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1. The origin of vertical temperature stratification in Esthwaite Water (Lake District. Cumbria. England) was obser\'ed.2. At the first stage of the spring warming of the whole water column some discontinuity in the vertical temperature distribution appears in the surface and bottom layers.3. Persistent summer stratification occurred for the first time al the lee end of the lake immediately after a change of wind direction, when the cumulative potential energy defieit dominated the cumulative kinetic energy by about IxU)"'*Jcm"4 . The results show the importance of the proper choice of initial conditions in compulations of total energy, as well as the necessity to consider the spatial distribution of potential and mechanical energy responsible for the summer stratification in the lake.

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The potential growth and yield of 1‐ and 2‐year‐old carp, Cyprinus carpio L., in climatic conditions similar to those of la Dombes were determined by a stochastic‐deterministic model, derived from data on growth of Polish carp cultured under intensive methods. The model's forcing functions are the water temperature, effective for carp body mass growth, recorded in ponds of la Dombes, the final fish density, the protein content in feeds and the initial body mass (for 2‐year‐old fish). It was assumed that the pond ecosystem does not limit fish growth and that the pond trophic chain works correctly. Quantification of relations between basic elements of the carp culture techniques enables prediction of the required final output from ponds, permitting the most efficient culture methods to be chosen.