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Four experimental diets with different inclusion levels of plant proteins and fish protein hydrolysates were compared with a commercial diet for meagre (Argyrosomus regius) ongrowing at optimal and suboptimal water temperature. Results in terms of growth in length and weight, conversion efficiency, dietary feed intake and utilization, body composition (whole fish and liver) as well as enzyme and immunological activities are presented. Fish growth was significantly reduced by the inclusion of plant proteins, although further addition of fish protein hydrolysates improved the results. Daily feed intake was not affected by plant protein inclusion in the diets, although the group fed the highest inclusion level showed lower ingestion than the rest of the groups, probably as a consequence of a reduced dietary palatability. The decrease in water temperature during the second part of the experiment had a negative effect on feed intake and fish growth. Gross visceral morphology of meagre fed the experimental diets was not affected, but muscle weight was significantly reduced. Whole body and liver composition was not affected with plant protein inclusion. However, the inclusion of fish protein hydrolysates resulted in a significant increase in fat content, especially in liver cholesterol and steryl esters, with a parallel reduction in protein. Brush border enzymes were affected by plant protein inclusion as well as serum lysozyme that significantly increased in the fish fed the highest inclusion level. As a conclusion, up to 315 g kg−1 plant protein (76.2% of total protein content) can be included in the diet for meagre without affecting growth or feed utilization. Higher inclusion levels can also be used if at least 5% fish protein hydrolysate is also included.

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