Hydroacoustic surveys of Otsego Lake s pelagic fish community, 2010 1 Holly A. Waterfield 2 and Mark Cornwell 3 INTRODUCTION Hydroacoustic surveys were conducted in May and November 2010 to estimate pelagic fish abundance in Otsego Lake (Otsego County, NY). Following their introduction in 1986, alewife (Alosa pseudoharengus) quickly became the most abundant forage fish in Otsego Lake (Foster 1990) and had major impacts on trophic relationships, chlorophyll a, water clarity, and hypolimnetic oxygen depletion rates (Warner 1999). Walleye (Sander vitreus) stocking resumed in 2000 (previously stocked through 1934) to establish a recreational fishery while at the same time introducing a predator which would potentially control the alewife population. The acoustic surveys reported on here are part of an ongoing effort to document changes in the pelagic fish community and lake trophic condition. Comparisons are made to the results of past hydroacoustic surveys (1996 to 2007) which were summarized by Brooking and Cornwell (2008). METHODS Acoustic surveys were conducted on the nights of 25 May and 11 November 2010, beginning at least 1 hour after dark. The May survey was conducted from north to south; the November survey was conducted from south to north, following a zig-zag pattern from shore to shore (Figure 1). Approximately east-west transects were connected with a diagonal zig to yield two sets of parallel transects for additional analysis of methods and statistical approaches. Data were collected using a BioSonics DtX echosounder with either a 123kHz (7.5 o beam) or 120kHz (7.7 o beam) transducer; data collection settings for each survey are listed in Table 1. Performance of each transducer was checked against a standard tungsten carbide sphere. Table 1. Data collection settings for 25 May and 11 November 2010 acoustic surveys conducted on Otsego Lake, NY. Survey Date Number of transects Downlooking Frequency (khz) Average Survey Speed (m/s) Pulse Duration (ms) Ping Rate (pps) 5/25/2010 14 123 2 0.4 4 11/11/2010 16 120 4 0.4 3 1 Made possible by 2007 NSF grant entitled Acquisition of hydroacoustic and associated instrumentation for fisheries research (DBI: 0722764). 2 Research Support Specialist. SUNY Oneonta Biological Field Station, Cooperstown, NY. 3 Assistant Professor, SUNY Cobleskill Department of Fisheries and Wildlife. Cobleskill, NY.
Figure 1. Bathymetric map of Otsego Lake, NY with acoustic transects surveyed May 25, 2010 (1-14 from north to south) and 11 November 2010 (1-16 from south to north). Hydroacoustic data were analyzed using the Sonar5-Pro post processing system (Sonar5) following conversion of the raw acoustic files to Sonar5-compatible formats (Balk and Lindem 2007). Only echoes from the open-water were analyzed; to achieve this, surface and bottom exclusion zones were established on each echogram. 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 actual detected bottom to avoid inclusion of bottom echoes in abundance estimates. Echograms were visually inspected to remove echoes from targets other than fish (i.e., submerged vegetation). Passive noise, calculated from passive listening data collected prior to each survey, was subtracted from all files before analysis. Fish density (#/ha) was calculated using the echo counting method in Sonar5 (Balk and Lindem 2007) based on a minimum in situ target strength threshold of -55 db. The TS threshold used in past analyses was -61dB; the higher threshold of -55 db was used to exclude an abundance of small targets (peak TS of -70 db) in the alewife abundance estimate (Figure 2). Target strengths of -61 to -55 db have been associated with alewife ranging in length from 1.5 to 2.0 cm (Warner 2002), a size class that would not be present at the time of either 2010 survey. Abundance estimates were determined for predator-sized (TS >-35 db) and preysized (TS -55dB to -34dB) targets.
Figure 2. Target strength distribution for even numbered transects surveyed 11 November 2010, Otsego Lake, NY. Note the abundance of low-intensity targets from -70 db to -56 db, overlapping with the standard minimum TS threshold of -61 db. Based on this distribution, the threshold used in this analysis was established at -55dB. Variable small mesh experimental gill nets were set concurrent with each survey. Each net was composed of seven 3-meter wide panels of different mesh sizes (6.2, 8, 10, 12.5, 15, 18.7, and 25mm bar mesh). Nets were 21 meters long by 6 meters deep and set from the surface downward or from the bottom upward. Three nets were set on the nights of 25 May and 11 November 2010 for about 12 hours each, set just before dark and pulled early morning. Fish were tallied by panel and vertical position (top, middle or bottom) in the net. Species, length, and weight were recorded for all fish caught. RESULTS AND DISCUSSION Lake-wide acoustic fish density in May was estimated to be 108 fish/ha for all fish (alewife and predators combined; 95% CI +/- 137 fish/ha) based on the 14 transects (Table 2). Fish density estimates varied among transects ranging from 0 fish/ha for several transects to 725 fish/ha for transect 14, the southern-most transect surveyed (Table 2). This is the lowest density estimate since spring surveys began in 2004 (Table 3; Brooking and Cornwell 2008). Figure 3 is a plot of acoustic targets based on target strength (db) their depth (m) in the water column relative to the surface. A grouping of targets having a range in TS typically associated with alewife was observed below 30m (Figure 3); given this overlap TS and 100% alewife in the gill nets, these targets contribute to the abundance estimate that is considered to be representative of alewife abundance. Gill nets caught 108 alewife, all concentrated in the surface net (fishing 0 to 6m); 75% of these were located in the top 2 meters, outside of the acoustically-surveyed portion of the water column. If this vertical distribution is representative of lake-wide alewife distribution, it is
possible that the acoustic density may actually be four times that estimated by acoustic survey. If that is the case, alewife density may be more in line with estimates from the recent past, at 400 fish/ha. Table 2. Fish abundance estimates of alewife and predator-sized targets for transects surveyed 25 May 2010, including lake-wide mean, standard error (SE) and 95% confidence intervals. Fish Density (f/ha) Transect alewife predators 1 48 0.0 2 601 0.0 3 7 2.0 4 6 8.0 5 20 2.0 6 3 8.0 7 2 2.0 8 5 1.0 9 0 0.0 10 3 14.0 11 0 0.0 12 27 0.0 13 23 5.0 14 725 0.0 mean 105 3.0 SE 63 1.1 95% CI 137 2.5 Table 3. Alewife abundance estimates (fish/ha) for spring surveys conducted between 2004 and 2010. Year Fish/ha # transects stdev 95% CI 2004 907 9 175 114 2005 236 9 214 137 2006 2522 10 1463 907 2007 1330 9 611 399 2010 105 14 236 137
Total acoustic fish abundance in November was estimated to be 279 fish/ha (alewife + predators; 95% CI +/- 249 fish/ha) across 16 transects (Table 4). This estimate is the lowest reported for a fall survey since acoustic surveys began on Otsego Lake in 1996 (Table 5). As seen in the May survey, density estimates varied greatly among transects, ranging from 16 fish/ha to 1492 fish/ha on transect 14, between Hyde Bay and the western shore of the lake (Figure 1). The majority of acoustic targets were observed at depths greater than 10m (Figure 4). As observed in May, a grouping of targets was observed around or below 35m depth and having in situ TS values within the range typically associated with alewife (Figure 4). A cluster of predator-size targets is seen in both transect sets between 20 to 35m depth, a depth range typically occupied by walleye (Potter et al. 2010). Corresponding gill nets caught a total of eight fish, all alewife in the deepest net, fishing 8-14m. Table 4. Fish abundance estimates of alewife and predator-sized targets for transects surveyed 11 November 2010, including lake-wide mean, standard error (SE) and 95% confidence intervals. Fish Density (f/ha) Transect "alewife" predators 1 91 0.0 2 53 0.0 3 53 0.0 4 16 0.0 5 104 13.0 6 93 0.0 7 73 0.0 8 82 3.0 9 46 0.0 10 30 2.0 11 54 0.0 12 108 1.0 13 240 5.0 14 504 9.0 15 1367 11.0 16 1481 27.0 mean 275 4.4 SE 116 1.9 95% CI 247 3.9
3a. 3b. Figure 3. Target strength (db) versus depth (m) relative to the surface for odd-numbered (a) and even-numbered (b) transects surveyed 25-26 May 2010. Table 5. Abundance estimates (fish/ha) from fall acoustic surveys conducted between 1996 and 2010. Modified from Brooking and Cornwell (2008). Fall Alewife Abundance Fall Predator Abundance Year #/ha # transects stdev 95% CI #/ha # transects stdev 95% CI 1996 5170 7 1434 1063 7.5 7 4.2 3.1 1997 2053 9 798 521 3.3 9 3.4 2.2 2000 1382 8 925 774 2001 8562 9 3811 2490 35.2 9 13.9 9.1 2002 10901 16 4886 2394 15.2 16 10.7 5.2 2003 3851 16 2901 1421 1.2 16 1.5 0.7 2004 2418 9 1571 1026 3.5 9 4.7 3.1 2005 9562 9 3555 2322 8.6 9 8.8 5.7 2006 1631 7 2713 2010 19.4 7 25.6 19 2007 3921 11 2524 1492 6.5 11 5.7 3.4 2010 275 16 464 247 2.9 16 4.3 2.3 2010 results show a drastic decrease in estimated spring and fall alewife density compared to past surveys (Tables 3 and 5), even when considering the confidence intervals and their reflection of the high degree of variability among transect density estimates. This decrease in estimated alewife abundance is corroborated by relatively low gill net catch for both surveys, as well as results documented in other ongoing studies, including increased mean alewife size concurrent with decreased trap net catch rate (Potter 2010) and the prevalence of alewife in stomachs of captured walleye (McBride and Cornwell 2008). Further refinement of methods to assess shallow fish targets concentrated in the upper 2 meters of the water column is underway by researchers around the Great Lakes region; employment of such methods will lead to more accurate estimates of alewife abundance in Otsego Lake.
4a. 4b. Figure 4. Target strength (db) versus depth (m) relative to the surface for odd-numbered (a) and even-numbered (b) transects surveyed 11 November 2010. REFERENCES Balk, H. and T. Lindem. 2007. Sonar4 and Sonar5 post processing systems, operator manual version 5.9.7, 420p. Lindem Data Acquisition Humleveien 4b. 0870 Oslo Norway. Bowers, B.E. and H.A. Waterfield. 2011. Investigating walleye (Sander vitreus) behavior and optimal forage theory in Otsego Lake, New York. In 43 rd Ann. Rept. (2010). SUNY Oneonta Biol. Fld. Sta., SUNY Oneonta. Brooking, T.E. and M.D. Cornwell. 2008. Hydroacoustic surveys of Otsego Lake, 2007. In 40 th Ann Rept. (2007). SUNY Oneonta Biol. Fld. Sta., SUNY Oneonta. Foster, J.R. 1990. Introduction of the alewife (Alosa pseudoharengus) into Otsego Lake. In 22 nd Ann. Rept. (1989). SUNY Oneonta Biol. Fld. Sta., SUNY Oneonta. Cornwell, M.D. and N.D. McBride. 2009. Walleye (Sander vitreus) reintroduction update: Walleye stocking, gill netting 2008. In 41 st Ann. Rept. (2008). SUNY Oneonta Biol. Fld. Sta., SUNY Oneonta. Potter, J. 2010. Littoral fish community survey of Rat Cove & Brookwood Point, summer 2009. In 42 nd Ann. Rept. (2009). SUNY Oneonta Biol. Fld. Sta., SUNY Oneonta. Warner, D.M. 1999. Alewives in Otsego Lake, NY.: a comparison of their direct and indirect mechanisms of impact on transparency and chlorophyll a. Occas. Pap. No. 32. SUNY Oneonta Biol. Fld. Sta., SUNY Oneonta.