Cardiac disorders in farmed adult brown trout, Salmo trutta L.

During summer in Brittany, France, sea farmed brown trout, Salmo trutta L., regularly experience a high mortality rate which is associated, at least i n part, with cardiac disorders (aneurysms and infarcts). The present study is preliminary to a more extensive research programme, the objective of whi ch is to determine to what extent the physiological performance of the card iovascular system of brown trout is affected by the environmental condition s the fish experience in farm cages. We conducted a 2-week in situ experime nt during which the heart rate of eight sea water acclimatized individuals was telemetered using acoustic tags. During the experimental period, water temperature ranged from 16.0 to 17.6 degreesC. Water oxygen saturation was above 80% at all times and salinity was very high (35.5 parts per thousand) but stable. Although they were unfed and not active, seven of out the eigh t tagged animals displayed near maximum heart beat frequencies, which range d between 83 and 98 beats per minute (bpm). On the other hand, the eighth a nimal exhibited medium-range heart rates (50-70 bpm). Using phase delay map s, we established that the maximum heart rate of brown trout at 17 degreesC was in the range of 96-100 bpm. This result suggests that in our experimen tal conditions, the heart rate of most of our inactive fish was between 85 and 100% of maximum myocardial performance. We hypothesize that the cardiac failures observed in brown trout during summer are most likely a result of strenuous workloads imposed on the cardiovascular system by a combination of elevated temperature, high salinity and possibly season-related decrease d hypo-osmoregulatory abilities.