David W. Willis

Relative Weight (Wr) Status and Current Use in Fisheries Assessment and Management

Condition assessment is commonly practiced by fisheries personnel as one tool for evaluating fish populations and communities. Several noninvasive condition measures are available for use, including Fultons condition factor (K), relative condition factor (Kn), and relative weight (Wr). The use of Wr as a condition measure has increased within several peer-reviewed journals. In 1995 to 1996, survey responses from agency personnel in 48 states indicated that 22 states used Wr as a standard technique, 18 states identified Wr use as occasional, whereas only eight states indicated no Wr use. The regression-line-percentile technique is recommended for developing standard weight (Ws) equations. There are currently Ws equations available for 52 species and three purposeful hybrids. Length-related trends in condition need to be evaluated prior to calculating a population mean Wr. Relative weight target ranges should be adjusted to meet specific management objectives. Relative weight values are influenced by seasonal dynamics. The uses of Wr may go beyond just a measure of fish “plumpness.” Relative weight can serve as a surrogate for estimating fish body composition, as a measure of fish health, and to assess prey abundance, fish stockings, and management actions.

The Relative Weight Index in Fisheries Management: Status and Needs

Abstract The relative weight (Wr) index allows easy interpretation of condition for fish of various species and lengths. Rapid proliferation of index use, with a piecemeal approach to research on the index, has led to a confusing array of equations and methods for developing these equations. We recommend that the regression-line-percentile (RLP) technique be adopted as a standardized method for developing standard weight (Ws) equations, as good statistical consistency has been demonstrated for this approach. Equations should be modeled on 75th-percentile-weight data for consistent interpretation, and should be developed from data sets that represent the entire geographical range of a species. Standard weight should be considered a benchmark for comparison rather than a management target. Although the concept of a generalized “optimal” target range for Wr is attractive, Wr targets should be established for specific management objectives. Standard weight equations should be evaluated for length-related bias...

Proposed Standard Weight (Ws) Equations and Standard Length Categories for 18 Warmwater Nongame and Riverine Fish Species

Abstract Relative weight (W r) is one of several condition indices used to assess the general health of fishes. Standard weight (W s) equations are required to calculate W r, but are unavailable for many nongame and riverine fish species. Therefore, we developed W s equations for the following taxa: longnose gar Lepisosteus osseus, spotted gar L. oculatus, common carp Cyprinus carpio, bigmouth buffalo Ictiobus cyprinellus, river carpsucker Carpiodes carpio, shorthead redhorse Moxostoma macrolepidotum, smallmouth buffalo I. bubalus, white sucker Catostomus commersoni, black bullhead Ameiurus melas, brown bullhead A. nebulosus, flathead catfish Pylodictis olivaris, white catfish A. catus, yellow bullhead A. natalis, white perch Morone americana, yellow bass M. mississippiensis, green sunfish Lepomis cyanellus, rock bass Ambloplites rupestris, and warmouth L. gulosus. These W s equations were evaluated with statistical validation approaches similar to previously defined regression-line-percentile equations. ...

Population Characteristics of Black Crappies in South Dakota Waters: A Case for Ecosystem-Specific Management

Abstract We sampled 22 populations of black crappie Pomoxis nigromaculatus from three ecosystem types (large impoundments, >40 ha; small impoundments, ≤40 ha; natural lakes) to determine the factors that influence population characteristics (recruitment, growth, size structure, and condition) in South Dakota. Recruitment variability was best correlated with the log10 of the shoreline development index (r = 0.63, df = 16) and the log10 of the watershed : lake area ratio (r = 0.89, df = 12). Mean back-calculated length at age was highly variable among ecosystems and was inversely correlated with the log10 of the catch per unit effort (CPUE; r = –0.35 to –0.69). Mean back-calculated length for all ages was positively correlated with mean relative weight (r = 0.48–0.78, df = 18-21). Proportional stock density and relative stock density of preferred-length fish were inversely correlated with log10 CPUE (Spearman correlation, rs = –0.31 to –0.83, df = 21) and were positively correlated with growth of black crap...

Seasonal Influences on Freshwater Fisheries Sampling Data

Abstract Fisheries managers often assess fish populations using catch per unit effort (CPUE), size and age structure, growth, and condition. For many freshwater fishes and common sampling gears, CPUE, size and age structure, and condition are highest in the spring and fall, while growth commonly is fastest during the summer growing season. However, there are exceptions to these general trends, especially in populations with erratic recruitment, growth, or mortality. At the least, CPUE, size and age structure, growth, and condition of fish should be expected to change with season, given the effects of variable recruitment, growth, and mortality. However, if recruitment, growth, and mortality are relatively stable, seasonal changes in sampling data occur due to changes in fish behavior caused by many factors (e.g., changes in temperature, turbidity, food availability, photoperiod, etc.). However, these patterns of change through the seasons should not necessarily be assumed to be the same for all fish speci...

Stock density indices: Development, use, and limitations

Abstract The purposes of this paper are to review the development and assess the utility of stock density indices. Stock density indices, specifically proportional stock density (PSD) and relative stock density (RSD), were developed to quantify length‐frequency data. Length categories for standardized determination of stock density indices were based on percentages of world‐record length for each fish species; five‐cell length categories have been proposed for many warm‐ and coolwater fishes, but few coldwater fishes. Both seasonal patterns in sampling data and gear‐related biases can affect length‐frequency data used to determine stock density indices. Stock density indices have been correlated with population dynamics (recruitment, growth, and mortality), relative abundance, and condition for many fish species; coefficients of determination typically are low, and much of the variability in the relations is unexplained. Stock density indices for predator and prey fish populations tend to be inversely rel...

Development and Evaluation of a Standard Weight (WS) Equation for Yellow Perch

Abstract Weight-length data for 78 populations of yellow perch Perca flavescens in 20 states and 6 Canadian provinces were used to develop a standard weight (Ws ) equation. We used the regression-line-percentile (RLP) technique, which provides a 75-percentile standard, to develop the Ws relationship. The proposed equation in metric units is log10 Ws = –5.386 + 3.230 log10 L ; Ws is weight in grams and L is total length in millimeters. The English equivalent of this equation is log10 Ws = –3.506 + 3.230 log10 L ; Ws is weight in pounds and L is total length in inches. These equations are proposed for use with 100-mm (4-in) and longer yellow perch. Relative weight (Wr ) values calculated with the proposed Ws equation did not consistently increase or decrease with increasing fish length. Mean population Wr values were significantly correlated with growth and size structure of yellow perch populations, but correlation coefficients were generally low.

Evaluation of Three Different Structures Used for Walleye Age Estimation with Emphasis on Removal and Processing Times

Abstract We compared the removal and processing times required when scales, sagittal otoliths, and dorsal spines were used as age estimation structures for 160 walleyes Stizostedion vitreum collected from six water bodies in South Dakota. Removal and processing times were calculated by 10 fish groups. Dorsal spines required the least amount of time for removal, followed by scales and otoliths. Whole-view otoliths required no further manipulation prior to estimating age, while the sectioning of dorsal spines and scale pressing required 12.5 and 16.6 min of additional processing time, respectively. Dorsal spines and scales also required significantly more time to read than otoliths. In terms of total processing time, whole-view otoliths proved the most time-efficient approach for estimating the age of walleyes. Scales were slightly more time-efficient than dorsal-spine sections, and sectioning otoliths would add additional processing time. Sectioning may not have been necessary in this evaluation because ag...

Relationship of Food Habits to Yellow Perch Growth and Population Structure in South Dakota Lakes

ABSTRACT We compared summer yellow perch (Perca flavescens) food habits among a wide range of perch population types in six South Dakota lakes. Chironomids were a major dietary component in all populations with mean relative importance (RI) values ranging from 29 to 55. Corixids were a major diety component for high-quality (large proprtion of fish ≥200 mm TL) yellow perch populations, with mean RI values ranging from 22 to 26. Zooplankters were important sources of prey for yellow perch in low-quality (low proportion of fish ≥200 mm TL) populations, with mean RI values ranging from 35 to 51. The fast growth of yellow perch in high-quality populations was attributable to a diet in which macroinvertebrates, specifically chironomids, amphipods and corixids, were the primary prey. Fish were not a substantial component of yellow perch diets in situations were perch attained large size (≥ 300 mm) and experienced fast growth. A significant negative correlation existed between yellow perch mean back-calculated l...

Reproduction and Recruitment of Gizzard Shad in Kansas Reservoirs

Abstract Gizzard shad (Dorosoma cepedianum) were collected from 19 Kansas reservoirs to evaluate reproduction and recruitment. Reproduction occurs yearly in large reservoirs (500-6,000 hectares) in Kansas, regardless of adult population structure, but survival to adulthood is sporadic. Gizzard shad ovaries were collected near the peak of the 1983 spawn from Melvern Reservoir, one of the 19 impoundments. Egg-diameter measurements from these ovaries had discrete modes, indicative of multiple-spawning capabilities. However, the gonadosomatic index (GSI) and trawling data were unimodally distributed in both 1983 and 1984, which indicated that a single spawning period extended from early May to mid-June. Statewide, proportional stock density (PSD) and mean relative weight (Wr) of adult gizzard shad the previous fall had significant influences on the quantity of young-of-the-year shad produced. Egg-diameter data from Melvern Reservoir gizzard shad indicated that the larger females contained mostly eggs of one s...