Abstract
Rainbow trout Oncorhynchus mykiss (Walbaum 1792) larvae that first started active feeding were separated into 3 replicate batches (1A, 1B, 1C) with 100 individuals each, while four days later, the fry that last started active feeding were also distributed in 3 replicate batches (2A, 2B, 2C) with 100 individuals. Four measurements were performed with an interval of four days between the first and second measurements, as well as between the third and fourth measurements, to compare the initial masses of individuals by groups (1ABC and 2ABC) on the same day from the beginning of active feeding. The average initial body weight of the fry per batch in each measurement was generally higher in the first group (1ABC) than in the second group (2ABC). However, when comparing the weight of both groups for the same number of days from the beginning of active feeding, it is noticed that the second group (2ABC) has a higher average individual weight than the first group (1ABC). There was no statistically significant difference (one-way ANOVA, p>0.05) in SGRw between the groups in the first three measurements, while there is a statistically significant difference (one-way ANOVA, p<0.05) in SGRw in the fourth measurement. Group 2ABC has a significantly higher SGRw (5.0064%day−1±0.05394) than group 1ABC (4.7711%day−1±0.01715). This leads to the conclusion that in the second group 2ABC, the backlog of 4 days in mass was compensated. It is also noted that mortality in the first group (1ABC) was only 1%, while in the second group (2ABC) it was 3.7%. Consequently, a higher density of the fry in the first group (1ABC) could have an impact on a lower growth rate.