Brian R. Murphy

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...

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...

Relationship of relative weight (Wr) to proximate composition of juvenile striped bass and hybrid striped bass

Abstract The relative weight (W r ) index is commonly used to assess fish condition. However, little is known about the relationship of W r to physiological condition. Whole-body proximate analysis quantifies the general chemical composition of fish but is impractical and too costly for large-scale application to natural populations. Relative weight may offer an alternative method to estimate body composition. We raised juvenile striped bass Morone saxatilis and hybrid striped bass M. saxatilis ♀ × M. chrysops ♂ under controlled conditions for a 12-week period and then determined their proximate compositions. Analysis of relationships between W r and proximate components indicated that W r may be used for estimating body composition and gross energy in juvenile striped bass and hybrid striped bass; W r was correlated with percent crude fat, crude protein, ash, visceral fat, and ash-free dry-weight gross energy. Additionally, W r was correlated with relative growth and the change in total length for the ex...

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.

Factors Regulating Florida Largemouth Bass Stocking Success and Hybridization with Northern Largemouth Bass in Aquilla Lake, Texas

Abstract We electrophoretically assayed four enzyme-encoding loci in, and determined the ages of, 1,534 largemouth bass Micropterus salmoides colleted from a new Texas reservoir (Aquilla Lake) over a 41-month period. We used these data to evaluate stocking success and subsequent hybridization of the Florida subspecies M. s. floridanus (1.5–6.5 cm total length and age 0 when stocked between 1983 and 1985) with the indigenous northern subspecies M. s. salmoides. After these stockings, genomic inflow into the population from Florida largemouth bass was rapid. In the 1986 year class, age-0 Florida largemouth bass, first-generation (F1) hybrids between the two subspecies, and second- or higher-generation (F x ) hybrids were numerically dominant (72%). For these individuals, the frequencies of Florida alleles at two diagnostic loci (fixed allelic differences between subspecies) were 0.51 and 0.52, respectively. Although the subspecies hybridized extensively in 1986, the population at age 0 did not conform to ex...

Proposed Revision of the Standard Weight (WS) Equation for Smallmouth Bass

Abstract Weight-length data from 50 populations of smallmouth bass Micropterus dolomieu (N = 6,731) in 19 states were used to develop a revised standard weight (WS ) equation for smallmouth bass. The regression-line–percentile (RLP) technique was used to develop the proposed 75th-percentile WS equation from length-weight regressions for individual smallmouth bass populations. The proposed Ws equation, log10 WS (weight, g) = –5.329 + 3.200 log10 L(total length, mm), is valid for fish 150 mm long and longer. The English-unit equivalent, log10 WS (lb) = –3.491 + 3.200 log10 L(total length, in), is valid for fish 6 in long and longer.

A Revised Standard Weight (Ws) Equation for Channel Catfish

ABSTRACT Weight and length data were obtained from state, regional, and provincial biologists for 241 channel catfish (Ictalurus punctatus) populations, representative of the species geographic range. We randomly selected 60% of the populations to develop a new standard weight (Ws) equation and used the remainder as an independent test set. The regression-line-percentile (RLP) technique was applied to the development data set providing a 75th percentile-based metric Ws equation of log10 Ws (g) = −5.800 + 3.294 log10 total length (TL, mm). The English equivalent is log10 Ws (lb) = −3.829 + 3.294 log10 TL (in). The remaining 98 populations were used to test both current and new Ws equations for length-related changes in relative weight (Wr) values. Regression of Wr values on TL by population showed that the current equation produced 15% more slopes that were significant (H0: β = 0) than did the new equation. An analysis of significant slopes produced from the RLP-generated equation showed a ratio of 31 posi...

Variation in the Weight-to-Length Relationship among Florida and Northern Largemouth Bass and Their Intraspecific F1 Hybrid

Abstract Weight–length regression equations and relative weights (Wr , the ratio of measured weights to standard weights at length) were compared among 1,401 electrophoretically assayed Florida largemouth bass Micropterus salmoides floridanus, northern largemouth bass M. s. salmoides, and their first-generation (F1) intraspecific hybrid in Aquilla Lake, Texas. Small Florida largemouth bass (101–300 mm total length, TL) displayed lower Wr values than small northern largemouth bass. Differences in Wr were not apparent between 301 and 500 mm TL. Significant subspecific differences in the weight–length slope and intercept coefficients were consonant with observed variation in Wr values. Among four year classes, seasonal differences in subspecific Wr consistently varied five to nine units for fish of age 1 and younger. Relative weight values for small F1 hybrids (101–200 mm TL) were intermediate between those of the two subspecies; they were similar to those of northern largemouth bass but higher than values f...

Experimental trophic ecology of juvenile largemouth bass,Micropterus salmoides, and blue tilapia,Oreochromis aureus

SynopsisThe potential for feeding competition between largemouth bass, Micropterus salmoides, and blue tilapia, Oreochromis areus, in Lake Fairfield, Texas was evaluated experimentally. Largemouth bass and blue tilapia were grown in cages alone and in combination with each other. The fish were allowed to feed on the natural food within the lake. Largemouth bass grown in combination with blue tilapia were significantly shorter and weighed less than largemouth bass grown alone. Blue tilapia grown in combination with largemouth bass were statistically significantly longer and heavier than blue tilapia grown alone. Largemouth bass grown alone had diets (volume and number of food items) significantly different than the largemouth bass grown with the blue tilapia. Largemouth bass fed primarily on chironomid larvae and pupae, and odonates, whereas blue tilapia consumed vegetable matter, detritus, and chironomid larvae. Length and weight differences between large-mouth bass grown alone and in combination with blue tilapia, in conjunction with the largemouth bass diet shift, support the theory that these two species compete for food resources.

Grass Carp (Ctenopharyngodon idella) in the Trinity River, Texas

ABSTRACT Recent confirmation of grass carp spawning in the river systems entering Galveston Bay is a serious concern to many fisheries ecologists. Researchers suspect a portion of recent losses in submersed vegetation and marsh habitat in Galveston Bay may be due to the presence of grass carp. Grass carp captured in the Trinity River were examined to determine population structure, ploidy, fecundity, and diet. Fish captured were primarily diploids and represented a broad range of cohorts, providing strong evidence that naturally spawned grass carp are being recruited to adult sizes in the Trinity River. No evidence of submersed estuarine vegetation exclusive to Trinity Bay was found in the gut contents of grass carp examined.