Standard weight equation for largemouth bass[1] and burbot[2](fish).
Standard weight in fish is the typical or expected weight at a given total length for a specific species of fish. Most standard weight equations are for freshwater fish species.
Weight-length curves are developed by weighing and measuring samples of fish from the population. Methods of obtaining such samples include creel surveys, or measurements of fish caught by commercial fishermen, recreational fishermen and/or by the researchers themselves. Some scientists use cast nets, trotlines, or other means to catch many individual fish at once for measurement. To determine a standard weight equation, several data sets or weight-length relationships representing a species across its range are used.
As fish grow in length, they increase in weight. The relationship between weight and length is not linear. The relationship between length (L) and weight (W) can be expressed as:
When the equation is for standard weight, the standard weight for a given length is written as Ws. The exponent b is close to 3.0 for most species. The coefficient a varies between species. If the exponent b is greater than three for a certain fish species, that species tends to become relatively fatter or have more girth as it grows longer. For largemouth bass, the value of b is 3.273. If the exponent b is less than three for a certain fish species, that species tends to be more streamlined. For burbot, the value of b is 2.898.[3] While the standard weight for a largemouth bass that is 500 mm long is about 2 kg, the standard weight for a burbot that is 500 mm long is only about 0.9 kg.
Standard weight curves are often based on the 75th percentile weight data rather than the average of all the data available. Murphy et al. (1991) suggested that it is preferable that standard weight equations represent the entire geographical range of a species, and that they be used for comparison purposes rather than management targets.[4] Practically, weight-length equations are often developed for sub-populations from specific geographic areas, but these are different from standard weight relationships.
^Henson, J. C. 1991. Quantitative description and development of a species-specific growth form for largemouth bass, with application to the relative weight index. Master's thesis, Texas A&M University, College Station.
^Fisher, S., D. Willis, and K. Pope. 1996. An assessment of burbot (Lota lota) weight-length data from North American populations. Canadian Journal of Zoology 74:570-575
^R. O. Anderson and R. M. Neumann, Length, Weight, and Associated Structural Indices, in Fisheries Techniques, second edition, B.E. Murphy and D.W. Willis, eds., American Fisheries Society, 1996
^Murphy, B. R., Willis, D.W., and Springer, T.A The Relative Weight Index in Fisheries Management: Status and Needs. Fisheries, 16(2):30-38, 1991
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