Some Uses of Nomographs in Fish Growth Studies

Abstract

OMOGRAPHS have been used in various fish growth studies, (Huntsman, 1918; Lea, 1919; Hile, 1941; Fry, 1943) but it is believed that their usefulness in this field has not been fully realized. The nomograph described in this paper has saved much time in the calculation of growth rates at the fisheries research laboratories of the Minnesota Department of Conservation. This nomograph is easily constructed and may be operated accurately even by relatively unskilled technicians. As here described, it couples the use of cards on which the scale measurements are marked as proposed by Lea (1919) with the simpler form of nomograph used by Hile (1941). When the true body-scale relationship is known, the growth rate is calculated with specially prepared nomograph tables. Huntsman (1918) used a cardboard curve on a nomograph to make calculations when the true bodyscale relationship was known. Special nomograph tables are simpler to use than the cardboard curve. Fry (1943) described a nomograph which can be used only for those cases in which the body-scale relationship can be described by a logarithmic straight line. The logarithmic straight line does not satisfactorily describe the body-scale relationship of all fishes (Creaser, 1926; Bennett, Thompson, and Parr, 1940; Carlander, 1944). Nomographs used in growth studies are based on the geometric principle that parallel lines cut proportionate segments off divergent lines (Fig. 1).