The Morphoedaphic Index—Use, Abuse, and Fundamental Concepts

Abstract

The morphoedaphic index (MEI) has been used globally as a fish-yield estimator with widely divergent results. With few exceptions, when the index did not predict fish yield with moderate precision, practitioners had not exercised carefully the criteria for application. The fundamental concept of the MEI embraces standard thermodynamic theory, that of matter (nutrients) transported by energy within an open system. Fish yield is a biotic output of the system. Constraints on the level of fish yield are determined by lake-basin morphometry and nutrient and energy availability. Of the three morphometric factors considered, mean depth, volume, and area, mean depth appears to be the variable most closely linked with production processes. The major direct effect on production by area is that expressed by the elementary mathematics of both size and of scale, particularly perimeter-to-area and area-to-volume ratios which change markedly in lakes of different total surface areas. Volumetric effects contain elements similar to those of both area and mean depth. A rational approach to fish-yield estimation may be made at three hierarchic levels: (1) a global level at which area and temperature assume major importance; (2) a regional level at which nutrients and mean depth become the critical variables as area and temperature are held constant; (3) an infra-regional level at which either a nutrient or a depth variable alone may be used for fish-yield estimates while the remaining variable is constant.