972. Effects of eutrophication on salmonid communities in oligotrophic lakes. J' Fish' Res. Bd. Canada 29:975-983.Oligotrophic lakes respond to progressive eutrophication by a sequence of predictable events. Increased nutrient loads and subsequent increased plant production result in alterations in the abiotic environment, including changes in the color and transparency of the water, increased turbidity, oxygen depletion in the hypolimnion, and increased chemical stratification. The physico-chemical changes precipitate biotic changes among the phytoplankton, littoral algae, zooplankton, and benthos. The salmonid community may respond initially with an increased body growth rate in various taxa and a higher incidence of parasitism, but later inhibition of natural reproduction occurs, and finally, the taxa are replaced by others that can survive in the changed environment.A relation between natural nutrient loading (expressed in terms of a morphoedaphic index) and yield (both quantitative and qualitative) is proposed as an aid to determining the natural successional status of a lake. Knowing the natural baseline of a particular lake the fisheries managers can judge the nature and size of responses due to cultural nutrient loading and then alter the rate of cultural nutrient loading to modify the ecological effects, or they can use biological engineering to capitalize on the present conditions'Among the most important effects of eutrophication is the increased vulnerability of sedentary discrete stocks to changes in other stresses such as fishing. Corsv, P. J., G. R. SraNcr,nn, D. A. Hunuv, .qNo A. M. McCoMers. 1972. Effects of eutrophication on salmonid communities in oligotrophic lakes. J. Fish. Res. Bd. Canada 29:975-983.
The sequence of fish species shifts since the 1930s in the Bay of Quinte is interpreted in terms of climatic changes and the influences of man. Among the latter, eutrophication is judged most important. The marked decrease in large piscivores in both eastern Lake Ontario and the Bay of Quinte as a consequence of these changes resulted in instability, exemplified by the explosions in populations of alewife (Alosa pseudoharengus) and white perch (Morone americana). Nutrients are imported to the bay, from Lake Ontario by way of alewife. This source has increased in the 1970s with the further decline of piscivores, and it brought about a greater fish production in the bay. We conclude that the Bay of Quinte and Lake Ontario interact significantly and should be studied as a single system.Because of its specialized feeding habits, the post-World War II rise of the walleye (Stizostedion vitreum vitreum) was related to events outside the Bay of Quinte, but its later decline was attributable to direct and indirect effects of eutrophication. Yellow perch (Perca flavescens), by contrast, were apparently not affected adversely by severe habitat changes induced by cultural eutrophication. Key words: Percidae, community ecology, species shifts, eutrophication, habitat changes
The effects of varying net length and set time on fish catches in gill nets were studied at two sites in the Bay of Quinte, Lake Ontario. A latin square design was used (four locations per site over four nights); either net length was varied, from 25 to 200 yd over 12 h, or set time was varied, from 1.5 to 12 h with 50 yd of net. Treatment, location, and date effects on catches from each site were analyzed by a multiple regression model. Net length effects on catch per unit of effort (CPUE) were negligible for total and some fish species catches; CPUE for walleyes Stizostedion vitreum decreased with increasing net length. Set-time effects on total CPUE were significant; the CPUE of white perch Morone americana increased with set time, but CPUEs for yellow perch Perca flavescens, alewives Alosa pseudoharengus, and walleyes decreased. The results were attributable to specific differences in activity patterns and net-saturation effects. We concluded that use of gill-net catches as indices offish abundance must be validated species by species.
A study of the external membranes of the developing oocyte of the brook trout, Salvelinus fontinalis, was made by means of electron microscopy. The membrane, zona radiata, which becomes "hardened" in mature eggs after they are shed into water, was observed to begin development at the bases of microvilli which project from the surface of the oocyte. This membrane grows until in the mature egg it is about 50 μ thick. The zona radiata is completely permeated by numerous pore canals. In immature oocytes, the pore canals contain microvilli which arise from the surface of the oocyte. The microvilli make contact with the follicular cells surrounding the developing oocyte. Morphological changes which occur in the zona radiata and the other layers of developing oocytes are described at several stages of development. The relationship between morphological changes in the membranes and the transport of nutrients to the developing oocyte is discussed.The nomenclature of the membranes of the mature trout egg is discussed in relation to the findings of the present study. It is concluded that primary membranes and perhaps secondary membranes are present in the mature trout egg.
The life history of eels in Lake Ontario was studied by measuring, tagging, and releasing about 1500 individuals between 1958 and 1966. The smallest eels captured were from 295 to 318 mm long. Presumably they entered the lake at about age III–IV at lengths of 200–300 mm. Otoliths of eels from the lake showed a range in age between IV and XVIII, while those of smaller eels from the Ottawa River ranged between age II and IX. The length–weight regressions showed two distinct growth phases: one from about 480 mm to 790 mm, the other from about 830 mm to 1080 mm. Growth of the orbital diameter, interorbital distance, and pectoral fin length in relation to total length was not obviously related to sexual maturity. Eels that emigrated from the lake had a greater weight–length ratio than those that remained in the lake for another year. Of 917 tagged eels transplanted within Lake Ontario, 28 were recaptured near and 18 recaptured far from the original capture site. Although the results are inconclusive, there is a suggestion that eels have home territories within the lake.
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