The morphoedaphic index (MEI), a fish yield estimator, was historically reviewed and evaluated. Background papers leading to the development of the MEI were discussed, as well as more recent advances. The MEI was defined and its criteria for use examined. The significance of its components — total dissolved solids and mean depth — were explained. The MEI was compared with related models developed by other workers. The global range of the MEI was outlined as well as restrictions on its use. Sustained fish yields at different MEI values for various climatic regions were predicted. Several special applications of the MEI were described, including: prediction of angling yield in north-temperate lakes; prediction of commercial fish yield on a newly created reservoir; indicator of ecological stresses and changing environments; and contrasting responses to cultural eutrophication in the littoral and profundal zones of lakes. The implications of the MEI for ecological theory were explored, and its potential for future application in management was outlined.

We propose that the optimum habitat of the percid fishes Perca flavescens, P. fluviatilis, Stizostedton vitreum vitreum, and S. lucioperca in lakes may be defined by the littoral and sublittoral environmental conditions equivalent to those in large, temperate rivers. Analogous habitat conditions include sand or gravel substrate, low current velocity, reduced light penetration (Stizostedion spp. only), temperatures optimal for growth and reproduction, and well-oxygenated spawning substrates. The species' evolutionary origins and reproductive patterns also reflect their riverine ancestral habitat. Evidence in support of the hypothesis is derived from the diversity of papers contributed to the PERCIS Symposium. Key words: Percidae, habitat, theory, Perca, Stizostedion

In many oligotrophic lakes intense fisheries exploitation preceded severe cultural eutrophication by several decades at least, except perhaps in stream and inshore areas near shore. Attempts to infer the effects of exploitation from historical fisheries data must allow for the high rates of capture associated with the "fishing-up" process that follows initiation of a new fishery, introduction of a new gear, change in capture or vessel technology, etc. The nature of the fishing-up process is examined in some detail. Interactions of new species introduction and of eutrophication with exploitation are outlined.Within the salmonids, different taxa appear to respond somewhat differently to exploitation stress. Some taxa appear to have greater capabilities to compensate for this stress than others. Responses of the taxa are examined in the context of well-known properties of their niches and population variables.Some inferences concerning responses at the community level are proposed. Compensatory capabilities seem directly related to the degree of eutrophy of the lake. Events such as the virtual disappearance of taxa, or eruptions of new taxa, and the relative variability in year-class abundance of dominant taxa, are examined in this context.

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Among the lake trout, Salvelinus namaycush (Walbaum), of eastern Lake Superior are some which, instead of spawning on reefs and shoals in the lake, enter certain rivers to spawn. Tagging and recovery studies were conducted on such fish seined from the Montreal and Dog rivers during the period 1951 through 1955. Male trout in Montreal River were found to mature at age 7 years when their average length was 25 inches. Females matured a year later when their average length was 28.5 inches. Trout in Dog River matured at the same ages but at lengths shorter by 3 to 4 inches, apparently because of slower growth during early life. Captures of the tagged trout in Lake Superior by sport and commercial fishermen showed that they frequented depths from 10 to 30 fathoms. Most of the trout recaptured were taken within 30 miles of the river in which they spawned. High recapture rates in the succeeding spawning season in the rivers at which they were tagged suggest that most of the trout return to the same river annually to spawn, and that the populations associated with each spawning run are discrete. The mortality rate of fish in the spawning run increased during the study period and reached an estimated 90 percent in both populations for the year 1954 to 1955. Spawning populations in both rivers declined from over 2,000 fish in 1952 to only a few fish in 1955. Catch of trout by sport and commercial fishermen was light, and was estimated to be less than 5 percent of the population each year. Incidence of scars caused by attacks of sea lamprey, Petromyzon marinus, greatly increased during the years when the trout population declined. It appears likely that the rapid decline of these populations of stream‐spawning lake trout was caused by the increase of sea lampreys in Lake Superior.