Alewife (Alosa pseudoharengus) density as a predictor of open water utilization by walleye (Sander vitreus) in Otsego Lake, NY

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1 Alewife (Alosa pseudoharengus) density as a predictor of open water utilization by walleye (Sander vitreus) in Otsego Lake, NY B.E. Bowers 1 INTRODUCTION Walleye (Sander vitreus) are typically associated with shoals, drop offs, or in aquatic vegetation near shore (Foust and Haynes 2007), while in some lakes and reservoirs walleye make more use of pelagic waters (Ager 1976; Festa 1987; Palmer 2005, Byrne et al. 2009). Walleye have been stocked at a target rate of 80,000/year from and 40,000/year since Here, walleye occupy rocky shoals, weedy littoral areas and epilimnetic pelagic waters, rarely occurring at depths below 15m (Stich et al. 2008, Byrne et al. 2009, Potter et al. 2010). Alewife (Alosa pseudoharengus) were first documented in Otsego Lake in 1986 (Foster 1990) and were dominant by the early 1990s (Harman et al. 2002). They are efficient epilimnetic planktivores that demonstrate diel patterns in vertical migration and schooling behavior (Appenzeller and Legget 1992; Luecke and Wurtsbaugh 1993). Habitat utilizations during summer stratification indicate spatial overlap between walleye and alewife. While walleye habitat utilization has been studied in Otsego Lake, the factors contributing to their open water habitat selection are unknown. In Otsego Lake all walleye with prey in their stomachs fed on alewives; in a study by Cornwell (2005) 65% of walleye stomachs contained alewife, while the remaining 35% were empty. In Lake Erie walleye were documented as changing prey selection during times of the year when certain species are more abundant, selecting alewives during the fall (Parsons 1971). Following the introduction and establishment of alewife to Lake Huron in 1988, walleye preyed on alewife, corresponding with increases in weight and body conditions of various walleye year classes (Cade et al. 2008). Porath (2003) found the introduction of alewife increased the body condition of walleye within Lake Mcconaughy, indicating walleye were foraging for alewife. It is currently hypothesized that walleye occupy open-water areas of Otsego Lake in order to forage on alewife. This study sought to assess the spatial overlap of alewives and walleye in the open waters of Otsego Lake using hydroacoustic fisheries surveys. From these surveys, alewife and walleye densities were estimated and targets corresponding to walleye were identified in order to evaluate alewife density at increasing distance from walleye targets. METHODS The acoustic data analyzed for this study were collected on 17 October 2007 (Brooking and Cornwell 2008). Data were collected using BioSonics DTX echosounder with a 123 khz, 7.8 o beam-width transducer mounted at a depth of 0.5 meter (Brooking and Cornwell 2008). A percid gill net survey was conducted on the 18, 19 and 20 of September 2007 by the New York State Department of Environmental Conservation on Otsego Lake (McBride, unpublished). Data 1 OCCA Harman Internship, summer Present affiliation: Plymouth State University, Plymouth, NH.

2 from this survey were used to estimate the percentage of predator targets in the pelagic water that were most likely to be walleye. Hydroacoustic data were analyzed using Sonar5-Pro post processing system (Balk and Lindem 2007). The surface exclusion zone was established from the surface to 2.5m to eliminate data from the transducer s near-field and surface noise. The bottom boundary included a margin of approximately 0.5 m above the bottom to avoid inclusion of bottom echoes in abundance estimates. Echograms were visually inspected to remove bad data regions and echoes from targets other than fish. Potential walleye targets occurring at depths between 2.5 and 15m were analyzed; shallow areas less than 15m in total depth were excluded to avoid non-walleye predator targets. Fish density (#/ha) was calculated using the echo counting method in Sonar5 (Balk and Lindem 2007) based on a minimum target strength threshold of -61dB. Abundance estimates were determined for predator-sized (TS >-37 db) and prey-sized (TS -61dB to -37dB) targets (Brooking and Rudstam 2009). The September 2007 NYSDEC percid gill net survey of Otsego Lake provided ground truthing of acoustic data and indicated that 95% of the predator targets can be assumed to be walleye. The TS range used to estimate alewife abundance was based on target strength data from caged alewives; 99% of the TS measurements were between - 61 and -37 db (Brooking and Rudstam 2009). The acoustic survey was analyzed in various ways to investigate the relationships between prey density and walleye presence/absence, walleye abundance, and distance from a walleye; each of these is described below. Walleye presence/absence and prey density: The survey was divided into segments 100 pings in length (~58 meters); each segment was analyzed twice, using the TS thresholds for predators to identify individual predator targets as well as the TS thresholds associated with alewife, to determine prey density. Adult alewife density (TS -51 to -43dB) was estimated in each 100-ping segment; average density was calculated for two categories: segments containing a predator target (assumed to be a walleye) and segments from which predators were absent. A total of 11 segments contained prey/alewife densities in the presence of walleye and 202 segments contained prey/alewife densities absent of walleye. Average prey density in the absence of walleye targets was based on 11 randomly-selected 60-m/100-ping segments from the 202 that did not contain walleye targets. Walleye abundance and prey density: The survey was divided into segments 60m in length (~100 pings); each segment was analyzed twice, using the TS thresholds for predators to identify individual predator targets as well as the TS thresholds associated with alewife, to determine prey density. Each segment was assigned to one of three categories depending on walleye abundance (multiple walleye present, single walleye present, no walleye present). The average alewife density for each category was calculated; a total of 2 segments contained multiple walleye, 7 segments contained a single walleye and 202 segments contained no walleye. Distance from walleye and prey density: Predator targets (> -37 db) were located. Alewife density was calculated in 20m increments from each predator target to 100m, for a total of 5 density estimates. The average density for each distance increment (+/-20m, +/-40m, +/-60m, +/-80m, +/-100m) was calculated for the 11 identified predator targets. The density estimates for each increasing distance increment include areas analyzed in the previous, and so presents an average over the entire area rather than for a discrete distance range.

3 RESULTS Higher alewife density was associated with the presence of walleye in Otsego Lake (Figure 1&2). Large portions of the limnetic zone of Otsego Lake contained no predator targets and lower prey densities. Segments of the hydroacoustic survey where walleye were present had an adult alewife density of 712 fish per hectare, while those where no walleye were present had an adult alewife density of 392 fish per hectare (Figure 1). Adult alewife density in the presence and absence of walleye were not significantly different (T-test >.05). Figure 1. Adult alewife density in the presence or absence of a predator in 60-meter segments of the limnetic zone of Otsego Lake, 17 October Bars indicate calculated standard error (SE). Alewife density was highest in segments containing multiple walleye and lowest in those where walleye were absent (Figure 2). Sixty-meter segments with multiple walleye had a prey density of 1163/ha, those with a single walleye had 930/ha and those where walleye were absent had only 561/ha (Figure 2). Prey density in the presence of multiple walleye (1083 fish/ha) and absence of walleye (561 fish/ha) were significantly different (T-test <.05). Note: the data for Walleye Absent and No Walleye in the analyses of walleye presence vs. absence and walleye abundance were derived from different groups of segments randomly-selected from the pool of 202. Figure 2. Prey density versus walleye abundance in 60-meter segments of the limnetic zone of Otsego Lake, 17 October Data were inaccessible for calculation of standard error.

4 Figure 3 illustrates average prey density at distance from a predator target; on average, alewife density was greatest within 20m of a predator target and decreased slightly with the inclusion areas at increasing distance (Figure 3). Prey density decreased by 143 fish/ha from the nearest to the farthest distance from a walleye target. Figure 3. Prey density at distance intervals from walleye targets in the limnetic zone of Otsego Lake, 17 October Bars indicate calculated standard error (SE) DISCUSSION Walleye inhabit open water regions of Otsego Lake and tend to be located in areas having above average alewife (prey) density, identifying the spatial overlap of alewife and walleye and its potential tie to forage potential for walleye. Higher alewife density corresponds with segments containing multiple walleye. Average adult alewife density decreases with increasing distance from a walleye target (Figure 3). However, the difference in density from the shortest to longest distance increment is less than the error associated with the density estimate. Use of discrete distance intervals may provide a better assessment of alewife density in proximity to walleye. Past diet studies of Otsego Lake walleye (in 2002 and 2007) have found that alewife are by far their preferred prey; 97% of stomachs with food items contained at least one alewife (Cornwell and McBride 2008). This dietary information, along with the density relationships documented in this study, indicate that alewife are likely a factor in the location of walleye, specifically those found in open water regions of Otsego Lake.

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