Cold-acclimated fish also recovered more slowly from anaemia. In addition, warm-acclimated fish maintained compact myocardium between 32% and 37% during anaemia, while cold-acclimated fish responded with an increase in compact myocardium (from 29% to 37%). Routine cardiac output (Q) was continuously monitored following a single PHZ injection to examine the initial cardiac response to anaemia. Contrary to expectations, acute anaemia did not produce an immediate, proportionate increase in routine Q. In fact, Q did not increase significantly until Hct had decreased to 10%, suggesting that rainbow trout may initially rely on venous oxygen stores to compensate for a reduced arterial oxygencarrying capacity. Thus, we conclude that myocardial oxygenation, acclimation temperature and cardiac work load could all influence anaemia-induced cardiac remodelling in rainbow trout.

The 34% increase in relative ventricular mass (Mrv) resulting from chronic anaemia (induced by an intraperitoneal injection of phenylhydrazine hydrochloride) was accompanied by a 117% increase in coronary vascular volume of diploid rainbow trout Oncorhynchus mykiss. Coronary vascular volume of normocythemic triploid fish was similar to that of normocythemic diploid fish despite a larger Mrv. These observations, in combination with previous studies, suggest that the vascularity of compact myocardium in O. mykiss can vary independently of Mrv.