Larval Sampling as a Fisheries Management Tool: Early Detection of ...

North American Journal of Fisheries Management 18:137–143, 1998 q Copyright by the American Fisheries Society 1998

Larval Sampling as a Fisheries Management Tool: Early Detection of Year-Class Strength STEVE M. SAMMONS1

AND

PHILLIP W. BETTOLI

Tennessee Cooperative Fishery Research Unit,2 Tennessee Technological University Cookeville, Tennessee 38505, USA Abstract.—Larvae of black crappies Pomoxis nigromaculatus, white crappies P. annularis, and white bass Morone chrysops were sampled in 1992–1996 from Normandy Reservoir, Tennessee, with a 1 3 2-m neuston net. Larval crappies were not captured in 1992 or 1993, but densities over the remaining three years varied over two orders of magnitude. Larval white bass were collected every year, but densities also varied over two orders of magnitude among years. Larval crappies recruited to the gear at 9 mm total length (TL), but few over 15 mm were collected. Larval white bass recruited to the gear at 7 mm TL and continued to be sampled by the neuston net at lengths up to 35 mm TL. Peak larval crappie density in the neuston net samples was an accurate predictor of geometric mean number of age-1 crappies per hectare 1 year later in midsummer cove samples (r 2 5 0.99, P 5 0.0001). Peak white bass density in the neuston net samples was an accurate predictor of geometric mean catch of age-0 white bass in fall gill-net samples (r 2 5 0.77, P 5 0.05). Larval sampling of these species over a few weeks each spring in Normandy Reservoir can accurately demonstrate the presence or absence of strong year-classes much earlier and with less effort than traditional sampling techniques such as fall gillnetting or cove sampling. Early detection of year-class strength via neuston-net sampling may allow managers to predict poor year-classes early in the year and initiate remedial actions such as supplemental stocking or regulation changes in a more timely manner.

Accurately predicting year-class strength of fishes is integral to successful management of fisheries. Traditional methods of stock assessment focus on sampling adult members of a population, but year-class strength of fishes is often fixed before the end of a cohort’s first growing season (Diana 1995). Therefore, in recent years more emphasis has been placed on sampling juvenile fishes in an attempt to index year-class strength of a cohort before the adult stage. Larval fish catch has been used to predict yearclass strength for a variety of marine fishes such as North Sea plaice Pleuronectes platessa (Rijnsdorp et al. 1985) and striped bass Morone saxatilis (Uphoff 1989). In freshwater lacustrine environments, ichthyoplankton sampling has not been as widespread, and many studies have focused on clupeids (Cada and Loar 1982; Gallagher and Conner 1983; Tisa et al. 1987; Van Den Avyle and Petering 1988; Michaletz et al. 1995). The utility of trawling to sample larvae of piscivorous gamefish species has not been extensively investigated. Few studies have demonstrated the ability of larval sampling to yield accurate predictions of year1 2

Corresponding author: [email protected] The Unit is jointly supported by the Tennessee Wildlife Resources Agency, the U.S. Geological Survey, and Tennessee Technological University.

class strength of freshwater sportfish, particularly in southeastern reservoirs. White crappies Pomoxis annularis and black crappies P. nigromaculatus (‘‘crappies’’) are two of the most popular species sought by anglers across the United States. In 1991, 28% of all anglers in the United States fished for crappies (USFWS and USCB 1993). Despite the popularity of crappies, fisheries biologists are often at a loss to successfully sample these species, particularly in systems with steep-sided basin morphologies common to many reservoirs. Crappies are known to exhibit extreme fluctuations in year-class strength (Mathur et al. 1979; Colvin and Vasey 1986; Colvin 1991; Mitzner 1991; Guy and Willis 1995), further hindering efforts to successfully sample them. White bass Morone chrysops are equally popular with anglers in some locales. Although not as well studied, white bass also show high variability in year-class strength (Bettoli et al. 1993). Reservoirs tend to be more unstable systems than natural lakes (Carline 1986); thus, crappie and white bass year-class strength may be even more variable and difficult to predict in these environments. Assessing fluctuations in fish abundance in hardto-sample systems can be difficult. Weak or missing year-classes may not be documented until fish have (or have not) entered the fishery. Therefore,

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a lag can occur between production of a poor yearclass, recognition of the problem, and any attempt at remediation (e.g., supplemental stocking or changes in regulations). Larval sampling may be a reliable method of predicting year-class strength of crappies and other limnetic gamefishes; with sufficient notice, remedial actions might be instituted in a more timely manner. This paper presents a method of sampling larval fishes with a neuston net to detect strong and weak year-classes of crappies and white bass in a southeastern reservoir. Study Area Normandy Reservoir is a 1,307-ha tributary impoundment on the upper Duck River in south-central Tennessee. Created in 1976, Normandy Reservoir is operated by the Tennessee Valley Authority for flood control and water supply. At full pool, Normandy Reservoir is 27 km long and it has a shoreline length of 116 km, a mean depth of 11.2 m, and a shoreline development index of 9.0. The water level is drawn down 5 m each fall and returned to full pool in the spring. Normandy Reservoir is a steep-sided basin, and crappies have been traditionally difficult to sample with typical gears such as trap nets (J. Riddle, Tennessee Wildlife Resources Agency, personal communication). We divided the lake into four study areas to assess spatial variation in larval fish catches. The four areas (Figure 1) represent nearly 50% of the total surface area of the reservoir. Methods Larval fish samples were collected from 1992 to 1996 in Normandy Reservoir. Samples were taken weekly in April and May, then biweekly through mid-August (total of 12 dates each year). On each date, 16 fixed stations (four sites in each study area) were sampled at night by towing a 1m 3 2-m floating neuston net of 1-mm mesh (bar measure) equipped with a flow meter (Michaletz et al. 1995). The net was towed with a 5.6-m boat powered by a 70-hp outboard motor. Tow duration was usually 5 min; however, duration was occasionally reduced to avoid clogging the net. Average tow volume was 312 m3 per sample; average tow speed was 0.57 m/s. Samples were fixed immediately with 10% formaldehyde and later preserved in ethanol. All specimens other than shad Dorosoma spp. were identified, counted, and measured. Larval crappies were not sorted to species because their meristic characters are very similar (Siefert 1969; McDonough and Buchanan 1991). Data were log10(catch 1 1)-

transformed to normalize them. We compared geometric mean density of larval crappies and larval white bass among years and study areas using repeated-measures analysis of variance (ANOVA) procedures (SAS Institute 1990; Maceina et al. 1994); all tests were performed at alpha 5 0.05. Coefficients of variation (100·SD/mean) associated with the peak catches each year of each species were calculated from untransformed data to assess precision of the neuston net samples. Cove rotenone samples were collected at three sites during the first week of August each year (Figure 1). Total area sampled was 2.11 ha, and maximum depth of the coves ranged from 4.6 m to 7.6 m. Each cove was blocked off and rotenone (1.5 mg/L, active ingredient) was applied with gaspowered pumps. Fish were collected for 2 d. Individual lengths and weights were recorded for all gamefish species and otoliths were removed for aging. Right sagittal otoliths were examined under a dissecting scope in whole view to determine the age of each fish examined. Peak geometric mean densities of larval crappies (larvae/1,000 m3) were compared to geometric mean densities of age-1 crappies (fish/ha) 1 year later in cove samples by linear regression (SAS Institute 1990). Because adult crappies were difficult to collect in Normandy Reservoir by traditional methods, the services of one angler who lived along the reservoir were used each spring since 1993 to provide an unbiased sample of angler-caught crappies. This angler was instructed, and permitted, to harvest all crappies regardless of size that he caught each spring. We removed ototliths for aging and measured the total length of each fish he caught. White bass were collected in horizontal gill nets set lakewide in early October. Eleven nets were deployed at fixed stations each year. We used sinking experimental monofilament nets 46.9 m long and 1.8 m deep comprising panels of 19, 25, 38, 51, 64, and 76-mm meshes (bar measure). Individual lengths and weights were recorded and otoliths were removed for aging. Catches of age-0 white bass were log10-transformed and compared to larval white bass catches by linear regression (SAS Institute 1990). Results and Discussion Larval crappies were not captured in 1992 or 1993; densities over the remaining three years varied over two orders of magnitude (Figure 2). Larval white bass were collected every year; however, densities also varied over two orders of magnitude (Figure 2). Larval crappies were only collected on

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FIGURE 1.—Study areas in Normandy Reservoir, Tennessee. Larval sampling sites are indicated by diamonds. Cove rotenone sites are indicated by CR and numbered as in Table 2. Dashed lines indicate basin boundaries.

one date in 1995, but were collected over 5 weeks in 1994 and 4 weeks in 1996. Virtually all crappies in Normandy Reservoir hatched in May all three years (Figure 2). Black crappies from a natural lake in Ontario had nearly identical hatching periods in successive years (Amundrud et al. 1974). Larval white bass were only collected on one date in 1992 and 1995, but were collected over 3 weeks in 1993, 5 weeks in 1994, and 7 weeks in 1996. White bass tended to appear in the samples earlier than crappies, but most were also collected in May

all five years (Figure 2). Seasonal succession of different species of larval fishes has been documented in many studies (Amundrud et al. 1974; Holland and Sylvester 1983; Tisa et al. 1987). In most systems, the first appearance of a species’ larvae varies by only 1–2 weeks from year to year. Lengths of larval crappies ranged from 6 to 23 mm, but few larvae over 15 mm were collected (Figure 3). Larval crappies recruited to the gear at 9 mm. Larval white bass recruited to the gear at 7 mm; however, larval white bass were sampled

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FIGURE 2.—Geometric mean densities (means of 16 samples) of larval crappies and white bass collected in neuston samples at Normandy Reservoir, Tennessee, 1992–1996. Crappies were not collected in 1992 or 1993. One white bass was collected on June 17, 1992. Two white bass were collected on May 8, 1995.

over a much wider length range (3–35 mm) than crappies (Figure 3). Mean length of larval crappies captured in the neuston net increased only slightly over time, whereas mean lengths of larval white bass increased steadily over time. This implies that crappies hatched over the entire time they were collected and were only briefly vulnerable to the neuston net, whereas white bass had a short hatching duration and were vulnerable to the neuston net for several weeks afterwards. Size biases of various ichthyoplankton sampling gears are well documented and tend to be speciesspecific (Colton et al. 1980; Cada and Loar 1982; Gallagher and Conner 1983; Thayer et al. 1983; Jessop 1985; Tomljanovich and Heuer 1986; Gregory and Powles 1988; Conrow et al. 1990). Most fish lose their vulnerability to tow-net capture rather quickly when they grow large enough to avoid the net. In many ichthyoplankton surveys, smaller nets (e.g., Tucker trawls) are towed at speeds of 1.0–1.3 m/s. Larval crappies may have been only briefly vulnerable to our gear, relative to other larval gears, because of our slower operating speed (average, 0.57 m/s). However, because we com-

FIGURE 3.—Length frequencies (total lengths) of larval crappies and white bass collected in neuston samples at Normandy Reservoir, Tennessee, 1992–1996.

monly caught larvae of other species (such as white bass, gizzard shad, and threadfin shad Dorosoma petenense) exceeding 15 mm, it is more likely that crappies switched from a pelagic existence to a more littoral one when they reached 9 mm, accounting for their sudden disappearance from our samples. Larvae of some fish species have ontogenetic shifts in their habitat preferenda (Werner 1967). White bass apparently remained limnetic throughout our sampling period, ceasing to be collected only when they had achieved a size large enough to avoid the net. Regardless of why crappies were only briefly vulnerable to the neuston net, peak larval crappie density in the neuston samples was an accurate predictor of geometric mean density of age-1 crappies in cove samples (r2 5 0.99, P 5 0.0001). McDonough and Buchanan (1991) also found a positive relationship between larval crappie density and catch of age-0 crappies in cove samples in Chickamauga Reservoir, a main-stem impoundment on the Tennessee River. Their data implied a curvilinear relationship between larval densities and age-0 abundance in cove samples, due to re-

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TABLE 1.—Geometric mean (GM) catches, their upper and lower 95% confidence limits (CL), and their coefficients of variation (CV 5 100·SD/mean) at peak abundance for larval crappies and white bass captured in neuston-net samples and for age-0 white bass captured in fall gill-net samples in Normandy Reservoir, Tennessee. Catch rates are expressed as fish/1,000 m3 for neuston-net samples and fish/net-night for gill-net samples. Lower CL

Upper CL

77 273 160

Crappies—neuston net 0.0 0.0 0.0 0.0 236.8 151.9 0.4 0.0 6.4 1.3

0.0 0.0 328.0 0.9 11.6

1992 1993 1994 1995 1996

400 229 100 400 87

White bass—neuston net 0.2 0.0 0.7 0.0 15.0 7.6 0.2 0.0 41.6 23.8

0.5 1.5 22.4 0.6 59.6

1992 1993 1994 1995 1996

332 132 92

White bass—gill net 0.1 0.1 0.6 0.1 4.6 2.1 0.0 0.0 2.0 0.9

0.2 1.2 8.9 0.0 3.5

Year 1992 1993 1994 1995 1996

CV (%)

111

GM catch

duced survival at higher densities. This may not be the case for Normandy Reservoir, because the highest larval density observed (236 fish/1,000 m3 in 1994) led to the highest density of age-1 crappies in cove samples 1 year later (1,398 fish/ha). Both maxima were more than one order of magnitude greater than densities of other year-classes (Tables 1, 2), yet survival of the fish did not appear to be reduced. However, we never observed intermediate densities (e.g., 50–200 fish/1,000 m3) of larval crappies in Normandy Reservoir. Therefore, we cannot conclude whether or not juvenile crappie survival was density dependent. The 1994 year-class of crappies accounted for over 98% of the total catch (N 5 497) by our spring angler during 1996 and 1997, and it currently supports the crappie fishery. Cove samples have provided valid estimates of crappie year-class strength in other systems (Miranda et al. 1991); however, neuston sampling provided less variable estimates of crappie year-class strength (Tables 1, 2) with less effort. For instance, collecting three cove samples on Normandy Reservoir required approximately 322 person-hours and cost approximately $3,500 (not including salaries). In contrast, the 12-week larval sampling was accomplished with only 96 person-hours of

TABLE 2.—Estimated densities of age-1 crappies collected in three cove samples from Normany Reservoir. Coves are arranged numerically from lower lake to upper lake. Geometric mean catch (GM catch) is from log10 (catch 1 1)-transformed data. Density (fish/ha) in cove sample: Year

1

2

3

GM catch/ha

1993 1994 1995 1996 1997

0 0 2,137 0 41

0 0 243 0 16

0 6 4,812 0 66

0.0 0.9 1,358.2 0.0 35.3

effort in the field, costing approximately $900. Processing time of larval samples in the lab varied with the amount of debris and fish in the samples, but was never more than 8 person-hours per sample week. Because year-class strength of both crappies and white bass in Normandy Reservoir appears to be set at the larval stage, neuston sampling over a limited duration (approximately 1 month) could detect strong and weak year-classes of these species in that system. Peak larval white bass density in the neuston samples (fish/1,000 m3) was an accurate predictor of geometric mean catch of age-0 white bass (mean length, 238 mm TL; range, 141–298 mm) in fall gill-net samples (r2 5 0.77, P 5 0.05). Fall gillnet samples are a standard tool for assessing yearclass strength of offshore species such as white bass in southeastern reservoirs (Hubert 1996). Unlike cove samples, gill-net samples generally are comparable to neuston sampling in terms of cost and effort. Also, gill-net samples and neuston net samples had similar coefficients of variation (Table 1). However, no additional effort had to be expended to collect larval white bass because they occurred concomitantly with larval crappies. A larval sampling program could take the place of gill-net sampling if the intent of gill-net sampling is simply to monitor white bass year-class strength. Precision of peak catches in the neuston net varied directly (coefficients of variation varied inversely) with the geometric mean catch of larval crappies and white bass (Table 1). The highest mean catches were always the most precise estimates. Even though the most precise estimates were still highly variable, with coefficients of variation of 77% and 87%, they were excellent predictors of age-1 (crappies) and age-0 (white bass) abundances 6–14 months later. Larval catch data are often highly variable due to the contagious distribution of ichthyoplankton

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(Thayer et al. 1983; Jessop 1985; Van Den Avyle and Petering 1988; Cyr et al. 1992), and larval fish data are highly susceptible to gear biases. Variables such as mesh size, tow speed and duration, and volume of water strained can affect vulnerability of larval fish to the gear. The time of day at which a sample is taken can be especially important, because various fish species are found at different depths throughout the diurnal cycle. For instance, more clupeid larvae are found near the surface during the day than at night (Cada and Loar 1982). Conversely, Holland and Sylvester (1983) found that larval crappies and larval white bass were more often found on the surface at dusk. Therefore, ichthyoplankton investigators must standardize not only gear and sampling protocol, but time of day for sampling as well—preferably to coincide with the greatest abundance of the target species at the sampling depth. If larvae are collected with a neuston net in a standardized manner, these samples should be no more variable than samples taken with traditional gears such as gill nets (Table 1) and much less variable than cove samples (Table 2). Management Implications Larval sampling with a neuston net is very simple, especially compared to sampling with cove treatments and trap nets. No special equipment such as winches and booms are required to tow a neuston net; the only requirements are that the boat used to tow the net be sufficiently powered ($70 hp) and have a keel or runners to exert pressure against the drag of the net. Identification of both crappie and white bass larvae is relatively easy, although identifying larval crappies to species is difficult. Because both crappies and white bass spawn early in the year, larval samples directed at these species would have little bycatch of more numerous species such as clupeids, allowing samples to be processed with minimal effort. A typical larval sample from Normandy Reservoir in early and mid-May would have fewer than 50 fishes per tow. Care given to operational methods will help minimize variability of the samples and provide fisheries managers with a method of obtaining an early indicator of year-class strength for crappies, white bass, and perhaps other fish species with limnetic larvae. Thus, we recommend that other biologists assess the utility of larval sampling to provide an early index of year-class strength of crappies and white bass in other lakes and reservoirs. The Tennessee Wildlife Resources Agency (TWRA) stocks more than one million fingerling

crappies each fall statewide, and TWRA biologists are seeking information to help them allocate these stockings in the most biologically and fiscally responsible manner. Early detection of weak yearclasses via neuston sampling would allow biologists to take appropriate remedial actions (such as supplemental stocking or changes in fishing regulations) in a more timely manner and perhaps avoid (or at least anticipate) future public relation problems. Acknowledgments Funding for this project was provided by the Tennessee Wildlife Resources Agency (Federal Aid in Sport Fish Restoration, project FW-6), the Tennessee Valley Authority, and the Center for the Management, Utilization, and Protection of Water Resources at Tennessee Technological University. We thank the many students, technicians, and interested onlookers who have contributed their time to make this project possible. This manuscript was greatly improved by comments and suggestions from three anonymous reviewers. References Amundrud, J. R., D. J. Faber, and A. Keast. 1974. Seasonal succession of free-swimming perciform larvae in Lake Opinicon, Ontario. Journal of the Fisheries Research Board of Canada 31:1661–1665. Bettoli, P. W., M. J. Maceina, R. L. Noble, and R. K. Betsill. 1993. Response of a reservoir fish community to aquatic vegetation removal. North American Journal of Fisheries Management 13:110–124. Cada, G. F., and J. M. Loar. 1982. Relative effectiveness of two ichthyoplankton sampling techniques. Canadian Journal of Fisheries and Aquatic Sciences 39:811–814. Carline, R. F. 1986. Indices as predictors of fish community traits. Pages 46–56 in G. E. Hall and M. J. Van Den Avyle, editors. Reservoir fisheries management: strategies for the 80s. American Fisheries Society, Southern Division, Reservoir Committee, Bethesda, Maryland. Colton, J. B., Jr., J. R. Green, R. R. Byron, and J. L. Frisella. 1980. Bongo net retention rates as effected by towing speed and mesh size. Canadian Journal of Fisheries and Aquatic Sciences 37:606–623. Colvin, M. A. 1991. Population characteristics and angler harvest of white crappies in four large Missouri reservoirs. North American Journal of Fisheries Management 11:572–584. Colvin, M. A., and F. W. Vasey. 1986. A method of qualitatively assessing white crappie populations in Missouri reservoirs. Pages 79–85 in G. E. Hall and M. J. Van Den Avyle, editors. Reservoir fisheries management: strategies for the 80s. American Fisheries Society, Southern Division, Reservoir Committee, Bethesda, Maryland.

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