Estuaries and Coasts (2010) 33:1216–1224 DOI 10.1007/s12237-010-9318-x NOTE Mainstem and Backwater Fish Assemblages in the Tidal Caloosahatchee River: Implications for Freshwater Inflow Studies Philip W. Stevens & Marin F. D. Greenwood & Charles F. Idelberger & David A. Blewett Received: 20 March 2009 /Revised: 20 March 2010 /Accepted: 13 June 2010 /Published online: 14 July 2010 # Coastal and Estuarine Research Federation 2010 Abstract Research strategies for investigating the economically important fishes in the region. For example, freshwater-inflow requirements of estuarine fishes often striped mullet Mugil cephalus and pinfish Lagodon rhom- integrate life-history information and correlative analyses of boides were more abundant along the river's mainstem; inflow and fish abundance. In tidal rivers, however, some common snook Centropomus undecimalis and bluegill fish have affinities for embayments, oxbows, and smaller Lepomis macrochirus were more abundant in the river's tributaries, often referred to collectively as river “back- backwaters. For those species that were more abundant waters”. The objective of this study was to determine along the mainstem of the river or showed no difference, whether freshwater and estuarine fish assemblages differed studies that measure changes in the distribution and between backwaters and the mainstem of the tidal Calo- abundance of these species with varying inflow along the osahatchee River, a highly managed river system located in mainstem of the river are justified. However, for species an urban setting in southwest Florida. Nonmetric multidi- that were more abundant in backwater areas, geomorpho- mensional scaling of 21.3-m seine data revealed that fish logical features should be considered in the design of assemblages did indeed differ between the backwater and studies that assess factors affecting fish use. mainstem habitats in each of three river sections. Univariate analyses identified species that differed in abundance Keywords Juvenile fish . Fish movement . Coastal between the habitats, which included ecologically and geomorphology. Oxbows . Embayments . Tidal tributaries Electronic supplementary material The online version of this article (doi:10.1007/s12237-010-9318-x) contains supplementary material, Introduction which is available to authorized users. Freshwater inflow from major rivers brings nutrients that P. W. Stevens (*) : C. F. Idelberger : D. A. Blewett Florida Fish and Wildlife Conservation Commission, Fish and fuel high levels of estuarine productivity and forms a Wildlife Research Institute, Charlotte Harbor Field Laboratory, salinity gradient that provides the physiochemical condi- 585 Prineville St, tions favorable for juveniles of many marine species (Day Port Charlotte, FL 33954, USA et al. 1989). Management of freshwater resources often e-mail: [email protected] results in conflicts among various user groups, including M. F. D. Greenwood fishing industries that depend on natural flow variability to Florida Fish and Wildlife Conservation Commission, provide the appropriate magnitude and timing of freshwater Fish and Wildlife Research Institute, inflow to maintain estuary-dependent fish stocks (e.g., 100 8th Avenue SE, St. Petersburg, FL 33701, USA Baisre and Arboleya 2006). There is now broad acceptance among scientists and water managers that riverine and Present Address: estuarine resources be considered legitimate users of water M. F. D. Greenwood supplies (Flannery et al. 2002; Arthington et al. 2006). One ICF International, 630 K Street, of the most important challenges facing estuarine scientists Sacramento, CA 95814, USA and managers is to determine the necessary magnitude and Estuaries and Coasts (2010) 33:1216–1224 1217 timing of freshwater delivery to the estuary while main- river. The Franklin Lock is farthest downstream and serves taining sufficient supply for human populations (Alber as the upriver boundary for tidal influence (Fig. 1). The 2002). upper tidal reaches of the river are relatively narrow Appropriate research strategies for investigating the (0.2 km), and the shorelines along the mainstem are highly freshwater-inflow requirements of estuarine fishes should altered as a result of channelization. Downstream of the integrate life-history information and correlative analyses of confluence with the Orange River, the river abruptly widens inflow and fish abundance (Robins et al. 2005). Before to more than 1.5 km, and the littoral zone is characterized such analyses can be conducted, however, scientists and by expansive, shallow flats (<2 m deep). The remaining managers must determine the appropriate sampling uni- course of the river passes through two major cities (Fort verse. Because inflow influences zones of primary produc- Myers and Cape Coral), where more than 50% of the linear tivity and environmental conditions, sampling fish shoreline has been hardened with seawalls and rip-rap assemblages along a gradient from tidal freshwater to (estimated from aerial photography). Shorelines of remnant higher salinities within river systems is particularly useful oxbows, embayments, and tidal tributaries are still com- for measuring changes in fish distribution and abundance posed of native vegetation, primarily red mangrove Rhizo- with varying inflow (Wagner and Austin 1999). phora mangle and are therefore conspicuous features Another consideration when defining an appropriate compared to the highly altered shorelines along the river's sampling universe is landscape features within the estuarine mainstem. Although extensive beds of the submerged grass systems. Such features within rivers include oxbows, Vallisneria americana were present in the river prior to embayments, and smaller tributaries often referred to 2000, the beds have been largely absent from the river collectively as river “backwaters.” In freshwater portions during the past decade (only 2% of the sampling sites in the of major rivers in the southeastern USA, backwater fish present study contained submerged aquatic vegetation). The assemblages have been shown to be substantially different scarcity of this grass is attributed to drought conditions and from those along the river's mainstem (Lehtinen et al. high salinities in the upper river that are intolerable to this 1997). Backwaters provide the primary juvenile habitat for freshwater species (Doering personal communication). Low several recreationally important fishes, including large- light availability can also limit growth (Hunt and Doering mouth bass Micropterus salmoides and bluegill Lepomis 2005). The Caloosahatchee River ultimately drains into the macrochirus (Scott and Nielson 1989; Nack et al. 1993; southern portion of the Charlotte Harbor estuarine system. Lehtinen et al. 1997). For estuarine species, recent studies Comparisons of water conditions and fish assemblages have shown that channel morphology accounts for recur- were made in three river sections (lower, middle, and ring patterns and large differences in fish use among upper) to account for differences in river morphology (e.g., intertidal creeks (Visintainer et al. 2006; Allen et al. oxbows in the upper river vs. tidal tributaries and embay- 2007). The authors of these studies recommended that ments in the lower river) and likely differences in fish geomorphological variation be considered when assessing assemblages along the environmental gradient of the river. factors affecting fish abundance along salinity and other The upper section was defined as the area from the Franklin environmental gradients. We hypothesize that freshwater Lock to the confluence of the Orange River, where a and estuarine fish assemblages, and species-specific abun- distinct change in river morphology occurs. The middle dances, likely differ between the backwaters and mainstem section was the area from the Orange River downstream to of tidal rivers. The objective of this study was to compare the urban centers of Ft. Myers and Cape Coral, and the the fish assemblages of the mainstem and backwater lower section extended from the urban centers to the river habitats of the tidal Caloosahatchee River, a highly mouth. managed river system located in an urban setting in southwest Florida. Field Sampling Monthly stratified-random sampling for fishes and selected Methods invertebrates was conducted in the tidal Caloosahatchee River using a small center-bag seine (21.3 m long× 1.8 m Study Site deep, 3-mm-stretch mesh) from November 2003 to December 2007. For the purposes of site selection, the The Caloosahatchee River is part of a cross-Florida canal river was divided into seven mainstem zones and three system that passes through Lake Okeechobee and connects backwater zones, and the number of sites sampled in each the intracoastal waterways of Florida's east and west coasts. zone was proportional to zone area to ensure adequate Water discharge to the Caloosahatchee River is regulated at coverage along the environmental gradient of the river. The three water-control structures (lock and dam) along the sampling locations were chosen randomly each month from 1218 Estuaries and Coasts (2010) 33:1216–1224 Fig. 1 Mainstem and backwater sampling sites in the Caloosahatchee River, Florida. River sections (lower, middle, upper) used in the analysis are delineated by vectors a universe of 0.2×0.2 km grids that contained shoreline. A practical taxonomic level (nomenclature for fishes follows total of 16 sites were selected along the mainstem of the Nelson et al. 2004), measured, enumerated,