Intestinal Blood Flow in Swimming Chinook Salmon Oncorhynchus Tshawytscha and the Effects of Haematocrit On Blood Flow Distribution

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Abstract
Blood flow in the intestinal artery (qia), the rate of oxygen consumption (V(dot)O2) and a number of haematological variables were measured in chinook salmon, Oncorhynchus tshawytscha, while they swam up to the critical swimming velocity (Ucrit). The fish used in this study had previously been exposed to one of two different exercise-training regimes, swimming for 8 months at either 1.5 bl s-1 (HS) or 0.5 bl s-1 (LS) (where bl is body length). During this period, growth rate was the same in both groups. At rest, qia was approximately 36 % of cardiac output. qia was inversely related to V(dot)O2, indicating that blood flow was gradually redistributed from the viscera as the oxygen demands of the locomotory muscles increased. Both V(dot)O2 and qia were relatively constant at swimming velocities less than 50 % Ucrit, but at Ucrit, qia had decreased by 60–70 % as V(dot)O2 reached a maximum. Blood flow redistribution away from the intestine contributed significantly to the oxygen supply for locomotory muscles, since it was estimated that the oxygen-transporting capacity of this redistributed blood flow was enough to support 12–18 % of the maximum internal oxygen consumption (total V(dot)O2 - gill V(dot)O2). Following exercise training, haematocrit (Hct) in the HS group (27.1 %) was significantly higher than in the LS group (23.3 %). However, neither the maximum V(dot)O2 nor Ucrit was significantly different in the two groups. qia was inversely related to Hct but, in spite of lower qia at rest, oxygen transport to the intestines was greater at all swimming speeds in the HS than in the LS training group. In addition, blood flow in the HS group was better maintained as the swimming speed was increased. As a result of the higher Hct in the HS-trained group, oxygen transport to the intestines was similar in both groups at their respective training velocities. Therefore, we suggest that, by increasing Hct and thereby maintaining oxygen delivery to the intestines, the HS group maintained normal intestinal function while swimming at the higher velocity, enabling overall growth rate to be the same as in the LS group.