Aspects of the biology of Pontoporeia hoyi Smith in Lake Winnipeg and a comparison of hexagenia limbata (Serville) and P. hoyi production

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Flannagan, John F.,
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During the open water season of 1969, three Birge-Ekman or Ponar grab samples were taken six times at up to fifty-five stations in Lake Winnipeg. Eighteen thousand one hundred and nine specimens of Pontoporeia hoyi Smith and one thousand and forty of Hexagenia limbata (Servilla) were identified from these samples. P. hoyi was collected in the greatest densities in the Narrows, was common in the North Basin and has almost disappeared from the South Basin. Multi-linear regression analyses of the density distribution of P. hoyi indicated significant negative correlations with depth, temperature and water transparency and a significant positive correlation with percent. sand on a whole lake basis. These four factors together explained 69.5% of the variation in the density data... Unlike populations in other shallow, warm, unstratified water bodies, the Lake Winnipeg population had a 2+ yr life cycle. P. hoyi were found to grow only in fall, winter and spring in the North Basin and only in spring and fall in the Narrows. It is suggested that this cessation of growth in both basins in the summer, and in the Narrows in the winter, prevents this population from reaching maturity in one year... Annual whole lake production, using the instantaneous growth method, was estimated at 67,015 tonnes of P. hoyi and 66,972 tonnes of H. limbata. These two species, which utilise similar food resources and which between them represent over 36%, by number, of the macrobenthos of the lake apparently coexist by dividing the resources of the lake temporally and physically: P. hoyi producing most of its biomass in the colder parts of the year in the North Basin and most of the Narrows; H. limbata growing only in summer and distributed throughout the South Basin, in the shallower parts of the Narrows, and in a very restricted area of the North Basin.