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Growth of black crappies in two west Tennessee impoundments with implications for a reduced minimum length limit.

Abstract--Slow growth of black crappies (Pomoxis nigromaculatus (Lesueur)) is common in small impoundments and often results in populations with small size structure. Poor growth of black crappies can be compounded by restrictive minimum length limits. Following anecdotal angler reports of abundant small black crappies in Brown's Creek and Maples Creek lakes, Tennessee, we used angling to sample black crappies from each lake in May and June 2012 to evaluate several growth metrics. We used length, weight, and back-calculated length at age data to estimate and compare size structure, body condition (relative weight), and growth of black crappies in each impoundment. Both impoundments have similar depths, watersheds, and fish communities, and were managed with Tennessee's statewide 254 mm minimum length limit for black crappies prior to March 2014. Black crappie growth rates were significantly greater in Maples Creek Lake compared to Brown's Creek Lake; however, both impoundments contained slow-growing black crappie populations. Size structure was similar between impoundments; both populations were dominated by 250-270 mm black crappies. Body condition of 200-250 mm black crappies was significantly greater in Maples Creek Lake (mean relative weight = 91) than in Brown's Creek Lake (mean relative weight = 85). Body condition did not differ for 250-300 mm black crappies between impoundments, and condition of all black crappies was below average (relative weight <100). Based on observed body condition, growth rates, and asymptotic length, our findings suggest that reducing the 254 mm minimum length limit may improve black crappie size structure and/or angler harvest in these impoundments by releasing potential density-dependent constraints on growth.

Introduction

Black (Pomoxis nigromaculatus (Lesueur)) and white (P. annularis (Rafinesque)) crappies are among the most pursued sport fish in the United States. Between 2001 and 2006, 25% of United States freshwater anglers (excluding the Laurentian Great Lakes) reported participating in black crappie fishing (USFWS, 2006). Because of their popularity, black crappies have been stocked into many small impoundments to provide recreational fishing opportunities (Gabelhouse, 1984; Boxrucker, 1987). Outcomes of stocking black crappies into small bodies of water have been mixed, due in large part to their highly variable spawning success (Gabelhouse, 1984). When recruitment is low, black crappies have often failed to produce the numbers of fish necessary to sustain a desirable fishery in small impoundments (McHugh, 1990). In contrast, high black crappie recruitment in small lakes and impoundments has frequently resulted in high density populations that exhibit slow growth with small size structure (Gabelhouse, 1984; Boxrucker, 1987).

Black crappie fisheries are harvest-oriented (Miranda, 1999; Boxrucker, 2002). Anglers desire high numbers of harvestable size fish, and fisheries managers are faced with the task of maintaining quality black crappie populations in various habitats, often under heavy exploitation (Miranda and Allen, 2000). Minimum length limits have been used to improve size structure in black crappie populations that exhibited rapid growth rates, low natural mortality, and have been negatively affected by exploitation (Allen and Miranda, 1995). Minimum length limits have been less effective for managing black crappie populations that did not meet these requirements because relatively few individuals survived long enough to exceed minimum length limit requirements when growth was slow or natural mortality was high (Allen and Miranda, 1995; Bister et al., 2002; Isermann et al., 2002). Also, minimum length limits may increase population density by protecting fish from angler harvest, compounding the problem of slow growth in systems where growth is density-dependent.

The Tennessee Wildlife Resources Agency (TWRA) owns and operates 18 small impoundments in middle and west Tennessee known as "Family Fishing Lakes," which are specifically managed to provide recreational fishing opportunities. Black crappie fishing regulations for most "Family Fishing Lakes" dictate a 30 fish daily bag limit (combined white and black crappies) and a 254 mm statewide minimum length limit. Anecdotal reports from anglers suggested that two of these impoundments in West Tennessee, Maples Creek Lake and Brown's Creek Lake, showed signs of slow-growing black crappie populations (e.g., small size structure). Our study objective was to estimate and test for differences between growth, body condition (relative weight, Wr (unitless index of body condition)), and size structure of black crappies in these impoundments in regards to the suitability of the 254 mm minimum length limit.

Methods

This study was conducted on Maples Creek Lake (39 ha, Carroll County, Tennessee, N 35.863110[degrees] W 88.292060[degrees], Mercator) and Brown's Creek Lake (67 ha, Henderson County, Tennessee, N 35.730986[degrees] W 88.292060[degrees], Mercator), both of which are impoundments with maximum depths of 4-5 m. Emergent macrophytes were present in some areas of Maples Creek Lake, but were limited to shoreline reeds in Brown's Creek Lake. The impoundments are separated by a distance of about 16 km and lie within the boundaries of Natchez Trace State Park. The watersheds of both impoundments are predominately forested. Both impoundments are stream-fed most of the year, have relatively stable water levels, and are eutrophic. Fish communities in both impoundments were typical of other TWRA "Family Fishing Lakes", which are stocked with largemouth bass (Micropterus salmoides (Lacepede)), bluegill (Lepomis macrochirus (Rafinesque)), redear sunfish (L. microlophus (Gnther)), blue catfish (Ictalurus furcatus (Valenciennes)), channel catfish (I. punctatus (Rafinesque)), and black crappie.

We used angling to collect black crappies on our study lakes. We chose angling as our sampling method because black crappie fishing is size selective for larger fish (> 200 mm) relative to other gears (Miranda and Dorr, 2000). Our objective was to primarily capture larger/older black crappies that would be the most useful for obtaining age, growth, and back-calculated length at age information. We obtained a scientific collection permit from TWRA to allow for collection of black crappies <254 mm, and all captured black crappies were measured for total length (TL, mm) and weighed (g), regardless of size. We used the same fishing tackle and method (trolling) in both impoundments to minimize variation in sampling technique. All black crappies were captured using no. 4 hooks with small (~50-75 mm) live golden shiners (Notemigonus crysoleucas (Rafinesque)) as bait. Black crappies were captured primarily from relatively deep (3.0-4.9 m) pelagic areas, often near the dams in both impoundments.

A total of 181 black crappies were collected on four sampling occasions in May and June of 2012. Representative subsamples of the fish collected

were used for age and growth analysis. Subsamples were selected by aging about 50% of each 10 mm size class. Saggital otoliths were removed from the subsample of fish (n = 94; Brown's n = 72; Maples n = 22) and placed into numbered envelopes without size information to minimize aging bias. Otoliths were polished whole and aged using whole view photographs taken with a dissecting microscope (Leica S8AP0) equipped with an image analysis system (Leica DFC295). We estimated age at capture for each fish, and length at age was back-calculated using enlarged photographs.

We used total length (TL) and weight of each fish to estimate population size structure and relative weight (Wr), respectively, for black crappies in each impoundment. We calculated Relative Size Distribution (Willis et al., 1993; Guy et al., 2007) of Quality (RSD-Q (RSD-Q = number of fish [greater than or equal to] Quality size/number of fish [greater than or equal to] Stock size)) and Preferred (RSDP (RSD-P = number of fish [greater than or equal to] Preferred size/number of fish [greater than or equal to] Stock size)) size black crappies for each lake (Stock = 130-199 mm, Quality = 200-249 mm, Preferred = 250-299 mm; Neumann et al., 2012). A chi-square ([chi square]) test was used to test for differences in size distribution between the two impoundments. We used the null hypothesis ([H.sub.0]) of no difference in size structure between the black crappie populations at the [alpha] = 0.05 level.

To examine the effectiveness of our collection method in sampling the black crappie populations, we visually compared our angling data to unpublished TWRA electrofishing data taken from both lakes from the previous three years (2009-2011). A length frequency histogram was created and used to examine the likelihood that substantial portions of the population existed outside the range of our sampled sizes (Fig. 1). Since we determined that there was no difference in size structure between lakes from our own data ([[chi square].sub.1] < 3.841, P > 0.05), we combined lakes for the examination. Because of TWRA's relatively small sample size (n = 41), we combined their electrofishing data from both lakes for Fig. 1. TWRA did not collect any black crappies outside of our 10 mm size classes (Fig. 1). Given the similarities between TWRA's independent electrofishing data and ours, angling effectively sampled the black crappie populations in these lakes.

We calculated Wr using the standard weight equation given by Neumann and Murphy (1991) (minimum length = 100 mm). We tested for differences in Wr of size classes between impoundments using a t-test ([H.sub.0] = no difference in Wr between impoundments, [alpha] = 0.05). Annuli measurements were used with the direct-proportion method to back-calculate length at age for each fish (Fig. 2). Mean back-calculated length at age data were used to fit the von Bertalanffy growth model (von Bertalanffy, 1938) and estimate model parameters [L.sub.[infinity]] (asymptotic or theoretical maximum length that a fish could obtain given adequate time), K (Brody growth coefficient; describes how quickly the asymptotic or theoretical maximum length is obtained), and [t.sub.0] (age at which length would be 0) for each population (Quist et al., 2012). We used a general linear model (GLM) with back-calculated length at age to test for differences in growth of black crappies between impoundments ([H.sub.0] = no difference in growth between impoundments, [alpha] = 0.05).

Results

In total, 181 black crappies were collected from Brown's Creek Lake (n = 127) and Maples Creek Lake (n = 54). Quality (200-250 mm) and Preferred (250-300 mm) were the only size classes represented by the samples in both impoundments. Black crappies from Maples Creek Lake ranged in length from 228-277 mm (mean length = 257 [+ or -] 1.57 mm). Black crappie lengths in Brown's Creek Lake ranged from 223-277 mm (mean length = 258 [+ or -] 0.81 mm) (Fig. 1).

Relative Size Distribution-Quality (RSD-Q) and Preferred (RSD-P) were 17 and 83 for Brown's Creek Lake black crappies, respectively. Maples Creek Lake black crappie RSD-Q and RSD-P were 22 and 78, respectively. Size structure of the black crappie populations did not differ between impoundments ([[chi square].sub.1] < 3.841, P > 0.05). Relative weight did not differ between impoundments ([T.sub.78] = 1.62, P [greater than or equal to] 0.05) or between Preferred-size black crappies in each lake (Brown's Creek Lake Preferred-size Wr = 83; Maples Creek Lake Preferred-size Wr = 84; [T.sub.61] = 0.34; P [greater than or equal to] 0.05). Relative weight of Quality-sized black crappies differed between lakes ([T.sub.27] = 3.98; P = 0.001). Quality-sized black crappies had greater Wr in Maples Creek Lake (91) than in Brown's Creek Lake (85).

A total of 72 and 22 black crappies were aged from Brown's Creek Lake and Maples Creek Lake, respectively. Black crappies ranged from age 2-6 years in Maples Creek Lake and from age 2-10 years in Brown's Creek Lake (Fig. 2). Growth rates of black crappie differed between lakes (GLM: P = 0.046; [F.sub.1,510] = 4 .0). Black crappies grew more rapidly in Maples Creek Lake compared to Brown's Creek Lake; however, growth of black crappies in both impoundments slowed considerably at sizes > 200 mm (Fig. 2). Estimated [L.sub.[infinity]] values were 257 mm for Brown's Creek Lake and 263 mm for Maples Creek Lake black crappies.

Discussion

Although growth of black crappies was greater in Maples Creek Lake, the difference was small and of little biological significance. Maples Creek and Brown's Creek Lake black crappies exhibited very slow growth at sizes > 200 mm (Brown's = 9.8 mm/year; Maples = 14.2 mm/year). Quality-size black crappies in Maples Creek Lake had higher Wr than similar size Brown's Creek Lake black crappies, potentially suggesting that 200-249 mm fish may have had greater food availability in Maples Creek Lake, contributing to their faster growth. Annual growth increments of both populations were reduced for individuals [greater than or equal to] 200 mm, resulting in populations with very similar size structures. In comparison, Allen and Miranda (1995) reported that average growth rates of age 4+ black crappies were 25.7 mm/year throughout the south-central United States. Relative weight of black crappies in both populations indicated poor body condition (average body condition Wr = 100).

Historically, black crappies have been managed with liberal regulations aimed to decrease abundance and intraspecific competition to improve population size structure (Allen and Miranda, 1995). Recent trends in black crappie management have shifted towards the use of minimum length limits to improve yield and size structure (Allen and Miranda, 1995; Isermann et al., 2002). Minimum length limits may prevent overfishing and improve the size structure of harvested fish by protecting smaller fish until they grow to desirable size. However, effectiveness of minimum length limits has varied among individual black crappie populations (Allen and Miranda, 1995; Bister et al., 2002). For black crappies, minimum length limits have been most effective for populations that exhibited rapid growth rates and whose size distribution had been negatively affected by exploitation (Colvin, 1991; Allen and Miranda, 1995). When applied to slow growing populations, minimum length limits may compound the problem of slow growth by protecting too many fish, which may result in increased intraspecific competition and further reductions in growth rates (Allen and Miranda, 1995; Bister et al., 2002; Isermann et al., 2002).

Beginning on March 1, 2014, TWRA lowered the minimum length limit from 254 to 203 mm for black crappies in Maples Creek and Brown's Creek lakes. We believe that size structure of black crappies and/or angler harvest will likely benefit from the reduced length limit. In the current study, growth of black crappies in both impoundments slowed considerably at lengths > 200 mm. Although our study did not estimate exploitation or mortality in these populations, growth alone can be a strong predictor of the effectiveness of minimum length limits for black crappies (Isermann et al., 2002). In general, restrictive minimum length limits have not been effective at improving slow-growing black crappie populations (Colvin, 1991; Allen and Miranda, 1995). Isermann et al. (2002) suggested that black crappie populations in Tennessee reservoirs needed to reach 254 mm by age 3 to realize an increase in yield from a 254 mm minimum length limit. Based on our growth rate observations, black crappies in Maples Creek Lake did not reach 254 mm until age 5 and black crappies in Brown's Creek Lake took [greater than or equal to] 7 years to grow to harvestable size. Our estimated von Bertalanffy growth model [L.sub.[infinity]] values indicated that the asymptotic length (mean length fish would reach if they were to grow for an infinitely long period) of black crappies was only slightly above 254 mm in both impoundments (Maples Creek Lake [L.sub.[infinity]] = 268 mm; Brown's Creek Lake [L.sub.[infinity]] = 257 mm). Black crappies would likely become legally harvestable in both impoundments at age 2 or 3 with a 203 mm minimum length limit. Increasing the proportion of legally harvestable fish would probably lead to a reduction in black crappie abundance, which could increase growth and size structure of survivors if growth is density-dependent in these impoundments. Even if growth and size structure does not improve as a consequence of decreased abundance, anglers will have an opportunity to harvest many black crappies that otherwise would likely have died before reaching harvestable size.

Population statistics drawn from our study relied upon black crappies captured by angling. Angling is known to be size selective and may have influenced our RSD values (Miranda and Dorr, 2000). We also acknowledge that our sampling method likely underrepresented fish <age 3. Nonetheless, we believe our sample closely represented the actual population, or is at least accurate enough to make useful management decisions. In comparing our data with TWRA data collected from 2009 and 2011, black crappie lengths from our sample were similar to those gathered by the TWRA, which samples "Family Fishing Lakes" using electrofishing in spring and fall each year. Thus, we believe that our age data, back-calculated length at age, and RSD values are reasonably accurate and suitable for comparison between these two impoundments. Also, our sample sizes were relatively small for estimating population characteristics. However, we believe the relatively narrow range of sizes and low variability of the samples collected by TWRA and our current study suggests stunted black crappie populations. Given this evidence, most captured black crappies would probably have continued to fall within this size range and our estimated population statistics would not have differed substantially, even if our sample size had been greater.

Another consideration is the aging technique we used. Aging with whole otoliths has been shown to underestimate age in older fish (Besler, 1999). However, Schramm and Doerzbacher (1982) found whole otoliths to be effective for aging and back-calculating length at age of black crappies in Florida. Given the higher latitude of our black crappie populations and associated shorter growing season, our otoliths should have been as easy to read, if not more so, as those in Florida. Otoliths showed distinct bands and ages were agreed upon with another reader for the subsamples of otoliths. We believe this technique was sufficiently accurate to compare and evaluate growth of the black crappie populations in these impoundments. Even if the ages of older fish were underestimated, our results would be conservative, and the management implications derived from them would still have merit.

Slow growth of black crappies in small impoundments has been attributed to overcrowding and/or a lack of suitable prey; black crappies are planktivorous as juveniles and primarily piscivorous as adults (Becker, 1983; Gabelhouse, 1984; Mosher, 1984; Boxrucker, 1987; Bister et al., 2002). Although the driver(s) of slow growth of black crappies in these two impoundments was beyond the scope of this study, our findings contribute to the fisheries management literature in further suggesting that growth of black crappies in small impoundments is often poor. Our findings are also important because Maples Creek Lake and Brown's Creek Lake are similar in physiography and management to many other "Family Fishing Lakes". Similar size structure and growth rates of black crappies between our two study impoundments indicate that similar situations could be occurring in other "Family Fishing Lakes". Future monitoring of black crappie populations in these and similar small impoundments will be critical to detect slow-growing black crappie populations and implement corrective management practices, such as reductions in minimum length limits.

Acknowledgments

We thank E. Box, J. and C. Turner, J. Mosier, and F. Lee for their valuable assistance with data collection and recording. We also thank D. Rizzuto and TWRA for providing valuable information and assistance with this study.

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Manuscript received 11 March 2015; Manuscript accepted 7 October 2015.

Chris T. Bailey, Bradley A. Ray *, and Greg G. Sass

The University of Tennessee, Department of Agriculture, Geosciences, and Natural Resources, Martin, TN 38237 (CTB, BAR) Present address, Wisconsin Department of Natural Resources, Escanaba Lake Research Station, WI54512 (CTB, GGS)
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Author:Bailey, Chris T.; Ray, Bradley A.; Sass, Greg G.
Publication:Journal of the Tennessee Academy of Science
Article Type:Report
Geographic Code:1U6TN
Date:Dec 1, 2015
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