All facts when ordered and synthesized with previously published information suggest that light is the principal abiotic controlling variable of the environment that determines temporal and spatial dimensions of feeding and reproduction in the walleye, Stizostedion vitreum vitreum. Underwater observations during the day revealed that adult walleyes were active in turbid regions of a lake but rested in contact with the substrate in clear-water regions. In the latter instance physical shelter served to shield the eyes of the walleyes from the ambient light. An inverse relation was noted between numbers of walleyes sighted per hour of transect swum and transparency levels of the water. In experimental angling during the open-water season, the total number of walleyes captured and the catch per unit of effort were positively related to illuminance levels. Winter angling through the ice showed a similar relationship, though at percentage light transmission levels one order of magnitude lower. Percentages of walleyes captured per [Formula: see text] increment during openwater at dusk were positively related to percentage decrease in illuminance for the same time intervals and inversely related to absolute levels of illuminance. Other authors have determined that nuptial activity also, and hence reproduction, is governed by variations in the ambient light regime. Key words: Stizostedion vitreum vitreum, walleye, light, behavior, photopic vision, tapetum lucidum, turbidity, preferred habitat, scotopic vision, feeding pattern
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.
A review of the literature indicated contrasts in retinal structure and function between perches (Perca spp.) and pikeperches (Stizostedion spp.). Foremost among these were differences in size and distribution of rods, size and shape of cones, extent of retinomotor responses, possession or lack of reflecting material, and relative concentrations of melanin. The perches are primarily active by daylight, whereas the pikeperches are most active during twilight or at night. The coincidental occupancy of shoal areas by yellow perch (Perca flavescens) and walleye (Stizostedion vitreum vitreum) at twilight is related to decreasing visual acuity of the former and the approach of optimum visual performance in the latter species. On this basis the two species have a classic predator–prey relationship. Key words: histology, light, Perca sp., photopic vision, predation, retina, Stizostedion vitreum vitreum, symbiosis, tapetum lucidum
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.
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