Size and hematological impact of the splenic erythrocyte reservoir in rainbow trout,Oncorhynchus mykiss.

Fish were sampled individually, at rest, following air exposures of up to 8 min, during recovery from a 5 min air exposure or after a 5 minute chase. The spleen was photographedin vivo at rest and following 5 min air exposure in one fish. The effect of individual versus serial sampling from the same tank and of MS222 anaesthesia was also examined. Spleen hemoglobin content (SpHb), spleen somatic index (100 × spleen weight/body weight; SSI), blood hemoglobin concentration (Hb), and hematocrit (Ht), were measured. Mean cell hemoglobin concentration (MCHC), erythrocyte reservoir size, and relative contributions of reservoir release, erythrocyte swelling, and plasma water loss to hemoconcentration were calculated. The splenic reservoir contained 0.54 g Hb/kg body (21% of total body Hb), most of which it released between 1 and 3 minutes after the onset of air exposure. The spleen released more than 95% of the erythrocytes it contained at rest within 8 min. The release accounted for 31% of the 5.65 g/dl rise in Hb and 23% of the 26.6% observed increase in Ht after 8 minutes of air exposure. The balance of the increase was caused by erythrocyte swelling and fluid shifts reducing plasma volume. Animals exercised for 5 min showed changes similar to those in fish air exposed for 5 min. Recovery of all parameters was complete in 3 to 6 h, with the exception of MCHC which recovered in 30 min. Serial sampling produced a decrease in SpHb, and R1Wt and induced a significant hemoconcentration. MS222 did not cause erythrocyte release, but failed to prevent it after handling. Many previous reports of Ht and Hb in resting fish are probably high because they were taken under conditions that would cause the spleen to release its contents.