Justification for Rainbow Trout stocking reduction in Lake Taneycomo Shane Bush Fisheries Management Biologist Missouri Department of Conservation December 14, 2017 Lake Taneycomo was formed by the construction of Powersite Dam (Ozark Beach Dam) in 1913 and is Missouri's oldest hydroelectric reservoir. The lake is riverine in nature, 22 miles in length and encompasses 2,080 surface acres. Prior to 1958, Lake Taneycomo supported one of Missouri's best warm-water fisheries. This changed in 1958 when Table Rock Dam, located in the headwater of Lake Taneycomo, began discharging cold hypolimnetic water into Lake Taneycomo. The discharge of cold water changed Lake Taneycomo into a cold-water environment, providing good conditions for trout. Rainbow Trout were first stocked into Lake Taneycomo in 1958 and Brown Trout were first stocked in 1980. Since that time over 45 million Rainbow and Brown Trout have been stocked (Figure 1). The trout fishery is maintained by stocking Rainbow and Brown Trout. Shepherd of the Hills Hatchery, located adjacent to Lake Taneycomo immediately below Table Rock Dam, supplies the majority of the trout stocked into the lake (475,000). Lake Taneycomo also receives approximately 225,000 Rainbow Trout per year from Neosho National Fish Hatchery as part of the mitigation agreement for the construction of Table Rock Dam. Rainbow Trout stockings peaked in 1984 when 1.6 million fish were stocked. The stocking rate was reduced to the current annual rate of 700,000 Rainbow Trout in 1992 to reduce the potential for overstocking. The monthly Rainbow Trout stocking schedule was also adjusted in 1997 to better correspond to seasonal fishing pressure. Planned monthly adjustments provide heavier stockings during periods of heavy angling use (Table 1, Figure 2). Brown Trout stocking has ranged from 67,000 in 1982 to 7,959 in 1992. The current stocking rate for Brown Trout is 10,000 annually. Natural spawning success for both trout species is very limited. Gammarus pseudolimnaeus were introduced into Lake Taneycomo in 1961 in an effort to improve the forage base for trout. By the late 1960s large numbers of G. pseudolimnaeus were present, and Rainbow Trout exhibited growth rates up to 0.7 inch per month. Large Rainbow Trout were abundant, and the lake quickly earned a national reputation for producing trophy Rainbow Trout. By 1986, the trophy Rainbow Trout fishery had declined, and few Rainbow Trout larger than stocking size were present. Invertebrate sampling indicated that the G. pseudolimnaeus population had declined by approximately 90%. It is not fully understood why the large decline in G. pseudolimnaeus and the accompanying decline in the quality of the Rainbow Trout population occurred. It was likely due to a complex set of factors including high trout stocking and harvest rates, low dissolved oxygen, siltation due to development in the watershed, increased abundance of white suckers, and lake bed disturbance due to gravel removal. On March 1, l997, a 12 to 20-inch slot length limit on Rainbow Trout and an "artificial lures" only restriction in the upper three miles of Lake Taneycomo from Table Rock Dam to Fall Creek
was implemented. The purpose of this regulation was to provide a refuge for Rainbow Trout to grow larger than stocked size and to provide anglers with trophy fishing opportunities. This regulation was also meant to allow for angler harvest of Rainbow Trout below 12 inches in order to maintain moderate densities of Rainbow Trout and reduce overcrowding. However, based on the most recent creel survey conducted in 2008-2009, nearly all Rainbow Trout caught in this regulation zone are released by anglers (Figure 3). Lake Taneycomo is Missouri's largest and most popular trout fishery. It annually receives in excess of 135,000 fishing trips and anglers catch an estimated 600,000 trout annually. However, angling fishing pressure and Rainbow Trout harvest rates have decreased while Rainbow Trout angler catch and release rates have increased since the regulations went into effect in 1997 (Figures 4-5). As of 2009, anglers only harvested approximately 200,000 of the 700,000 Rainbow Trout stocked annually. Additionally, Rainbow Trout electrofishing catch rates for all sizes of Rainbow Trout have continued to rise since 1997 (Figure 6). Based solely on the decline in angling fishing pressure and harvest rates combined with steadily increasing electrofishing catch rates, it appears that the stocking rate of 700,000 Rainbow Trout annually may be contributing to overcrowding in Lake Taneycomo and may also be inhibiting the growth rates and reducing the size of Rainbow Trout available for anglers to catch. The Missouri Department of Conservation (MDC) cold water hatchery system experienced a loss in the number of Rainbow Trout in the hatcheries in the fall and winter of 2015. This loss resulted in a 17 percent reduction in the number of Rainbow Trout stocked statewide in 2016, including Lake Taneycomo. Only 575,139 Rainbow Trout were stocked into Lake Taneycomo in 2016. Despite this decrease in the number of Rainbow Trout stocked in 2016, the densities of Rainbow Trout above Fall Creek remained consistent and comparable to 2015 (Table 2). Angler creel survey data was not available for 2016, however angler reports indicated that the size and quality of Rainbow Trout caught in 2016 was the best it had been in many years. Data from MDC s annual electrofishing sample in August 2016 confirmed these angling reports as the number of Rainbow Trout greater than 16 inches (PSD16) was 25 percent, the highest value ever recorded (Table 2, Figure 7). An inverse relationship exists between PSD16 values and stocking rates as illustrated in Figure 7 wherein the PSD16 always increases when fewer Rainbow Trout are stocked. A similar relationship also exists between PSD and Rainbow Trout stocking rates as illustrated in Figure 8. A roving angler creel survey began on Lake Taneycomo in January 2017 and will continue through December 2018. In addition to a standard set of questions, anglers were asked the following question: Sampling data indicates that when moderately fewer rainbow trout are stocked, the size and condition of trout improves with little impact to angler success. Would you support a moderate reduction in stocking rates of rainbow trout if it resulted in higher quality fish? Out of 8,149 responses, 70 percent of anglers said they would support a moderate reduction in stocking rates, 19 percent said they would oppose a reduction, and 11 percent had no opinion. Given the implications for improving Rainbow Trout growth when fewer Rainbow Trout are stocked and the majority of support from anglers, considering a reduction in the number of Rainbow Trout stocked into Lake Taneycomo per year is appropriate. The apparent benefits
associated with reducing the number of Rainbow Trout stocked into Lake Taneycomo by 17 percent in 2016 are evident. It is likely that reducing the stocking rate in the future could achieve even higher improvements in the quality of the Rainbow Trout fishery. The following adjustment to the Lake Taneycomo Rainbow Trout stocking regime is proposed: Reduce Rainbow Trout stocking in Lake Taneycomo from 700,000 to 560,000 annually beginning in 2019. This adjustment would reduce the number of Rainbow Trout stocked into Lake Taneycomo by 20 percent each month (Table 3) and would not only benefit the size structure of the Rainbow Trout population in Lake Taneycomo, but would also ease the burden on the MDC Coldwater Hatchery System by requiring fewer trout to be produced for Lake Taneycomo each year. The MDC Coldwater Hatchery System is currently operating at maximum capacity. The additional Rainbow Trout currently produced in the MDC system could be allocated to other trout areas in Missouri. This would be particularly beneficial during years when unforeseen circumstances inhibit trout production in the hatchery system, such as the floods in 2015 and 2017. In addition to the benefits listed above, reducing the number of Rainbow Trout stocked into Lake Taneycomo each year would eventually stabilize the overall density of the Rainbow Trout population present in the lake. By doing so, this could result in additional food and habitat availability for Rainbow Trout and possibly lead to higher numbers of trophy-sized Rainbow Trout in Lake Taneycomo that are currently being suppressed by these density dependent factors. While the overall size distribution of Rainbow Trout is good in the regulation area above Fall Creek, the number of Rainbow Trout greater than 18 inches present is very low and few Rainbow Trout over the legal-size limit of 20 inches are caught during electrofishing surveys each year (Figure 9). Evaluation of this stocking reduction will be accomplished both through annual electrofishing surveys and angler creel surveys every six years. The current angler creel survey that began in January 2017 will continue through December 2018. The standard commitment of 700,000 Rainbow Trout is scheduled to be stocked in conjunction with one of the years of the creel survey and this stocking rate will be reduced to 560,000 in conjunction with the other year of the creel survey. By doing so, we will be able to determine if angler catch rates change as a result of the change in the stocking regime. Continuing the current stocking rate of 700,000 Rainbow Trout per year on Lake Taneycomo could eventually lead to a decline in the Rainbow Trout fishery due to overstocking and should not be continued based solely on tradition. With continued monitoring, data may indicate that additional changes in the stocking regime are warranted and additional changes may need to be considered for the future to maximize the potential of the Lake Taneycomo trout fishery.
Table 1. Monthly Rainbow Trout stocking schedule for Lake Taneycomo. back to text January 46,800 February 39,100 March 54,500 April 60,100 May 67,100 June 81,000 July 81,000 August 81,000 September 67,100 October 60,100 November 39,100 December 23,100 TOTAL 700,000
Table 2. Electrofishing capture rates and size structure indices of stock size Rainbow Trout (>11 inches) in August electrofishing samples, taken above Fall Creek in the regulation area, 1996-2016. back to text *For Zones 1 and 2 only Year Number/Hour PSD-13 PSD-16 1996 27 9 0 1997 121 31 2 1998 147 47 5 1999 238 54 9 2000 202 25 3 2001 205 34 5 2002 334 62 13 2003 260 55 14 2004 331 44 10 2005 315 59 12 2006 226 53 6 2007 302 56 4 2008 187 67 7 2009* 449 62 11 2010 275 72 19 2011 338 60 16 2012 278 58 9 2013 271 53 6 2014 225 60 9 2015 314 66 14 2016 296 63 25
Table 3. Revised monthly Rainbow Trout stocking schedule for Lake Taneycomo. back to text January 37,440 February 31,280 March 43,600 April 48,080 May 53,680 June 64,800 July 64,800 August 64,800 September 53,680 October 48,080 November 31,280 December 18,480 TOTAL 560,000
Figure 1. Rainbow and Brown Trout stockings into Lake Taneycomo, 1958-2016. back to text 80000 70000 2008-2009 Average Monthly Estimated Fishing Hours and Rainbow Trout Stockings 60000 50000 40000 30000 20000 Total Number of Rainbow Trout Stocked Estimated Angler Fishing Hours 10000 0 Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Figure 2. Estimated monthly angler fishing hours and number of Rainbow Trout stocked at Lake Taneycomo, 2008-2009. back to text
Figure 3. Percentage of Rainbow Trout released by anglers in creel surveys, 1993-2009. back to text Figure 4. Angler fishing hours per acre and Rainbow Trout electrofishing catch rates, 1993-2009. back to text
Electrofishing Capture Rates (No./Hr.) Figure 5. Rainbow Trout catch, release, and harvest rates from creel surveys, 1993-2009. back to text Lake Taneycomo Rainbow Trout Electrofishing Catch Rates 350 300 250 200 150 100 50 All Sizes CR 13 CR 16 0 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014 2016 Year Figure 6. Electrofishing capture rates of Rainbow Trout sampled in Lake Taneycomo 1996-2016. back to text
Figure 7. Proportional Stock Density (PSD) of stock size Rainbow Trout (11 inches) greater than or equal to 16 inches in electrofishing samples and Rainbow Trout stockings from 1996-2016. back to text
Figure 8. PSD of stock size Rainbow Trout (11 inches) greater than or equal to 13 inches (PSD) and 16 inches (PSD16) in electrofishing samples and Rainbow Trout stockings from 1996-2016. back to text
Fish/Hr Fish/Hr Fish/Hr 50 45 40 35 30 25 20 15 10 5 0 RAINBOW TROUT LENGTH FREQUENCY SPECIAL MANAGEMENT ZONE LAKE TANEYCOMO - 2016 Effort = 4.1 hr. 296 Total RBT/hr. N = 1226 PSD (13) = 63% PSD (16) = 25% PSD (20) = <1% 0 3 6 9 12 15 18 21 24 Length (Inches) 50 45 40 35 30 25 20 15 10 5 0 RAINBOW TROUT LENGTH FREQUENCY SPECIAL MANAGEMENT ZONE LAKE TANEYCOMO - 2015 Effort = 3.4 hr. 314 Total RBT/hr. N = 1077 PSD (13) = 66% PSD (16) = 14% PSD (20) = <1% 0 3 6 9 12 15 18 21 24 Length (Inches) 50 45 40 35 30 25 20 15 10 5 0 RAINBOW TROUT LENGTH FREQUENCY SPECIAL MANAGEMENT ZONE LAKE TANEYCOMO - 2014 Effort = 3.8 hr. 225 Total RBT/hr. N = 865 PSD (13) = 60% PSD (16) = 9% PSD (20) = 0% 0 3 6 9 12 15 18 21 24 Length (Inches) Figure 9. Length frequencies for Rainbow Trout sampled above Fall Creek 2014-2016. back to text