Chapter 14: Length, Weight, and Associated Indices
Robert M. Neumann, Christopher S. Guy, and David W. Willis
Length and weight data provide statistics that are cornerstones in the foundation of fisheries research and management. The objectives of this chapter are to present information on the uses of length and weight data as well as the calculation and interpretation of structural indices from such data.
The numbers and sizes of fish in a population determine its potential to provide benefits for commercial and recreational fisheries. Length and weight data also provide the basis for estimating growth, standing crop, and production (tissue growth) of fishes in natural waters as well as in fish hatcheries and laboratories. The rate of annual production is determined by the rate of reproduction (number of viable offspring each year), the rate of growth (change in weight of individuals), and the rate of mortality (loss of numbers in an age-group). These functional rates determine population dynamics over time, as well as structural attributes such as density, biomass, and length frequency at any point in time.
One challenge for a fisheries manager is to identify problems and opportunities presented by existing population structures. Effective adjustments of functional attributes, such as altering mortality rates with length-limit regulations, can achieve a more favorable population structure to meet the management objectives for that fish population and assemblage.
Homer S. Swingle and Richard O. Anderson provided the primary impetus for understanding the utility of weight and length data in fisheries assessment. These scientists showed us what could be learned from assessment of fish asemblages in small impoundments. Small impoundments are convenient experimental mesocosms for basic research on ecological concepts and principles as well as for applied research on fish population dynamics and fisheries management. Beginning in the 1940s, state and federal programs stimulated the construction of many small impoundments in the southern USA for soil conservation, irrigation, or wildlife needs. An entomologist at Auburn University, Swingle was concerned about these waters as habitats for mosquitoes that could cause the spread of malaria. His early observations led to the conclusion that ponds with fish had few or no mosquitoes. This led to further studies of how these waters might best be used to produce fish as food for rural populations. The results of management efforts in experimental ponds were often evaluated by census of the fish assemblages after the ponds were drained.