E¡ects of size grading and stocking density on growth and survival of 3.5-month-old signal cray¢sh (Pacifastacus leniusculus Dana) juveniles were investigated in a 3 Â 2 factorial design with size group (small, large and mixed) and stocking density (25 and 200 individuals m À 2 ) as factors for 90 days. Carapace length and wet weight growth of the juveniles were a¡ected by the presence or absence of other size groups, stocking density and animal size. All factors had statistically signi¢cant interactions, which make the interpretation of the results more complicated. The smaller cray¢sh grew faster in the absence of large animals and larger animals bene¢ted from the presence of smaller cospecies. Increasing density decreased the growth in length and weight. Survival (82^100%) was good in all groups. Subsequently, the within population dynamics was studied by selectively removing the largest individuals (selectively removing (SR) group). E¡ects on growth and survival of the remaining juveniles were evaluated on 6-monthold P. leniusculus juveniles reared in plastic tanks (100 juveniles m À 2 ) for 122 days. Size distribution manipulation was performed at days 34 and 70. Juveniles in SR group grew signi¢cantly faster in comparison with a control group (RR), from which juveniles were removed randomly. Also, the survival rate was signi¢cantly higher in the SR group than in the RR group. In conclusion, size grading of juvenile P. leniusculus either at the start of the experiment or repeatedly during the experiment increased growth performance of smaller animals in length and weight.

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The inert marker method for individual food intake measurement of signal crayfish (Pacifastacus leniusculus Dana) juveniles (carapace length (CL) ranged from 16.5 to 25.6 mm and wet weight (WW) from 0.88 to 4.47 g) was evaluated in a series of experiments. The measurement technique was tested with two types of diet: a gelatin‐based semi‐moist diet and an agar‐based dry diet. Diets were labelled with X‐ray dense ballotini glass beads. After feeding, juveniles were X‐rayed, and the amount of ingested glass beads in the digestive track was counted from the X‐ray images. The method proved reliable only with the gelatin diet. In the ingested gelatin food, 92.80±8.55% of the glass beads originally in the food were found in comparison with 40.99±24.72% for the agar diet. Subsequently, the growth efficiency of the gelatin diet was compared with typically used raw fish diet (roach, Rutilus rutilus L.), and possible harmful effects of X‐raying on the growth and survival of the juveniles were tested on 2‐year‐old crayfish (CL 22.90±1.66 mm and WW 3.15±0.71 g). Roach‐fed juveniles grew significantly faster than the gelatin‐fed counterparts. However, the growth performance and survival of the juveniles on gelatin diet was considered good enough to allow long‐term growth experiments with individual feed intake monitoring, e.g. for studies on social behaviour of the crayfish. The results of this study confirmed that the X‐radiography technique together with a semi‐moist gelatin‐based diet makes it possible to measure individual food consumption of freshwater crayfish juveniles reared communally.

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