fish recruitment is influenced by river flows and floodplain inundation ...

RIVER RESEARCH AND APPLICATIONS

River Res. Applic. (2012) Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/rra.2604

FISH RECRUITMENT IS INFLUENCED BY RIVER FLOWS AND FLOODPLAIN INUNDATION AT APALACHICOLA RIVER, FLORIDA A. C. DUTTERERa*, C. MESINGb, R. CAILTEUXc, M. S. ALLENd, W. E. PINEe and P. A. STRICKLANDc a

Florida Fish and Wildlife Research Institute, Florida Fish and Wildlife Conservation Commission, Gainesville, FL USA Division of Habitat and Species Conservation, Florida Fish and Wildlife Conservation Commission, Midway, FL USA c Florida Fish and Wildlife Research Institute, Florida Fish and Wildlife Conservation Commission, Quincy, FL USA d Fisheries and Aquatic Sciences Program, University of Florida, Gainesville, FL USA e Wildlife Ecology and Conservation, University of Florida, Gainesville, FL USA

b

ABSTRACT High human demand for limited water resources often results in water allocation trade-offs between human needs and natural flow regimes. Therefore, knowledge of ecosystem function in response to varying streamflow conditions is necessary for informing water allocation decisions. Our objective was to evaluate relationships between river flow and fish recruitment and growth patterns at the Apalachicola River, Florida, a regulated river, during 2003–2010. To test relationships of fish recruitment and growth as responses to river discharge, we used linear regression of (i) empirical catch in fall, (ii) back-calculated catch, via cohort-specific catch curves, and (iii) mean total length in fall of age 0 largemouth bass Micropterus salmoides, redear sunfish Lepomis microlophus and spotted sucker Minytrema melanops against spring–summer discharge measures in Apalachicola River. Empirical catch rates in fall for all three species showed positive and significant relationships to river discharge that sustained floodplain inundation during spring–summer. Back-calculated catch at age 0 for the same species showed positive relationships to discharge measures, but possibly because of low sample sizes (n = 4–6), these linear regressions were not statistically significant. Mean total length for age 0 largemouth bass in fall showed a positive and significant relationship to spring– summer discharge; however, size in fall for age 0 redear sunfish and spotted sucker showed no relation to spring–summer discharge. Our results showed clear linkages among river discharge, floodplain inundation and fish recruitment, and they have implications for water management and allocation in the Apalachicola River basin. Managed flow regimes that reduce the frequency and duration of floodplain inundation during spring–summer will likely reduce stream fish recruitment. Copyright © 2012 John Wiley & Sons, Ltd. key words: fish recruitment; floodplain inundation; river fish monitoring; streamflow management; largemouth bass Micropterus salmoides; redear sunfish Lepomis microlophus; spotted sucker Minytrema melanops Received 25 June 2012; Accepted 31 July 2012

INTRODUCTION Water allocation and operation of regulated rivers has become an increasingly contentious subject worldwide, primarily because of increasing competition between human users of aquatic ecosystems and regulatory mandates to maintain ecological integrity of aquatic ecosystems (Poff et al., 2003). With the increasing freshwater demand by humans commonly surpassing supply in many river basins, trade-offs among water allocation options become inevitable. Informed water allocation practices and policies depend on understanding how aquatic ecosystems function under natural flow regimes as well as knowledge of ecosystem response to flow modification (Richter et al., 2003). Streamflow is considered to be among the most influential factors that shape biotic communities in lotic environments (Poff and Ward, 1989; Poff et al., 1997). It can influence *Correspondence to: A. C. Dutterer, Fish and Wildlife Research Institute, Florida Fish and Wildlife Conservation Commission, 7922 NW 71st St, Gainesville, FL 32653, USA. E-mail: [email protected]

Copyright © 2012 John Wiley & Sons, Ltd.

community structure (Stanford and Ward, 1983, Rogers et al., 2005) as well as growth (Sammons and Maceina, 2009), reproduction (Smith et al., 2005) and mortality (Tramer, 1978) of stream biota. As regulation of rivers has increased, there is a large and growing body of research that has demonstrated responses of aquatic ecosystems to modified streamflow (Murchie et al., 2008). In low-gradient river floodplain systems, wet season high flows usually provide annual connectivity and inundation of the floodplain (Welcomme, 1979). Annual flooding is considered to be a major driver of productivity in river floodplain systems (Junk et al., 1989), and it is common that fish species in these systems display behavioral adaptations to exploit annual flooding events (Welcomme, 1979; Bayley, 1988; Kwak, 1988; Balcombe et al., 2005). Commonly, fish in river floodplain systems respond to rising water levels and floodplain inundation as cues for spawning (Agostinho et al., 2004). As spawning and nursery habitat for river fish assemblages, inundated floodplain habitats provide food and complex cover for refuge from predation (Balcombe et al., 2005). Consequently, annual variation in fish recruitment

A. C. DUTTERER ET AL.

is often influenced by river water level and floodplain inundation (Raibley et al., 1997; DiCenzo and Duval, 2002; Smith et al., 2005; Janac et al., 2010). The Apalachicola River is a highly regulated river in Florida with a large, undeveloped floodplain. Wet season high flows in the Apalachicola River historically provided annual periods of inundation and connectivity to floodplain habitats (Light et al., 1998). Although the magnitude and the duration of floodplain inundation vary from year to year, some degree of floodplain inundation has occurred with near-annual regularity, and the ecology of floodplain biota is reflective of regular flooding (Light et al., 1998). Considering the pervasive linkage of fish productivity and river floodplain inundation in other large river–floodplain systems (Welcomme, 1979), this relationship may extend to the fish assemblage at Apalachicola River as well, and the annual degree of floodplain inundation may influence stream fish population vital rates, such as recruitment and growth. However, recent research has shown that the frequency and the magnitude of floodplain inundation have declined in recent years at Apalachicola River relative to historical records (Light et al., 2006). These results cause significant concerns because decreases in connectivity between river main stem and floodplain habitats may lead to reduced ecosystem function or loss of biodiversity in these habitats (Ward et al., 1999). In light of this downward trend in floodplain inundation and ongoing debates over water management and allocation within the Apalachicola– Chattahoochee–Flint (ACF) river system, our objectives were to determine fish and streamflow relationships at Apalachicola River by relating indices of fish recruitment and measures of age 0 fish growth to river discharge and floodplain inundation. The understanding of the relationship of interannual fluctuations in fish recruitment and condition relative to river discharge will help resource managers gauge the potential effects of managed flow regimes. STUDY AREA The Apalachicola River is the largest river in Florida in terms of mean annual discharge [630 m3∙s 1 (22,300 ft3∙s 1); Light et al., 1998]. It is formed by the confluence of the Chattahoochee and Flint rivers near the Florida–Georgia border and flows 170 km through the Florida panhandle to the Gulf of Mexico (Figure 1). In total, the ACF basin encompasses an area of approximately 50,700 km2, and it includes 16 dams (Ward et al., 2005). Jim Woodruff Lock and Dam, the most downstream dam in the ACF basin, impounds the confluence of the Chattahoochee and Flint rivers (forming Lake Seminole in 1957), and its discharge marks the beginning of the Apalachicola River. Owing to the extensive dam complex within its basin, streamflow in the Apalachicola River is highly regulated. Copyright © 2012 John Wiley & Sons, Ltd.

Figure 1. Map of Apalachicola River and its adjacent floodplain,

with sampling reach denoted (32–128 rkm). Spatial data for the Apalachicola River floodplain was derived from a forest map by Leitman (1984) that was digitized and modified for use by Darst and Light (2008)

The Apalachicola River floodplain is the largest river floodplain in Florida, and having minimal development, it remains one of the most the most extensively forested floodplains in the contiguous United States. The freshwater, nontidal floodplain ranges from 1.6 to 8 km in width and represents an area of roughly 332 km2 (Light et al., 1998). Floodplain habitats range from deep and often connected oxbow lakes to Tupelo-cypress (Nyssa aquatic, Nyssa ogeche and Taxodium distichum) swamps to mixed bottomland forests (Light et al., 1998). Typically, highest flows occur in the Apalachicola River during late winter and spring (January–April). This seasonal window of high flows varies in magnitude and duration yet provides inundation of some portion of floodplain habitats during most years (for further explanation of the relationship of river discharge and floodplain inundation, see Discharge Measurement section; Light et al., 1998). The freshwater fish assemblage of the Apalachicola river– floodplain system is one of the most diverse of all Florida rivers (Bass, 1983). Basin-wide, there have been 116 fish species documented, and within the Apalachicola River section (below Jim Woodruff Lock and Dam), 86 species have been found (Yerger, 1977; Bass, 1983). A large proportion of these fishes (80%) have been linked to River Res. Applic. (2012) DOI: 10.1002/rra

APALACHICOLA RIVER, FLORIDA: FISH RECRUITMENT RELATED TO RIVER FLOWS

floodplain habitats at some point during their life history (Light et al., 1998). At least 45 species are known to use the Apalachicola River floodplain for spawning and nursery habitats based on larval fish light trap collections from 2002 to 2007 (Walsh et al., 2009; Burgess et al., 2012). Therefore, the connection and the inundation of floodplain habitats are most likely very important in shaping the structure of the freshwater fish assemblage at Apalachicola River.

METHODS Fish collection We sampled stream fish at Apalachicola River with boat electrofishing during September and October of each year during 2003–2010. Our sampling spanned from river kilometer (rkm) 128, where floodplain expansion begins, downstream to rkm 32, which excluded areas of tidal influence in the lower river (Figure 1). We used 10-min electrofishing transects as sampling replicates for measuring fish catch per unit effort (CPUE). To ensure that our sampling was representative of the overall fish assemblage, our sampling effort was stratified equally between main stem and connected backwater habitats (50 transects each; 100 transects total per year), as many species routinely migrate between these habitats (Burgess et al., 2012). For each year of sampling, we randomly reselected sampling transect locations within each of the two strata. Our electrofishing sampling was focused exclusively toward shoreline habitats (≤10 m from bank). Habitats along channel margins generally included sandbars, overhanging riparian vegetation, woody debris or exposed cypress root structures. During sampling, we encountered depths that ranged from less than 0.5 m, along inside river bends, to 6 m, along outside river bends. Habitat composition and depth varied within sampling transects as well as among sampling transects during each year; however, the suite of conditions for each variable was similar among years of sampling. Our boat electrofisher used two bowmounted anode arrays, spaced 2 m apart, and two bowpositioned netters. Electrical output (pulsed direct current, 60 Hz) was standardized to produce approximately 6000 W of potential energy transfer to targeted fish. All fish collected were identified to species and measured for total length (TL) and weight, except for during preliminary sampling in 2003–2004, when only largemouth bass Micropterus salmoides were enumerated. Fish aging We selected largemouth bass, redear sunfish Lepomis microlophus, and spotted sucker Minytrema melanops as species for which recruitment could be related to river Copyright © 2012 John Wiley & Sons, Ltd.

discharge. These species were selected because they are native to the ACF basin, relatively common and represent different feeding guilds or trophic levels within the fish assemblage (largemouth bass, piscivore; redear sunfish, insectivore/molluscivore; spotted sucker, algivore/detritivore). To construct age-length keys (Ricker, 1975), we retained approximately 10 individuals per 1-cm size group for aging during each sampling season. To ensure that our age-length keys were representative of the overall population of each species within the river, we generally used equal contributions of individuals from main stem and backwater strata for aging analyses. In addition, to reduce any potential biases of a single locale, individuals retained for aging were generally selected from different sampling transects. We aged largemouth bass and redear sunfish by counting annular rings in whole and transverse sectioned sagittal otoliths via dissecting microscopes. To age spotted sucker, we counted annular rings of whole and sectioned lapilli otoliths. The verification of annular ring formation in largemouth bass otoliths was established by Taubert and Tranquilli (1982), and the validation of these methods for aging largemouth bass was conducted by Hoyer et al. (1985). Verification that rings in redear sunfish sagittal otoliths are formed annually and that interpretation of these rings is a valid estimator of age was established by Mantini et al. (1992). For spotted sucker, the use of otolith annuli as an age estimator has not been yet validated in peer-reviewed literature. However, monthly marginal increment analyses of spotted sucker at Apalachicola River, Florida indicated that ring formation in lapilli otoliths occurred on an annual cycle (Strickland PA, unpublished research). Furthermore, the use of annular rings in lapilli otoliths has been used as an age estimator in other Catostomidae species (Thompson and Beckman, 1995; Terwilliger et al., 2010). Therefore, we interpreted rings on spotted sucker as annuli for this study. Discharge measurement Apalachicola River discharge data were provided by the US Geological Survey as measured at the long-term surfacewater gauge near Chattahoochee, Florida (station number 02358000), located 1 km downstream of Jim Woodruff Lock and Dam. Our measure of streamflow was the proportion of days during 1 March–30 September when mean daily discharge was ≥460 m3∙s 1 (16,400 ft3∙s 1). The discharge criterion of 460 m3∙s 1 corresponds to the median daily discharge during 1922–1995, and it is close to an inflection point in the relationship of discharge and floodplain inundation, which occurs at approximately 370–400 m3∙s 1 (13,000-14,000 ft3∙s 1; Light et al., 1998). Lower than 370 m3∙s 1, less than 3% of the floodplain is inundated, and discharge is largely confined within the channels of the Apalachicola main stem and major tributaries. Higher than 370 m3∙s 1, discharge increasingly inundates floodplain River Res. Applic. (2012) DOI: 10.1002/rra


habitats (Light et al., 1998). The median daily discharge value (460 m3∙s 1) that we used as a discharge criterion is slightly above the inflection point in the relationship of discharge and floodplain inundation and corresponds to inundation of approximately 10% of the Apalachicola River floodplain (Light et al., 1998). The temporal window of our discharge criterion begins in March, typically the onset of stream fish spawning in Apalachicola River (Pine et al., 2006; Walsh et al., 2009; Burgess et al., 2012), and includes flows affecting young-of-the-year fishes until the onset of annual sampling in the fall. Analyses We used linear regression to assess the relationship of fish recruitment as a response to Apalachicola River discharge during spring and summer. We used two methods of indexing recruit abundance: (i) empirical measures of age 0 electrofishing CPUE (individuals per hour) and (ii) backcalculated estimates of age 0 CPUE via cohort-specific catch curves. Because we collected age-specific catch data during six consecutive years, we were able to track relative abundance of multiple cohorts through time; thus, enabling use of cohort-specific catch curves (Tetzlaff et al., 2011). Also, the empirical CPUE of age 0 fish could be influenced by annual changes in the catchability of young, small fish. Therefore, we used the back-calculated method as a second method to evaluate trends. Cohort-specific catch curves consisted of the linear regression of loge transformed mean CPUE as a response to cohort age. We used the y-axis intercept of the linear regression equation for each cohortspecific catch curve as the back-calculated catch of that cohort at age 0. In our analyses, we only included backcalculated recruit indices that were calculated from cohortspecific catch curves that had three or more years of consecutive catch and showed significant and negative linear regression lines (i.e. an indication of mortality). To be valid, the method of back calculation of recruit abundance via cohort-specific catch curves assumes (i) survival is constant among years, (ii) catches are proportional to abundance, (iii) effort is consistent among years or catch is standardized relative to effort and (iv) annual sampling catchability is constant (Tetzlaff et al., 2011). This method improves the accuracy of recruitment indices over catch curves conducted across cohorts (Tetzlaff et al., 2011), and it provided an additional measure of recruitment to the annual age 0 CPUE data. We considered linear regression lines to be significant at a = 0.1. In addition to evaluating juvenile abundance indices as a response to river discharge, we also evaluated juvenile stream fish growth as a response to river discharge. Specifically, we used linear regression to evaluate mean TL in fall of age 0 individuals as a response to a spring–summer Copyright © 2012 John Wiley & Sons, Ltd.

discharge metric. Our use of relative abundance of age 0 fishes (empirical CPUE) in fall as an index of recruitment for each year assumes similar over-wintering survival among years. However, because size-specific survival can be influential to juvenile cohorts, high juvenile abundance alone does not necessarily infer higher recruitment (e.g. Van Horne, 1983). In other words, a juvenile cohort could be very abundant in the fall but occur at smaller than normal body size and suffer a higher than normal overwinter mortality rate, reducing the cohort’s actual contribution to the adult population. Therefore, knowledge of the relationship of age 0 stream fish size in fall and spring–summer discharge patterns would provide insight to the validity of drawing inferences from abundance of juvenile fish cohorts in fall.

RESULTS We sampled largemouth bass from 2003 to 2010, and during that time frame, empirical catch rates of age 0 largemouth bass (individuals per hour) ranged from 6.19 (2004) to 54.93 (2005). Redear sunfish and spotted sucker were sampled during 2005–2010. During that time frame, the age 0 catch rates of redear sunfish ranged from 0.12 (2007) to 15.28 (2005) and those of spotted sucker ranged from 0.12 (2010) to 9.73 (2009). Apalachicola River discharge measures (proportion of days with discharge ≥ 460 m3∙s 1) during spring and summer of sampling years ranged from 0.09 (2007) to 0.89 (2003), indicating both persistently high and low streamflow conditions during our study duration. Linear regression showed a positive and significant relationship between age 0 catch and spring–summer river discharge for all three of the species investigated (all P ≤ 0.0691; Table I; Figure 2, left panels). Back-calculated catch of age 0 fish abundance had lower sample size than the age 0 fish CPUE data, but the results generally corroborated the age 0 CPUE catches (Table I; Figure 2). Back-calculated catch of age 0 largemouth bass included cohorts from 2001 to 2006 and ranged from 11.71 (2001) to 63.70 (2003). For redear sunfish, backcalculated catch included 2003–2006 cohorts and ranged from 3.48 (2006) to 62.24 (2003). For spotted sucker, back-calculated catch included cohorts 2002–2004 and 2006 and ranged from 4.98 (2006) to 85.56 (2003). Spring–summer discharge measures (proportion of days ≥ 460 m3∙s 1) that corresponded to back-calculated catch ranged from 0.12 (2002) to 0.89 (2003), indicating high and low streamflow periods, were encompassed in time frames for back-calculated catch. Linear regression lines for back-calculated age 0 catch against spring–summer river discharge showed positive relationships; however, linear models were not significantly different from a zero slope River Res. Applic. (2012) DOI: 10.1002/rra


Table I. Linear regression equations for catch of age 0 stream fish and Apalachicola River discharge (proportion of days during 1 March–30 September when mean daily discharge ≥ 460 m3∙s 1) for each species and abundance estimation method with R2, P and n Species Largemouth bass Redear sunfish Spotted sucker

Recruitment metric Empirical CPUE Back-calculated CPUE Empirical CPUE Back-calculated CPUE Empirical CPUE Back-calculated CPUE

Regression equation Age Age Age Age Age Age

0 CPUE = 50.149 Discharge + 1.252 0 CPUE = 37.514 Discharge + 14.153 0 CPUE = 20.199 Discharge 2.383 0 CPUE = 26.086 Discharge + 24.711 0 CPUE = 12.881 Discharge 1.258 0 CPUE = 85.174 Discharge + 7.764

R2

P

n

0.73 0.41 0.73 0.15 0.60 0.65

0.0068 0.1689 0.0311 0.6120 0.0691 0.1933

8 6 6 4 6 4

Figure 2. Plots of age 0 stream fish catch during fall against spring–summer river discharge at Apalachicola River, Florida. Empirical catch rates are shown in the left panels, and back-calculated catch rates are shown in the right panels. Only significant regression lines are included (P < 0.1)

line (all P between 0.1689 and 0.6120; Table I; Figure 2, right panels). Sample sizes for the regressions were low (4–6 points), resulting in low statistical power. Copyright © 2012 John Wiley & Sons, Ltd.

Results of linear regression of mean TL for age 0 stream fish in the fall against spring–summer discharge patterns varied by species (Table II; Figure 3). The mean TL of age 0 River Res. Applic. (2012) DOI: 10.1002/rra


Table II. Linear regression equations for mean TL of age 0 stream fish and Apalachicola River discharge (proportion of days during 1 March– 30 September when mean daily discharge ≥ 460 m3∙s 1) with R2, p and n Regression Equation

R2

P

n

TL = 36.014 Discharge + 91.908 TL = 13.639 Discharge + 61.384 TL = 2.914 Discharge + 98.321

0.53 0.07 0.01

0.042 0.603 0.837

8 6 6

Species Largemouth bass Redear sunfish Spotted sucker

largemouth bass was positively and significantly (P = 0.042) related to spring–summer discharge, whereas linear regression of age 0 redear sunfish and spotted sucker with spring– summer discharge showed no relationship (both P ≥ 0.603; Table II; Figure 3).

DISCUSSION We found positive relationships between age 0 stream fish catch in fall and spring–summer discharge measures in the Apalachicola River, Florida. Other studies have reported similar relationships between strong year-classes of fish and elevated water levels or streamflows for multiple riverine or river influenced habitats, including estuaries (Staunton-Smith et al., 2004), reservoirs (Maceina and Stimpert, 1998; DiCenzo and Duval, 2002; Maceina, 2003), rivers (Bonvechio and Allen, 2004; Smith et al., 2005) and large river floodplain systems (Raibley et al., 1997; Janac et al., 2010). Furthermore, the interconnection of fish recruitment, streamflow and floodplain inundation is consonant with previous fish community research at Apalachicola River. Walsh et al. (2009) showed extensive use of floodplain habitat by larval stream fish during spring and summer, and Pine et al. (2006) and Burgess et al. (2012) reported high use of inundated floodplain habitat by telemetered adult stream fish during the spring spawning season that was coincident with the collection of high numbers of larval fishes representing numerous species (n = 45) among light trap catch in the floodplain. Combined, these results provide strong implication that floodplain connection and inundation are important for stream fish communities at Apalachicola River, Florida. River floodplain connection and inundation provides critical exchange of energy and nutrients between river main stem and floodplain ecosystems (Junk et al., 1989). Bolstered production in inundated floodplain habitats provides abundant food sources for young-of-the-year stream fishes (Bayley, 1988; Junk et al., 1989), and inundated floodplain vegetation creates complex structural habitat, providing refuge from predation (Savino and Stein, 1982; Rozas and Odum, 1988). Therefore, increases in the spatial and temporal breadth of floodplain inundation, as mediated by river discharge, likely explain stream fish recruitment and streamflow relationships. Copyright © 2012 John Wiley & Sons, Ltd.

Size-selective mortality can be very influential on juvenile fish cohorts (Miller et al., 1988; Sogard, 1997). Generally, the largest individuals within a cohort enjoy the greatest chances of recruiting to adult populations, as larger body size usually translates into decreased vulnerability to starvation, predation or environmental extremes (Sogard, 1997). Previous research has downplayed the role of sizeselective overwinter survival on cohorts of juvenile fishes in Florida (Rogers and Allen, 2009). However, because our sampling occurred in fall, before any potential effects of size-selective survival associated with winter conditions, we thought it was important to investigate fish size as a response to spring–summer discharge patterns. Our results indicated that years with higher river discharge and sustained floodplain inundation during spring–summer allowed age 0 largemouth bass to grow to a larger size in the fall relative to lower discharge years. However, age 0 redear sunfish and spotted sucker growth did not appear to be influenced by spring–summer discharge patterns. These results show that individuals belonging to the large cohorts of fish produced during years of high discharge during spring–summer have average to larger than average body size in fall. On the basis of body size, we would expect large cohorts produced during high flow years to have similar overwinter survival, or greater for largemouth bass, as during years with lower spring–summer discharge patterns. Thus, large cohorts in fall would be expected to remain large cohorts entering age 1. This was corroborated by the similarity of results between empirical catch and back-calculated abundance for age 0 fishes at the Apalachicola River. Our use of catch rate as an index of abundance for age 0 fishes requires acknowledgment that multiple factors other than true abundance can affect catch rates. Physical habitat variables (i.e. water temperature, conductivity, clarity, habitat complexity and flow velocity), fish size, fish seasonal behaviour patterns and sampling crew have been shown to affect the fraction of a fish stock caught per unit effort (i.e. catchability; Hardin and Connor, 1992; Hilborn and Walters, 1992; Reynolds, 1996; Bayley and Austen, 2002). Timing of our sampling was standardized to fall (September and October), thus reducing the variable influence of factors such water temperature and fish seasonal behaviour patterns. Typically, streamflows are relatively stable during fall at Apalachicola River (Light et al., 1998), and we standardized River Res. Applic. (2012) DOI: 10.1002/rra


Figure 3. Plots of mean TL of age 0 stream fish in fall against spring–summer river discharge at Apalachicola River, Florida, with linear regression lines. Only significant regression lines are included (P < 0.1)

sampling to occur only during discharges where flows were largely confined within the banks of the main stem and major distributary channels. Thus, river stage did not vary substantially across years or during sampling periods, minimizing its influence on our catch rates. Also, our use of back-calculated age 0 catch rates based on cohort-specific catch curves relied on the catch of a cohort over multiple years of sampling and therefore reduced the influence of year to year variation in environmental conditions on fish catchability (Tetzlaff et al., 2011). For cohorts that Copyright © 2012 John Wiley & Sons, Ltd.

allowed our use of empirical catch and back-calculated catch as indices of recruit abundance, our results were similar, indicating that fluctuations in our catch reflected actual fluctuations in fish abundance and not annual fluctuations in catchability. The flow metric we used was the proportion of days during spring–summer (1 March–30 September) with river discharge ≥460 m3∙s 1(16,400 ft3∙s 1), but our results were robust to other flow measures. The value of 460 m3∙s 1 was the median river discharge at Apalachicola River during 1922–1995 as reported by Light et al. (1998), and above this discharge threshold, the wetted area of inundated floodplain increases substantially (Light et al., 1998). However, we explored alternate flow levels (e.g. 364–1,700 m3∙s 1) within the 1 March–30 September time frame without seeing substantial changes in our results. In contrast, we explored the use of flow metrics from smaller time frames (e.g. March–May instead of March–September), and shorter time frames resulted in poorer fit of linear models. Therefore, river flows that inundate the Apalachicola River floodplain during spring spawning and maintain portions of inundated floodplain during summer for nursery habitat appear to have the strongest relationship with high stream fish recruitment. Our results have implications for water management and allocation in the ACF basin. We found clear linkages between flows of the Apalachicola River and recruitment of fish in the system, and water management operations and upstream consumptive uses that substantially reduce the number of days with flow exceeding 460 m3∙s 1 (16,400 ft3∙s 1) would be expected to reduce fish recruitment. These findings are important and critical to water management within the ACF basin as Light et al. (2006) identified recent downward trends in streamflow at Apalachicola River. We observed fish recruitment response to streamflow across multiple trophic levels within the fish community; therefore, effects of reduced flow likely would be widespread within the Apalachicola River fish community. These biological effects should be considered for streamflow management and water allocation within the basin. Our results indicate that fish monitoring programs that measure both fish CPUE and age composition data can be useful for evaluating management of flow in regulated river systems. Considering the decreasing trend in river flows and the high demand for water use within the basin, continued monitoring of stream fish assemblages at Apalachicola River is recommended. ACKNOWLEDGEMENTS

Partial funding for this research was provided by the Aquatic Habitat Restoration and Enhancement Subsection of the Habitat and Species Conservation Division of the Florida Fish and Wildlife Conservation Commission. River Res. Applic. (2012) DOI: 10.1002/rra


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River Res. Applic. (2012) DOI: 10.1002/rra