Recombinant bovine somatotropin more than doubles the growth rate of coho salmon ( Oncorhynchus kisutch) acclimated to seawater and ambient winter conditions

Sub-adult coho salmon ( Oncorhynchus kisutch) were tested in a culture situation to assess whether somatotropin therapy would improve growth performance during their first “sea-winter”. Untreated fish and sham-treated fish did not differ significantly in their growth performance. Over the 12-week course of the experiment, the mean increase in fork length for these groups was 9.5% (±6.0 SD) while the mean increase in weight was 29.6% (±24.1 SD). Relative to these control groups, the administration of recombinant bovine somatotropin by injection, cholesterol pellet or mini-osmotic pump resulted in substantially better growth. Mean increases in fork length ranged from 16.5% (±5.5 SD) to 23.0% (±8.7 SD) while mean increases in weight ranged from 52.2% (±31.2 SD) to 65.5% (±44.0 SD). Specific growth rate was elevated 2–4 weeks after the initiation of somatotropin therapy and the effect did not diminish over the 8-week treatment period. Subsequent to hormone withdrawal, the growth rates of somatotropin-treated fish were no longer distinguishble from those of the controls. Somatotropin was noted to decrease the rate of growth depensation, yielding a more uniform group of salmon with respect to size. Condition factor was decreased by the hormone treatment. It was concluded that somatotropin treatment can produce a significant increase in the growth rate of coho salmon, both after they have made the transition to salt water and during the winter months, when environmental conditions are sub-optimal for growth.