INDEXING YEAR-CLASS STRENGTH OF CRAPPIES USING LARVAL SAMPLING IN TENNESSEE RESERVOIRS

Larval crappies were sampled from three reservoirs (Douglas, Region IV; Percy Priest, Region II; Barkley, Region 1) in 1998 and 1999 to determine if year-class strength could be predicted from larval densities. All samples were taken by towing a 1 x 2 m neuston net weekly at 14 to 16 sites per reservoir. All samples were conducted at night except for Barkley Reservoir, where the same 14 sites were sampled during the day and night to examine diel differences in catch. Catch of crappie larvae in neuston samples reflected catch of age-0 crappies collected in fall trapnetting samples only in Douglas Reservoir. Lack of a precise measure of year-class strength in Percy Priest Reservoir did not allow similar comparisons in that system. However, larval crappie catches in 1999 (a dry spring) were half that in 1998 (a wet spring); similar patterns have been documented in other Tennessee reservoirs. Catch of crappie larvae was low in both day and night samples in Barkley Reservoir in both years, despite high catches of age-0 crappies in fall trapnets in 1999. It appears that neuston net sampling, developed on Normandy Reservoir in the mid-90s, has application in tributary storage impoundments across the state, and could be a useful tool for fisheries managers desiring to index year-class strength of crappies. However, use of this method in mainstem impoundments like Barkley Reservoir may be limited.

The Tennessee Wildlife Resources Agency currently stocks approximately one million crappies annually; however, a systematic evaluation of this program has not been conducted. We assessed year-class contribution, initial post-stocking mortality, and predation upon recently stocked crappies in seven Tennessee impoundments. An oxytetracycline (OTC) marking technique was used to assess the year-class contribution of stock crappies in three large Tennessee reservoirs and one small impoundment. Marking efficacy ranged from 97-100% and marks persisted for at least 80 weeks. Crappies marked and stocked in Normandy Reservoir during October-December 1997 represented 90% of age-1 fish collected in August 1998 retenone samples and 70% of the age-2 crappies collected in a 1999 spring electrofishing sample. Initial post-stocking survival was assessed by placing fish in net pens for approximately 24 hours. Post-stocking mortality rates ranged from 0-95%, averaged 16%, and were most heavily influenced nby loading time and hauling density. Predation on recently-stocked crappies was evaluated by stomach analysis of potentially predacious fish collected by electrofishing 100-m transects proximal to stocking sites. Occurrence of stocked crappies in predator stomachs ranged from 14 to 41% among five systems; predation rates could be high in systems with high densities of predators such as Woods Reservoir and Lake Graham. Stocked BNC were on average 30% smaller than wild age-0 black crappies and 40% smaller than wild age-0 white crappies collected from eight reservoirs at roughly the same time the BNC were stocked, which may increase their risk of predation. Adding to this risk is the fact that the fish are often stocked in late fall, which is a time when major prey species such as shad are offshore and inshore prey density is low. By changing seasons or life stages when crappies are stocked, managers may make this program more widely successful, and benefit the crappie fisheries of more reservoirs in Tennessee.

We evaluated the effect of harvest restrictions on the crappie fisheries in 12 large Tennessee reservoirs. A Beverton-Holt equilibrium yield model was used to predict and compare the response of these fisheries to three size restrictions: the current 254-mm total length limit, a 229- mm limit and a 178-mm limit (i.e., no size limit). The predicted responses of crappie fisheries to size-limits differed among reservoirs and varied with rates of conditional natural mortality (CM). In general, Tennessee Reservoirs fell into one of three groups. In the first group (Normandy, Woods, Chickamauga, Cherokee, and Douglas Reservoirs), a 254-mm size restriction would benefit the fishery if CM was low (30%) and may not harm the fishery if CM was close to 40%. However, at a CM of 50%, yield was adversely impacted by size restrictions, and the decrease in number harvested was severe enough to overcome improvements to the size structure of the population. In the second group (Barkley, Kentucky, J. Percy Priest, Dale Hollow, Watts Bar, and Norris Reservoirs), size limits would provide benefits if CM was low (30%). However, when CM was higher, decreases in the number harvested under a size restriction would probably outweigh any benefits gained by improving the size structure of the population. In any case, a 229-mm size restriction would be better on these lakes than the current 254-mm restriction. The last group, consisting only of Tellico Reservoir, showed no benefits to yield or size structure of the population under any size restriction at any level of CM. Mean number harvested also decrease under all size restriction scenarios. In Tellico Reservoir, the fishery would be best managed with no size limit.

The main factor affecting the response of crappie populations to size limits was growth. With the exception of Cherokee and J. Percy Priest Reservoirs, populations that recruited to the 254-mm size limit in under three years (Normandy, Woods, Chickamauga, and Douglas Reservoirs) appeared to benefit from the size limit under low and intermediate levels of CM. Populations that recruited to the 254-mm size limit in three or four years (Barkley, Kentucky, Dale Hollow, Watts Bar, and Norris Reservoirs) appeared to be better served by the 229-mm limit under low and intermediate levels of CM. With the except of Barkley Reservoir, these populations recruited to the 229-mm limit in less than three years. In Tellico Reservoir, fish did not recruit to the 254-mm size limit until 4.5 years of age, and no size restriction appeared to benefit the population. The use of a statewide length limit to regulate crappie harvest assumes that crappie population dynamics are similar among the major of regulated waters and that the response of these fisheries to such a regulation would be fairly homogenous among systems. Our findings indicate that site- specific regulations may provide more effective management strategies. Based on model results, the response of Tennessee reservoir crappie fisheries to size and exploitation rates determined the exact response of the population to different size restrictions. Accurate estimates of conditional natural mortality and exploitation rates for crappies in Tennessee reservoirs are lacking and are essential to improve model applicability.

Black crappies and white crappies were sampled to index recruitment in eight reservoirs (four mainstems, four tributary storage impoundments) across the state of Tennessee. Crappie recruitment variation in two reservoirs was estimated from historical catch of age-0 fish in fall trapnet samples. In the remaining six reservoirs, variation in recruitment was assessed by examining residuals generated from catch curves. Mean daily discharge and reservoir storage volume values were obtained for each reservoir for three time periods each year: pre-spawn (1 January to 31 March), spawning (1 April to 31 May) and summer (1 June to 30 September). A combined model for three of four tributary storage impoundments revealed a strong positive relationship between year-class strength and discharge in the pre-spawn period. Discharge data were not available for the fourth tributary impoundment; however, year-class strength was negatively related to storage volume of the reservoir in the pre-spawn period. Crappie recruitment in the four mainstem impoundments was highest at intermediate levels of discharge, and a weak inverse relationship existed between crappie recruitment and mean daily discharge during the spawning period. No other relations were found between crappie recruitment and other hydrological variables in any reservoir. Crappie recruitment was linked to reservoir hydrology; however, the critical time and nature of the relationship (positive or negative) differed between tributary storage impoundments and mainstem impoundments. Thus, it is likely that crappie populations will rarely have strong year classes simultaneously over a wide geographic area, or even within a single watershed.

Population characteristics of crappies were examined from twelve Tennessee reservoirs sampled either in spring or fall with either trapnets or electrofishing. Total annual mortality (A) was estimated in eight of the twelve study reservoirs; high recruitment variability prevented estimation of A in four reservoirs. Total annual mortality ranged from 54-75% and averaged 66%. Total annual mortality was not correlated to mean length at age-3 among reservoirs sampled in the fall. Total annual mortality from age-2 to age-5 for black crappies ranged from 52-79% and averaged 64%; total annual mortality for white crappies ranged from 35-68% and averaged 54%. The distributions of lengths at age-3 were variable across reservoirs in terms of range and skewness, often encompassing the entire length range seen for the whole sample. Most of the variability in growth among reservoirs sampled during the fall was explained by an inverse relationship with chlorophyll-a concentrations. Mean relative weights of both species were not related to mean length at age-3 in reservoirs sampled during the fall. Species composition varied greatly among reservoirs, from systems dominated by black crappies (e.g., Dale Hollow, Cherokee, Chickamauga) to systems dominated by white crappies (e.g., Tellico, Percy Priest, Woods). Based on mean lengths at age-3, black crappies grew slower than white crappies in Barkley, Chickamauga, Kentucky, Normandy, and Woods Reservoirs; no significant differences in growth between species were detected in Douglas and Watts Bar Reservoirs. The percentage of black crappies in each reservoir was negatively correlated with chlorophyll-a concentrations.

Crappie species compositions in concomitant electrofishing and trapnet samples were relatively similar in Kentucky and Barkley Reservoirs. However, in Woods Reservoir black crappies and black-nosed crappies represented 63% of the trapnet sample but only 38% of the electrofishing sample. Similarly, black crappies represented 81% and 91% of the catch in trapnet samples from Normandy Reservoir in 1996 and 1997, respectively, but only 46% and 41% in spring angling samples taken concurrently. Species compositions in Normandy electrofishing samples taken at roughly the same time of year in subsequent years resembled those of the angling samples in 1996 and 1997. More white crappies were caught in the headwaters and fewer in the lower reaches of Douglas Reservoir; black crappie catch rates were similar among areas. More white crappies were caught in the headwater reaches of Kentucky Reservoir than in the middle and lower reaches. Catch rates of black crappies were higher in the lower and headwater reaches than in the middle reach of Kentucky Reservoir. Length-frequencies of crappies collected in concomitant electrofishing and trapnet samples in Kentucky and Barkley Reservoirs were distinctly different; electrofishing collected larger fish than trapnets in both systems.

We feel that the best sampling regime for assessing crappie populations in Tennessee reservoirs would be to use trapnets only to index year-class strength, coupled with concurrent electrofishing in the fall to collect larger individuals for age and growth analyses. All mainstem reservoirs can be sampled in this way, and some tributary impoundments may also fall into this category. In systems where fall larval sampling is ineffective (e.g., most tributary storage impoundments), we suggest that larval sampling be used to index year-class strength and spring electrofishing be used to obtain age and growth data.

Larvae were sampled over six years from Normandy Reservoir, a 1,307-ha tributary storage impoundment and over two years from Barkley Reservoir, a 23,458-ha mainstem impoundment in Tennessee. Larvae were collected from 16 sites stratified over 4 areas in Normandy Reservoir and 14 sites stratified over embayment and main channel habitats in Barkley Reservoir. Sites were sampled both day and night in Barkley Reservoir; sites were sampled only at night in Normandy Reservoir. In Barkley Reservoir, suckers and cyprinids were always more abundant in the main river channel than in embayments. In contrast, shad Dorosoma spp., sildersides, longperch Percina caprodes, and sunfish Lepomis spp. were usually more abundant in embayments. Larval fish distribution patterns in Normandy Reservoir were generally more homogenous. Larval distribution patterns in Barkley Reservoir resembled that of large rivers and were consistent with spawning requirements for each group, and were likely a reflection of spawning habitats used by adults. Embayments provided important habitats for many fish species at the spawning and larvae stages. Main river channel habitats supported fewer species of larvae, but appeared to be an important spawning area for native suckers and cyprinids. Catches of larval fish in Barkely Reservoir were usually higher at night than during the day for all groups of larval fish; however, variation did occur among groups, years and study areas. Stratifying samples among habitats is essential to accurately assess larval fish communities in large systems such as Barkley Reservoir, but is less important in systems such as Normandy Reservoir, with a more homogenous distribution of larvae. However, both types of systems should be sampled at night to maximize the number of individuals and taxa collected.

Population characteristics of crappies were examined from twelve Tennessee reservoirs sampled either in spring or fall with either trapnets or electrofishing. Total annual mortality (A) was estimated in eight of the twelve study reservoirs; high recruitment variability prevented estimation of A in four reservoirs. Total annual mortality ranged from 54-75% and averaged 66%. Total annual mortality was not correlated to mean length at age-3 among reservoirs sampled in the fall. Total annual mortality from age 2 to age 5 for black crappies ranged from 52-79% and averaged 64%; total annual mortality for white crappies ranged from 35-68% and averaged 54%. The distributions of lengths at age-3 were variable across reservoirs in terms of range and skewness, often encompassing the entire length range seen for the whole sample. Most of the variability in growth among reservoirs sampled during the fall was explained by an inverse relationship with chlorophyll-a concentrations. Mean relative weights of both species were not related to mean length at age 3 in reservoirs sampled during the fall. Species composition varied greatly among reservoirs, from systems dominated by black crappie (e.g., Dall Hollow, Cherokee, Chickamauga) to systems dominated by white crappies (e.g., Tellico, Percy Priest, Woods). Based on mean lengths at age-3, black crappies grew slower than white crappies in Barkley, Chickamauga, Kentucky, Normandy, and Woods Reservoirs; no significant differences in growth between species were detected in Douglas and Watts Bar Reservoirs. The percentage of black crappies in each reservoir was negatively correlated with chlorophyll-a concentrations.

Crappie species compositions in concomitant electrofishing and trapnet samples were relatively similar in Kentucky and Barkley Reservoirs. However, in Woods Reservoir black crappies and black-nosed crappies represented 63% of the trapnet sample but only 38% of the electrofishing sample. Similarly, black crappies represented 81% and 91% of the catch in trapnet samples from Normandy Reservoir in 1996 and 1997, respectively, but only 46% and 41% in spring angling samples taken concurrently. Species compositions in Normandy electrofishing samples taken at roughly the same time of year in subsequent years resembled those of the angling samples in 1996 and 1997. More white crappies were caught in the headwaters and fewer in the lower reaches of Douglas Reservoir; black crappie catch rates were similar among areas. More white crappies were caught in the headwater reaches of Kentucky Reservoir than in the middle and lower reaches. Catch rates of black crappies were higher in the lower and headwater reaches than in the middle reach of Kentucky Reservoir. Length-frequencies of crappies collected in concomitant electrofishing and trapnet samples in Kentucky and Barkley Reservoirs were distinctly different; electrofishing collected larger fish than trapnets in both systems.

We feel that the best sampling regime for assessing crappie populations in Tennessee reservoirs would be to use trapnets only to index year-class strength, coupled with concurrent electrofishing in the fall to collect larger individuals for age and growth analyses. All mainstem reservoirs can be sampled in this way, and some tributary impoundments may also fall into this category. In systems where fall sampling is ineffective (e.g., most tributary storage impoundments), we suggest that larval sampling be used to index year-class strength and spring electrofishing be used to obtain age and growth data.