. Similar increases were not observed in NR/WR, however, where salinities reached 10-15 psu.

Contact Information: Jennifer Rehage, USGS, Everglades National Park Field Station, 40001 State Road 9336, Homestead, FL 33034 USA, Phone: 305-242-7834, Fax: 305-242-7836, Email: rehagej@fiu.edu

(This abstract is from the 2006 Greater Everglades Ecosystem Restoration Conference.)

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