Experiments to determine the growth rate of eels (Anguilla anguilla L.) at different temperatures are described and show the optimum temperature for growth to be 22-23' C. The ultimate upper lethal temperature was found to be 38' C and the critical thermal maximum varied from 33 to 39' C for fish acclimated at 14 to 29' C. An attempt was also made to determine lower lethal temperatures. Eels enter a state of torpor at temperatures varying from 3' C for fish acclimated at 29' C to less than 1' C for fish acclimated at 23' C or below. The results have been used to estimate the growth rates expected from eels cultured in power station cooling water using different types of temperature control.
Fish survival, growth and productivity are dependent on both biological and environmental factors. The latter can be distinguished as edaphic (which includes water quality) and morphometric (which includes lake and stream morphology). Climatic factors are relatively less significant in temperate waters, except over a long time scale, or over wide geographic areas. In the conditions characteristic of acid lakeslow pH, low calcium and high aluminium concentrations-survival may be reduced, growth may be affected and, consequently, productivity will be low. In spite of inconsistencies in the literature, it can be shown that at low pH ( _N 4.9, a limiting concentration of calcium is about 1 mg litre-1 and aluminium can have adverse effects at 250 pg litre-1 within a limited range of pH. These three factors appear to interact. Taking age specific fecundity of adults and survival of eggs and fry, it is possible to predict the limiting conditions for maintenance, growth or decline of a fish population. At pH 4.5 and calcium < 0.8 mg litre-1 it may be expected that many lakes will be fishless.
Yearling brown trout, Salmo trutla L., were exposed to various concentrations of inorganic aluminium ((r3.7 p~ I -')over a pH range of4.34.5 in a flow-through bioassay apparatus using synthetic test media. Low pH, in the absence of aluminium, produced little effect on growth or survival except at the lowest pH tested (4.3). At pH less than 5.5, concentrations of total aluminium in excess of I p~ I-(27 pg 1 -') were found to retard growth. The effects of a given aluminium concentration were markedly reduced at pH above 5.5.The change in aluminium toxicity with pH must be related to changes in aluminium chemistry. When growth rates are correlated with the different aluminium species, calculated using thermodynamic equilibrium constants given in the literature, it appears that the AI(OH)2 + species is the most toxic, with a small contribution also coming from polymeric complexes.
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