2012 population estimate of walleye (Sander vitreus) in Otsego Lake Jonas Willson 1, John R. Foster 1 and Mark D. Cornwell 1 Abstract: The goal of this study was to estimate the adult walleye population in Otsego Lake. Adult walleye were captured and marked during their spawning run in tributary streams using backpack electrofishers and in the lake using an electrofishing boat during March and April 2012. A recapture sample was collected in May 2012 by electrofishing the entire circumference of the lake using a boat electrofisher. The 2012 adult walleye population in Otsego Lake was estimated to be 10,520 fish. This estimate indicates that the walleye population has continued to increase from the 2008 and 2009 levels. INTRODUCTION In the mid-1900s walleye (Sander vitreus) were abundant in Otsego Lake, but by 1990 the walleye population had become extirpated (Lehman et al. 1991). Following the unauthorized stocking of the alewife (Alosa pseudoharengus) into Otsego Lake (Foster 1989) and its subsequent irruption (Foster and Gallup 1990), detrimental impacts from this species were observed on the lake ecosystem (Foster 1995, Wilson and Warner 1998, Harman et al. 2002). Starting in 2000 a walleye stocking program has been carried out as a management strategy to control the abundance of alewife and to reduce the detrimental impact of this species on the ecology of Otsego Lake (Cornwell and McBride 2007). In order to provide a critical measure of the success of past and future walleye stocking efforts in Otsego Lake an efficient and accurate measure of walleye population abundance was needed. Initially population estimates followed the Percid Sampling Manual and were based on catch per unit effort data from boat electro-fishing and gillnetting (Cornwell and McBride 2007). However, the population estimates using mark and recapture techniques (Lydon et al. 2008, Peck et al. 2010) were significantly different than the number of walleye indicated by electro-fishing and gillnet catches (Cornwell and McBride 2007). Since mark and recapture population estimates are considered to be more reliable than those based on catch per unit effort data, this study utilized mark and recapture data to directly measure the abundance of adult walleye in Otsego Lake (Peck et al. 2010, Rogers et al. 1992). The goal of this study was to determine the size of the adult walleye population in Otsego Lake in 2012. These data were compared to estimates made of the walleye population in 2008 and 2009. 1 Fisheries & Aquaculture Program, State University of New York at Cobleskill, NY 12043
METHODS & MATERIALS This study was conducted on Otsego Lake (42.40⁰ N, 74.55⁰ W), Otsego County, New York. Otsego lake has a surface area of 1,711 ha., a maximum depth of 50.5 m and an elevation of 364.2 m (Harman et al. 1997). Marked Sample Smith-Root and Halltech backpack shockers were utilized at night to capture and mark 269 walleye in Cripple Creek (below Clarke Pond), Shadow Brook (at Mill Road) and Hayden Creek (County Road-53) from 20 March to 11 April 2012 (Figure 1 and 2). The marking effort in the streams resulted in fewer fish than needed. On 17 and 25 April 2012 the rocky shoals and Sunken Island (Figure 2) were sampled with a Smith-Root electro-fishing boat. Night (2030-2400 hrs) electrofishing resulted in the capture and marking of 257 additional walleye. All walleye captured were checked for previous marks (fin clips, VIEs, jaw tags, hole punches), measured, sexed and checked for ripeness before being hole-punched in the soft dorsal fin and released. Figure1. Marked sample collected in Otsego Lake tributaries using backpack electro-fishers in March and April 2012. Recapture Sample The recapture sample was collected by shocking the whole circumference of Otsego Lake with a Smith-Root electro-fishing boat (Figure 2). Recapture data were collected on 1 and 2 May 2012 and 9 and 10 May 2012 between 2030 and 2400. The 139 walleye in the recapture sample were checked for marks and tags, measured, sexed and released. Six recaptured walleye were marked in 2012.
Figure 2. Otsego Lake and its three main tributaries, Otsego County, NY.
RESULTS & DISCUSSION In the spring of 2012, a total of 526 walleye were marked. However, only 6 of the 139 recaptured walleye were previously marked fish. Unfortunately, the accuracy of the population estimate is highly dependent on the number of marked fish recaptured. Too small of a recapture sample can result in a biased population estimate. A minimum of 8 marked fish was needed in the recapture sample to provide an unbiased estimate of the walleye population in Otsego Lake (Brower et al. 1998). This should be kept in mind, when evaluating the 2012 estimate of the adult walleye population in Otsego Lake that was calculated using Bailey s modification (Bailey 1951) of the Petersen Mark and Recapture formula. In 2012 the estimated number of adult walleye in Otsego Lake was 10,520 fish (Figure 3). This indicates that the 2012 walleye population was substantially larger than the previous population estimates for 2008 (Lydon et al. 2009) and 2009 (Peck et. al 2010). Another population estimate should be made in 2013 to determine if this trend to increasing walleye abundance is real. Figure 3. Otsego Lake adult walleye population estimates for 2008, 2009 and 2012. The walleye population in Otsego Lake is maintained through stocking. Future fisheries management decisions regarding walleye stocking require a good estimation of their population, as well as its major prey the alewife. Alewife populations are monitored using hydroacoustic surveys (Brooking and Cornwell 2005, 2007; Waterfield and Cornwell 2010) and all indications are that they have dropped to very low levels in recent years (Waterfield 2013). With large enough sample sizes, mark and recapture data can be utilized to accurately monitor the size of the walleye population. These data are necessary in order to evaluate the effectiveness of walleye as a biological control agent for alewives.
ACKNOWLEDGEMENTS The SUNY Cobleskill Department of Fisheries and Wildlife provided the equipment used to conduct the survey as well as transportation to and from the study locations. Ben German, Anthony Bruno, Stephan Bence, Ryan Cuer, Dan Drake, Matt Bowker, Pat Keating, Alex Phillipcheck, and Eric Malone assisted in the data collection. LITERATURE CITED Bailey, N.T.J. 1951. On estimating the size of mobile populations from recapture data. Biometrika. 38: 293-306 Brower, J. E., J. H. Zar, and C. N. von Ende. 1998. Field and laboratory methods for general ecology. 4 th ed. Boston: McGraw-Hill. 273 pp. Brooking, T.E., and M.D. Cornwell. 2007. Hydroacoustic surveys of Otsego Lake, 2007. In 40 th Ann. Rept. (2007). SUNY Oneonta Biological Field Station, SUNY Oneonta. Brooking, T.E. and M.D. Cornwell. 2005. Hydroacoustic survey of Otsego Lake, 2005. In 37 th Ann. Rept. (2004). SUNY Oneonta Biol. Fld. Sta., SUNY Oneonta. Cornwell, M.D., and N.D. McBride. 2007. Walleye (Sander vitreum) reintroduction update:walleye stocking, gill netting and diet analysis 2007. In 40 th Annual Report. SUNY Oneonta Biol. Fld. Sta., SUNY Oneonta Foster, J.R. 1989. The introduction of the alewife into Otsego Lake. In 22 nd Ann. Rept. SUNY Oneonta Biol. Fld. Sta., SUNY Oneonta. pp 107-111. Foster, J.R. 1995. The fish fauna of Otsego Lake. In 28 th SUNY Oneonta Biol. Fld. Sta., SUNY Oneonta. pp 202-220. Harman, W.N., L.P. Sohacki, M.F. Albright, D.L. Rosen. 1997. The State of the Otsego Lake1936-1996. Occasional Paper No. 30. SUNY Oneonta Biol. Fld. Sta., SUNY Oneonta. Harman, W.N., M.F. Albright, and D.M. Warner. 2002. Trophic changes in Otsego Lake, NY following the introduction of the alewife (Alosa pseudoharengus). Lake and Reservoir Management 18(3):215-223. Lehman, K., W. Williams and J. Foster. 1991. Extinction of walleye (Stizostidion vitreum) in Otsego Lake, NY. In 23rd Ann. Rept. SUNY Oneonta Biol. Fld. Sta., SUNY Oneonta. Peck, D.J., J.R. Foster, J.C. Lydon, K. Poole and M.D. Cornwell. 2010. The
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