The behavioural and metabolic responses of the largemouth bass Micropterus salmoides (Lacépède) to temperature were determined to define optimal thermal conditions. The final preferendum of largemouth bass juveniles determined with acute and gravitation methods was independent of the method (29.0–28.1 °C). The displacement velocity in the horizontal thermal gradient of bass juveniles was 22.4 cm h⁻¹ in the light phase and 22.6 cm h⁻¹ in the dark phase. Oxygen consumption rates in the largemouth bass increased significantly (P<0.05) from 48.8 to 69.4 mg O₂ Kg⁻¹ h⁻¹ with an increase in the acclimation temperature from 20 to 33 °C. The temperature quotient (Q₁₀) in the juveniles was 1.37–2.00 in the range of acclimation temperatures of 26–29 and 29–32 °C. The optimum temperature range for growth calculated using Jobling's equation was 28.1–28.6 °C and for Q₁₀ values 26–29 °C. The results are discussed in relation to the use of this information in aquaculture.

This study assesses the effect of temperature and fluctuations in salinity on the nitrogen excretion and osmoregulation of Penaeus vannamei juveniles to determine the lowest stress combination so that these can be used to optimize production of the Mexican strain in culture. The ammonium excretion rate of juveniles acclimated to 20, 24, 28 and 32°C was measured. Fluctuating salinity levels were applied to these animals in a sequence of 40%o, 33, 25, 18, 11, 18, 25, 33 and back to 40%o. The results indicate that when the salinity was reduced from 40 to 11%> the ammonium excretion of the shrimp was reduced. The osmotic concentration of the animals was hyposmotic as the salinity decreased from 40 to 25%o, hyperosmotic during the 18–11–18 in %> interval and hyposmotic as the salinity increased from 25 to 40%> again. The range of isomotic points over this range of salinity was 712–777mmol Kg^‐1. The ammonium excretion of P. vannamei exposed to these experimental conditions can be attributed to the process of osmoregulation because excretion was increased when the shrimp were hyper‐regulating and reduced when they were hypo‐regulating. Based on our results, the animals experience the lowest stress in a temperature between 27 to 30°C and a salinity close to the isosmotic point between 25 ‐ 27%o. We propose that this should prove to be the optimal temperature and salinity regime for culturing the Mexican strain of P. vannamei.