MUSCLE ORNITHINE DECARBOXYLASE ACTIVITY AS AN INDICATION OF RECENT GROWTH IN PRE-SMOLT ATLANTIC SALMON, SALMO-SALAR

Omithine decarboxylase (ODC) is a key regulatory enzyme in the growth process of higher vertebrates. Epaxial muscle ODC activity was correlated to specific growth rate for weight (r2=0.70) in 55 juvenile Atlantic salmon subjected to four different photoperiods during their first autumn of growth, the period of time when hatchery-reared Atlantic salmon typically develop a bimodal length-frequency distribution. The correlation was strongest for fish which developed such a bimodality, which was the case for those exposed to a natural photoperiod or to a 12 h light: 12 h dark (LD 12:12) photoperiod (r2=0.77 in both cases). Larger (upper mode) fish had significantly higher muscle ODC activity than smaller (lower mode) fish (P<0.001 and P<0.05, respectively, for the two photoperiods), by greater than an order of magnitude. Weaker correlations were found for fish exposed to LD 16:8 and LD 8:16 photoperiods (r2=0.53 and 0.47, respectively). In these cases, clear bimodality was not observed for fork length, and larger fish did not have significantly higher muscle ODC activity than smaller fish. These results suggest that epaxial muscle ODC activity is a good indication of short-term changes in growth rate in fish, such as are seen during the recruitment of fish into the upper mode of the bimodal length-frequency distribution typically observed in juvenile hatchery-reared Atlantic salmon.