Piscine macrophage aggregate parameters as health monitors: effect of age, sex, relative weight, season and site quality in largemouth bass (Micropterus salmoides)

The utility of macrophage aggregate (MA) parameters as indicators of fish health and/or environmental stress was tested in largemouth bass (Micropterus salmoides) collected from Par Pond, a cooling reservoir for the Savannah River nuclear power plant. Initially, the effects of sex, age and season were evaluated using relative weight (W_r) in both healthy (W_r > 80) and stressed (W_r < 80) bass. There was no sex effect. A number of MA parameters, primarily hepatic and splenic no./mm² increased linearly with age in the healthy fish. These correlations were not apparent in the stressed bass. A comparison of stressed and healthy fish of the same age showed that stressed bass had significantly more aggregates in liver and spleen as well as higher relative amounts of hepatic iron. The only seasonal difference noted in healthy bass was that fish collected in the spring had significantly less relative iron than those collected in the fall. We then tested the sensitivity of these parameters by comparing fish of similar age and W_r from two sites - the Hot Dam area which is thermally impacted and the Cold Dam arca, an unimpacted site. Fish collected from the impacted area had significantly more aggregates in the liver than those collected from the Cold Dam area. Thus, we believe these parameters to be very sensitive indicators of environmental stress.