The relation of area to depth was studied for 500 lakes. Up to 0.3 km.2 the mean depth is constant at 3 to 4 m. This is probably an artifact due to omission of shallow lakes from study. From 0.3 to 300 km.2 the relation of depth to area is linear on a log—log plot. In the largest lakes the depths increase less proportionally.Of the three ways to study fish productivity in lakes, one, the estimate of standing crops, is usually attempted at mean depths around 1 m. Another, angling returns, centers on lakes of 3.5 m., while commercial reports come from lakes of 6.5 m. average depth, and extend to the deepest lakes.Fish records can be conveniently divided into three trophic levels, short food chain, e.g. carp, intermediate, e.g. bluegills and long, e.g. trout, bass. Records are interconvertible by use of factors. By factoring, a productivity index is calculated for some 150 lakes.The productivity index is found to be inversely proportional to mean depth on a log—log plot. Thus by correction of the PI it is possible to derive a theoretical value, called the quality index, for a standard lake 5 m. deep.The QI is intended to screen the effect of depth out of the PI and so disclose the inherent capacity of the lake. The QI values vary around unity. Lakes of mean depth under 2.5 m. are factored as of that depth, it being assumed that primary productivity due to photosynthesis goes on all the way to the bottom.A study of bottom fauna in 250 lakes in several regions gave no indication of a relation between depth and productivity.
