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Site Characteristics and Prey Abundance at Foraging Sites Used by Lesser Scaup (Aythya affinis) Wintering in Florida
Garth Herring and Jaime A. Collazo

Southeastern Naturalist, Volume 8, Number 2 (2009): 363–374

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2009 SOUTHEASTERN NATURALIST 8(2):363–374 Site Characteristics and Prey Abundance at Foraging Sites Used by Lesser Scaup (Aythya affinis) Wintering in Florida Garth Herring1, 2,* and Jaime A. Collazo1 Abstract - We examined site characteristics and prey abundances where wintering Aythya affinis (Lesser Scaup; hereafter scaup) foraged within three regions of the Indian River Lagoon system in central Florida. We observed that scaup concentrated in the Indian and Banana rivers; however, density of prey items did not differ between foraging sites and random sites. We also found that site characteristics were similar between foraging and random sites. Differences in site characteristics between random points across all three regions did not explain the distribution of foraging scaup (no scaup foraged in the Mosquito Lagoon); however, prey densities were approximately 3 times lower in the Mosquito Lagoon region. Our study suggests that current habitat conditions within the northern Indian River Lagoon system meet the overwintering requirements of scaup; however, prey densities in the Mosquito Lagoon may have been too low to be profitable for foraging scaup during the period of our study. Introduction Understanding how avian species distribute themselves is fundamental to understanding the use and significance of habitats throughout the annual cycle. For example, during the breeding season many birds must locate highquality foraging patches to accumulate nutrient reserves for breeding and brood rearing (see Afton and Ankney 1991, Alisauskas et al. 1990, Ankney and Afton 1988). Energetic demands of birds are not as high during the winter season; consequently, birds may only need to sustain a level of condition that allows them to survive and then initiate spring migration. Regardless of lower physiological requirements of birds during the wintering period, individuals still need to select foraging patches that allow them to acquire sufficient food resources for each day during the entire wintering period. For many species, the importance of habitat conditions on wintering sites remains poorly understood, and may well be an important consideration for conservation efforts, particularly for species that have demonstrated longterm declines. The combined continental population of Aythya affinis (Eyton) (Lesser Scaup) and A. marila (L.) (Greater Scaup) has declined since the late-1970s (Afton and Anderson 2001, Austin et al. 2000, US Fish and Wildlife Service 2008) and currently is below the North American Waterfowl Management Plan goals (US Fish and Wildlife Service 2008). Afton and Anderson (2001) 1US Geological Survey, North Carolina Cooperative Fish and Wildlife Research Unit, North Carolina State University, Raleigh, NC 27695. 2Current Address - US Geological Survey, Western Ecological Research Center, Davis Field Station, One Shields Avenue, University of California, Davis, CA 95616. USA*Corresponding author - gherring@ucdavis.edu. 364 Southeastern Naturalist Vol. 8, No. 2 concluded that the long-term population decline was due to a decline in Lesser Scaup. Population and habitat trends underscore the importance of identifying habitat characteristics that provide sufficient energetic needs and pre-migratory nutrient reserves on wintering grounds. While studies have suggested that habitat conditions on wintering grounds currently may be sufficient to meet wintering requirements of scaup (see Herring and Collazo 2004, 2005, 2006), understanding exactly what those habitat conditions are now is critical so that they can be used as a benchmark for potential future impacts that may infl uence wintering scaup. The Indian River Lagoon system (IRL) of central Florida provides overwintering habitat for approximately 15% of the continental population (Austin et al. 1998, Bellrose 1980). Yet, despite its importance, there are no available data on foraging-site characteristics or quality across the IRL system (e.g., density of scaup prey items). Data on foraging-site characteristics could aid in understanding underlying factors that might infl uence wintering scaup foraging-site selection and the biological processes that might infl uence the quality of foraging sites. Our objectives were to: 1) estimate potential scaup prey densities throughout the northern IRL system, 2) compare prey densities at sites used by foraging scaup and those we selected randomly, 3) determine if foraging sites have unique attributes (i.e., depth, vegetation structure) that might help scaup identify them as higher quality sites, and 4) assess diet of wintering scaup to verify our prey-density findings. Study Area We conducted our research at Merritt Island National Wildlife Refuge (MINWR) during the winter of 2001 (Fig. 1). The refuge is located on the Atlantic Coast approximately 1 km east of Titusville, FL (28°40'N, 80°46'W). The refuge envelops the John F. Kennedy Space Center on Merritt Island, covering 55,039 ha of the National Aeronautical and Space Administration’s 57,000 ha in Brevard and Volusia counties. Our study area was part of the Indian River Lagoon (IRL), a series of three distinct, but interconnected, estuarine systems, which extend 250 km from Ponce Inlet to Jupiter Inlet on Florida’s east coast. The IRL includes the waters of the Indian and Banana rivers and the Mosquito Lagoon. We focused our study in the northern IRL, covering approximately 25% of the IRL from State Road 528 north in the Indian and Banana rivers, and the southern half of the Mosquito Lagoon (Fig. 1). Merritt Island proper is a barrier island complex separated from the mainland by the Indian River and from Cape Canaveral by the northern Banana River. The island is composed of sandy beaches, dune systems, hammocks, lagoons, and 72 salt marsh impoundments jointly managed by the Brevard Mosquito Control District, National Park Service, NASA, and US Fish and Wildlife Service. IRL waters tend to be shallow, aeolian lagoons with depths averaging 1.5 m (maximum = 9 m in dredged areas). Accordingly, depths throughout our study are consistent with suitable foraging depths for 2009 G. Herring and J.A. Callazo 365 Lesser Scaup (Austin et al. 1998, Bellrose 1980). Salinities range from 10 to 42 ppt (Provancha and Sheidt 2000). Estuarine wetland habitats within our study site covered approximately 400 km2. Common macrophytes included Halodule wrightii Aschers (Shoal Grass), Syringodium filiforme Kuetz (Manatee Grass), Halophila engelmannii Aschers (Star Grass), Halophila decipiens Ostenf (Paddle Grass), and Ruppia maritime L. (Widgeon Grass) (Virnstein and Carbonara 1985). Figure 1. Location of the Lesser Scaup foraging (star) and random sites (asterisk), and whole-bird collection sites (×) in the northern Indian River Lagoon system, FL in 2001. Map shows the Merritt Island National Wildlife Refuge, and the three estuary systems: Indian River, Mosquito Lagoon, and Banana River. 366 Southeastern Naturalist Vol. 8, No. 2 Methods Scaup foraging sites We identified scaup foraging sites using aerial surveys conducted from a Bell 212 (UH-1N) helicopter at an altitude of 150 m and speed of 100 km/hr. We flew surveys approximately every 3 weeks, between 21 December 2000 and 8 March 2001 (n = 5), coinciding with the period scaup were observed on our study area. We began surveys at 0800 hr because scaup routinely foraged during the morning at all sites surveyed (Herring and Collazo 2005). The area surveyed included most of the highest scaup-use areas in the IRL region (Herring and Collazo 2005). Specifically, we covered all open estuarine regions of the Mosquito Lagoon, Banana River north of SR 528, and Indian River north of SR 528, using east–west transects approximately 3 km apart (Fig. 1); we alternated flight direction on transects for each survey. We estimated numbers of foraging scaup and recorded their locations during surveys; GPS coordinates were determined for the center of observed fl ocks. We considered only scaup fl ocks in which ≥50% of individuals were observed diving as the helicopter approached. The normal response of scaup to disturbance was fl ight rather than diving; thus, we are confident that fl eeing scaup were not confused with feeding scaup. Scaup at MINWR frequently used the deeper habitats of the IRL, requiring diving to feed (Herring and Collazo 2005, 2006). Flock size encountered during surveys averaged 1094 ± 89 SE (range = 7–4180). We arbitrarily chose to collect data at foraging sites from scaup fl ocks with ≥500 birds. Flocks of this size were scattered over an area of approximately 200 m by 200 m. The location and dimension of these foraging groups was our primary sampling unit (site). Site characteristics We measured vegetation characteristics, water depth, and prey communities at all sites (n = 19) within one week of the aerial surveys. At each site, we collected data at five locations using self-contained underwater breathing apparatus (SCUBA). The first sample point was located at the center point of the fl ock, based on the GPS coordinates from the aerial survey; the remaining four sampling points were located 100 m in each of the four cardinal directions from the center point. We used a 1-m2 sampling frame subdivided into 10-cm2 quadrats at each of the five locations at a foraging site. We lowered the 1-m2 sampling frame to the bottom at each site, aligning the west side of the quadrat in a north–south direction using a dive compass. We recorded the depth at the sampling sites by lowering a weighted measuring line and recorded the distance from the water’s surface to the bottom of the water column. We estimated the percent cover by counting the total number of 10-cm2 plots with submerged aquatic vegetation present. We recorded the average height (cm) and stem density (shoot count) of vegetation in four randomly selected 10-cm2 quadrats within the 1-m2 plot. We used mean values to better estimate site characteristics at the 1-m2 scale. 2009 G. Herring and J.A. Callazo 367 We identified random sampling sites on the same day by traveling 1 km on a random bearing from the center point of the foraging site. Scaup exhibit small home ranges during winter (home-range core use = 2.7 km2 ± 0.5 SE), with minimal movement from foraging sites on a daily basis (<3 km; Herring and Collazo 2005), suggesting that our random sampling sites were within the area that scaup foraged during a day. If we were unable to travel 1 km in a random direction, we selected another random bearing that met this criteria. We sampled random site characteristics using the same protocol as used for foraging sites; paired random and foraging sites were sampled on the same day. No foraging fl ocks with ≥500 scaup were located in the Mosquito Lagoon (see results), thus we randomly selected 8 sites to sample using a geographic information system (Arcview v. 3.3, ESRI, Redlands, CA); this number is close to the midpoint of the number of fl ocks sampled at Indian and Banana rivers. Prey density We collected 3 random, 5-cm deep × 10-cm diameter cores (392.5 cm3) at each 1-m2 plot where vegetation was assessed. Core samples included above-sediment macrophytes and benthos. This depth of benthos was 100% accessible to scaup and within their normal foraging depth (Richman and Lovvorn 2004). While core samples included both macrophytes and benthos, we acknowledge that highly mobile invertebrates (e.g., amphipods) probably were under sampled, and thus we constrained the analysis and interpretation to benthic and epibenthic fauna. Scaup diet in the IRL during this study was predominantly gastropods and bivalves (see Results), so prey-density results were meaningful during the period of our study. We strained core samples through a 60-μm sieve and stored them in a ziplock bag in 95% ethanol. Benthic macroinvertebrates samples were identified (most to species or genus) and counted within two weeks of sampling. Data were summarized as mean density per core sample at the m2 scale, and then averaged across the five 1-m2 plots sampled per site, and then extrapolated to a density/m2 estimate. We did not assess aggregate dry mass (biomass; Prevett et al. 1979) due to concerns of the high mass of shelled bivalves biasing the value of species (Custer and Custer 1996). Scaup diet To validate that prey items collected for prey-density estimates were in fact consumed by wintering scaup, we collected scaup (n = 57) at MINWR by either jump or pass shooting from the open-water areas in the Indian River, Banana River, and Mosquito Lagoon between 8 January and 3 March 2001. While efforts were made to collect scaup over the entire winter period, most scaup were collected over a short period (≤1 week) in each of the estuary sites. We assumed that diet selection varied little over the winter period given the large amount of available foraging habitat. We injected 95% alcohol into the esophagus immediately after recovering birds to minimize post mortem digestion. To avoid biasing results (Afton et al. 1991, Swanson and 368 Southeastern Naturalist Vol. 8, No. 2 Bartonek 1970), we used contents from the esophagus and proventriculus. Diet samples were identified, enumerated, and summarized as percent occurrence (Swanson et al. 1974) in scaup foraging sites. All research techniques were approved by the North Carolina State University, Institutional Animal Care and Use Committee (Protocol 01-144-0), and conducted under US Fish and Wildlife Service research permit 773137, and Florida Fish and Wildlife Conservation Commission, scientific collecting permit WXO1671. Statistical analyses Site characteristics. We divided the study period into three 26-day periods (early: 21 Dec–15 Jan, mid: 16 Jan–10 Feb, late: 11 Feb–8 Mar). We used a multivariate analysis of variance (MANOVA; JMP 2001) to test for differences between site types (foraging and random), season (early, mid, and late), region (Indian River, Banana River), and all interactions for % plant cover, water depth, plant shoot count, and plant height. Because we did not locate and subsequently sample scaup foraging sites in the Mosquito Lagoon, we tested separately for differences in site characteristics at random sites across all three estuaries using a MANOVA for the above response variables and main effects and their interactions. This approach allowed us to examine potential differences across estuaries that we could not detect based simply on our foraging sites, which were only located in the Indian and Banana rivers. We dropped interaction terms from final models if they were found not to be significant. Prey density. We used a general linear model (GLM; JMP 2001) to determine if total prey density differed by region, site type (foraging and random), and season. All data met assumptions of equal variance (Levene’s test; JMP 2001), and residuals were normally distributed. The critical level of all statistical tests was set at 0.05. We report least square means with one standard error (± 1 SE) from our final model, and estimate prey density at the m2 scale. Results Scaup foraging sites Scaup numbers on our first and last survey were low (250 and 2250, respectively) and peaked in late January (32,698). Mean number of scaup per survey was 12,625 ± 6062 SE. We located 19 foraging fl ocks of scaup with ≥500 individuals in the Banana River (n = 5) and Indian River (n = 14; Fig. 1). Mean fl ock size was 1094 ± 89 SE. Total cumulative numbers of scaup in the Mosquito Lagoon never exceeded 100 individuals during the entire study period. Site characteristics Foraging and random. We found no effect of site type (MANOVA: F4, 33 = 0.40, P = 0.80) on site characteristics, but did detect a significant effect of region (MANOVA: F2, 33 = 0.28, P < 0.01) and season (MANOVA: F2, 33 = 3.13, P < 0.01) on measures of % cover, depth, stem counts, and plant heights. 2009 G. Herring and J.A. Callazo 369 Univariate tests revealed that sites in the Indian River were deeper than the Banana River, % cover was higher early in the season and was higher in the Banana River, as were stem counts and shoot heights (Table 1). Random. We found no effect of season (MANOVA: F8, 36 = 0.1.54, P = 0.171) on site characteristics, but did detect a significant effect of region (MANOVA: F4, 18 = 29.32, P < 0.01) on measures of % cover, depth, stem count, and plant height. Univariate tests revealed that % cover was highest in the Banana River, but was similar between the Indian River and Mosquito Lagoon (Tables 1, 2). Plant height was higher in the Banana River and Mosquito Lagoon, while stem densities were highest only in the Banana River (Tables 1, 2). Prey density The three most common prey items collected with core sampling were Tellina tampaensis (Tampa Tellin; 67% of core samples), Cylichnidae Acteocina spp. (barrel-bubble snails; 14% of core samples), and Cerithium muscarum (Fly-specked Cerith; 5% of cores samples). We detected no differences in prey densities between scaup foraging sites (mean = 2154 m2 ± 254 SE) and random sites (mean = 1602 m2 ± 336 SE; F1, 36 = 1.90, P = 0.17). Prey densities were higher in the Indian (mean = 2546 m2 ± 240 SE) and Banana Rivers (mean = 2312 m2 ± 398 SE) than in the Mosquito Lagoon (mean = 774 m2 ± 454 SE) (F2, 36 = 5.80, P = 0.006). We failed to detect any difference in prey densities across the three seasonal periods (F2, 36 = 2.88, P = 0.07). Scaup diet We collected 57 scaup from the Banana River (n = 24), Indian River (n = 14), and Mosquito Lagoon (n = 19); 51 contained diet samples. Diet samples were evenly split by sex (26 M and 25 F). We recorded 16 different food items in their esophagus and proventriculus. Invertebrates comprised 99.5% of all items; the remaining 0.5% was made up of macrophyte seeds and vegetation parts (Table 3). The most frequent invertebrates consumed were T. tampaensis (26% occurrence), Assiminea succinea (Atlantic Assiminea; 18% occurrence), and Anomalocardia auberiana (Pointed Venus; 14% occurrence), accounting for 57% of all prey eaten (Table 3). Discussion The similarity of foraging conditions across the Indian and Banana rivers was underscored by the lack of differences in structural and prey-density levels between used and potential (random) foraging sites in these two estuaries. These contrasts also provided an opportunity to gain insights about foraging-site selection. Our results suggest that vegetation structure and other physical attributes were not important discriminators of foraging sites used by scaup. We suspect that lack of differences in prey density was real. Given that scaup had small core home ranges (mean = 2.7 km2) and moved short distances during the diel period (mean = 2.7 km; Herring 2004), we 370 Southeastern Naturalist Vol. 8, No. 2 Table 1. Multivariate analysis of variance model results for site characteristics by site type (random or foraging), region, and season at Lesser Scaup sites during winter 2001 in east-central Florida. Model results are defined as comparisons between foraging and random sites in the Indian and Banana rivers and among random sites only in the Indian River, Banana River, and Mosquito Lagoon. Source df F P Foraging and random: Indian and Banana rivers % cover Site type 1 0.003 0.95 Region 1 170.7 <0.01 Season 2 5.9 <0.01 Error 33 Total 37 Plant height Site type 1 0.0005 0.98 Region 1 250.1 <0.01 Season 2 4.1 0.02 Error 33 Total 37 Shoot count Site type 1 0.04 0.83 Region 1 347.7 <0.01 Season 2 3.5 0.03 Error 33 Total 37 Water depth Site type 1 0.21 0.64 Region 1 25.3 <0.01 Season 2 1.1 0.33 Error 33 Total 37 Random: Indian River, Banana River, and Mosquito Lagoon % cover Region 2 23.18 <0.01 Season 2 0.94 0.40 Error 27 Total 30 Plant height Region 2 7.38 <0.01 Season 2 1.40 0.26 Error 27 Total 30 Shoot count Region 2 11.86 <0.01 Season 2 1.44 0.25 Error 27 Total 30 Water depth Region 2 2.69 0.08 Season 2 0.75 0.47 Error 27 Total 30 2009 G. Herring and J.A. Callazo 371 suspect that the high densities of prey overall likely allowed scaup to remain within these core patches without depleting the available prey during the period of this study. It is important to note that our study was limited to one year; thus, our results have to be tempered with the prospect that in other years scaup may be able to deplete prey densities at foraging patches. Alternatively, scaup may have remained at foraging sites until a givingup density was reached, and then abandoned the sites. If this were the case, based on chance alone, it might be expected that some of the 19 random sites, with five independent replicates within, might have had elevated prey densities simply by chance. However, this was not the case, with estimates of variance being very similar between random and foraging sites. Further, while we did not quantitatively record whether scaup were still foraging at sites or how many (when we sampled scaup), we did frequently observe foraging scaup near the study foraging sites when we arrived to sample them (G. Herring, pers. observ.). Table 2. Least squares mean ± SE of site characteristics: plant cover, shoot height, shoot count, and water depth at random sites in the Indian and Banana rivers and Mosquito Lagoon, during winter 2001 in east-central Florida. Mosquito Banana River Indian River Lagoon Foraging and random: Indian and Banana rivers % cover (± SE) 77.0 ± 8.7 1.0 ± 6.5 Plant height (cm ± SE) 13.7 ± 1.1 1.50 ± 0.8 Shoot count 8.4 ± 0.6 0.2 ± 0.4 Water depth (m ± SE) 1.2 ± 0.8 1.8 ± 0.1 Random: Indian River, Banana River, and Mosquito Lagoon % cover (± SE) 69.0 ± 13.7 3.0 ± 10.0 21.2 ± 10.8 Plant height (cm ± SE) 8.50 ± 3.3 0.50 ± 2.4 2.96 ± 2.65 Shoot count 6.1 ± 1.8 0.6 ± 1.4 1.5 ± 1.5 Water depth (m ± SE) 1.3 ± 0.3 1.8 ± 0.2 1.6 ± 11.0 Table 3. Summary of Lesser Scaup diets during winter 2001 in the Indian River Lagoon system, FL. # of items Food item % occurrence in all scaup Tellina tampaensis Conrad (Tampa Tellin) 26 95 Assiminea succinea Pfeiffer (Atlantic Assiminea) 18 66 Anomalocardia auberiana d’Orbigny (Pointed Venus) 14 50 Palaemonetes intermedius Holthuis (Grass Shrimp) 9 32 Parastarte triquetra Conrad (Brown Gemclam) 8 30 Cylichnidae Acteocina spp Gray (Barrel-bubble Snail) 8 30 Granulina hadria Dall (Hadria Marginella) 7 27 Cerithium muscarum Say (Fly-specked Cerith) 5 18 Marginella apicina Menke (Common Atlantic Marginella) 4 14 Amphipoda spp. (scuds) 4 15 Astyris lunata Say (Lunar Dovesnail) 1 3 Melongena corona Gmelin (Crown Conch) <1 1 Caecum pulchellum Stimpson (Beautiful Caecum) <1 11 Melampus bidentatus Say (Coffee Bean Snail) <1 1 372 Southeastern Naturalist Vol. 8, No. 2 Differences in prey densities between the infrequently used Mosquito Lagoon and the continually and heavily used Indian and Banana Rivers appear to be directly related to prey densities within each area. Overall, prey densities were approximately 3 times higher in both the Indian and Banana River estuaries, as compared to Mosquito Lagoon. Again, it is important to note that our study occurred during only one year, so results may not refl ect long-term patterns of prey densities or wintering scaup use of these estuaries. Herring and Collazo (2004) also observed in their home-range analysis that radio-tagged scaup rarely used the Mosquito Lagoon during the winter of 2002. While these results combined do not provide any insight into longterm effects, they do provide an understanding of why wintering scaup may not have used the Mosquito Lagoon during our study period. Recent research demonstrated that boat traffic by both commercial fishers and recreational users was higher in the Mosquito Lagoon than in the areas scaup used in the Indian River (Holloway-Adkins et al. 2006). However, Holloway-Adkins et al. (2006) did not measure the response of increased boat traffic on wintering scaup, so increased boat traffic cannot be linked directly to lower wintering scaup use of the Mosquito Lagoon. We did not measure the effects of human disturbance, but other studies have shown that they can be detrimental to foraging scaup (see Havera et al. 1992, Knapton et al. 2000, Korshgen et al. 1985). Future studies are needed to assess whether human disturbance does play a role in the patterns of use within the IRL by wintering scaup. We found that scaup diets were similar to those reported for Louisiana (Afton et al. 1991, Harmon 1962, Rogers and Korschgen 1966) and South Carolina (Hoppe et al. 1986), in that principal food items were animal matter. This finding contrasts with other studies in Louisiana (Chabreck and Takagi 1985), Michigan (Jones and Drobney 1986), and South Carolina (Perry and Uhler 1982), where aquatic macrophytes were a primary food item for wintering scaup. Differences in diet might be related to site characteristics and associated biotic community. Anteau and Afton (2004) and Herring and Collazo (2006) found that scaup did not increase nutrient reserve levels prior to migration from the wintering grounds in Louisiana and Florida. Consequently, wintering scaup may only need to meet maintenance levels of reserves, likely obtainable from aquatic macrophytes, invertebrates, or a combination of both. Current habitat conditions (e.g., prey abundance) appear to meet the wintering requirements— high survival (Herring and Collazo 2004), small home ranges (Herring and Collazo 2005), and sufficient nutrient reserve acquisition (Herring and Collazo 2006)— of scaup in the IRL. Maintaining elevated densities and accessibility of invertebrates in areas with minimal human disturbance may be important to continued use of this wintering site by scaup and other avian species. Greater knowledge of the invertebrate community in the Mosquito Lagoon may yield more definitive insights on the relative low scaup use of this vast potential wintering site. 2009 G. Herring and J.A. Callazo 373 Acknowledgments Funding was provided by the St. Johns River Water Management District and US Fish and Wildlife Service Region IV. The North Carolina Cooperative Fish and Wildlife Research Unit and North Carolina State University provided graduate support and facilities. The North Carolina Cooperative Fish and Wildlife Research Unit is jointly supported by the North Carolina Wildlife Resources Commission, North Carolina State University, US Geological Survey, and Wildlife Management Institute. We appreciate the support of M.B. Epstein (Senior Refuge Biologist) and the MINWR staff. We thank our field research crew: Brenda Collado, Erin McDonald, and Larissa Miller. 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