Mark-recapture and catch per unit effort measures of walleye (Sander vitreus) abundance in Otsego Lake, NY

In this study, the adult population of Otsego Lake walleye was estimated in order to evaluate the success of the walleye stocking program. Spawning walleye were trap netted at five locations in April 2008, marked with a dorsal fin punch and visible implant elastomer, and recaptured in May using boat electro-fishing and in September by NYSDEC experimental gill net survey. Catch per unit effort as measured by spring boat electro-fishing (24.6 fish/hr) and gill netting (11 fish/net) following the standard percid sampling manual, indicate that Otsego Lake’s walleye population density was average compared to other New York State lakes. However, a Petersen population estimate (6,428 walleye with a 95% confidence interval of ± 987) produced by the combination of electro-fishing and gillnet recapture data indicated a low density of 3.76 walleye per hectare. These findings indicate that estimates of walleye abundance utilized by fisheries researchers and managers to measure ecological interactions and stocking success are more reliable if based on data derived from mark and recapture studies rather than catch per unit effort. INTRODUCTION Walleye were once an important component of Otsego Lake’s fishery, but were extirpated by the 1970’s presumably due to fry predation by the accidentally-stocked cisco (Coregonus artedii, Lehman et al. 1991). Since 2000, an extensive stocking program has been undertaken to re-introduce walleye to Otsego Lake (Cornwell 2005). The primary goal was to reestablish an historical walleye fishery, with a secondary goal of installing a partial bio-control of the alewife (Alosa pseudoharengus, Cornwell and McBride 2007). Since their illegal introduction in 1986 (Foster 1989), alewives have caused substantial detrimental impacts to the ecology of Otsego Lake, including the decline of many fish populations as well as decreased mean secchi disc readings and zooplankton abundance, increased chlorophyll a measurements, areal hypolimnetic oxygen deficits (AHOD), and phytoplankton abundance (Harman et al. 2002). However, alewives do provide a beneficial food source for larger predators, such as lake trout and walleye. Stomach samples collected from captured Otsego Lake walleye show that they forage on alewife almost exclusively (Cornwell and McBride 2007). Since walleye stocking began, hydro acoustic, trap net, and gill net data have displayed moderately decreasing densities 1 Fisheries and Wildlife Department, State University of New York at Cobleskill, NY 12043. 2 Cornell Biological Field Station, Bridgeport, NY 13030 of alewife (Brooking and Cornwell 2007; Cornwell 2005). Large zooplankton populations such as Daphnia spp. have increased in correlation to decreasing alewife abundance (Albright 2007). Thus ecological evidence indicates that stocking walleye has had a beneficial impact on Otsego Lake’s ecosystem (Cornwell 2005). However, in order to develop a true evaluation of the success of the walleye stocking program, walleye abundance needs to be measured. Fisheries researchers and managers commonly rely on measures of fish abundance based on catch per unit effort (CPUE) rather than on fish population estimates, because CPUE require less effort and expense (Harley et al. 2001). However, repeated measures of CPUE may not accurately reflect population abundance (Beverton and Holt 1957). Thus, while the overall goal of this project was to evaluate the walleye stocking program, in order to meet that goal, an accurate estimate of the adult walleye population was determined through mark and recapture analysis. The resulting estimate of walleye density (fish/ha) was then compared to standard gill net and electro-fishing CPUE measures of abundance. MATERIALS & METHODS Otsego Lake (42.40oN, 74.55o W), located in Otsego County, New York, is a relatively young glacially-formed, oligotrophic lake, with a maximum depth of 50.5 m (Harman et al. 1997). The lake’s four primary tributaries, Shadow Brook, Hayden, Cripple, and Leatherstocking Creeks were used to trap net walleye en route to spawn (Figure 1). Sunken Island, a rocky shoal at the north end of the lake, was also used as a trap net site, since walleye are known to congregate at that location (Stich et al. 2007). The Sunken Island shoal and the entire shoreline of the lake were boat electro-shocked for recapture samples. Trap nets were deployed immediately following ice-out on 9 April 2008 and were checked daily until 25 April. All walleye were marked during this period. Recapture samples were collected with a boat electro-fisher on the nights of 9, 10, 15, and 31 of May 2008. A different section of the lake was shocked each night. All captured walleye were double-marked by a dorsal fin hole-punch at the base of the third spine and with visible implant elastomer (VIE) posterior to the left eye. Although Thompson et al. (2005) concluded that fin-clipping is a more desirable method of marking walleye, all advanced pond fingerlings stocked into Otsego Lake have been fin clipped, thus a different marking method was needed. Each capture site hosted a different elastomer color. Jaw-tags supplied by NYSDEC were also attached to 490 legal-sized walleye to measure angler exploitation rates. Data from NYSDEC’s warm-water gillnet survey performed in the fall of 2008 were collected and used to supplement the population estimate. Petersen mark-recapture and Schnabel multiple census population estimates were calculated using Bailey (1951), Ricker (1975), and Schnabel (1938), with reference to Clopper and Pearson (1934) for confidence intervals. The 2008 Otsego Lake walleye surveys were compared to Canadarago, Cayuta, and Oneida Lake’s respective walleye data (Brooking et al. 2007; VanDeValk et al. 2008). Figure 1. Trap net (marking) locations at four Otsego Lake tributaries and Sunken Island.