Utility of splenic macrophage aggregates as an indicator of fish exposure to degraded environments

The utility of splenic macrophage aggregates (MAs) as an indicator of fish exposure to degraded environments was evaluated in several species of estua rine fishes as part of the Environmental Protection Agency's Environmental Monitoring and Assessment Program-Estuaries (EMAP-E). Using image analysis, we measured the number and mean size of MAs per square millimeter on tissu e sections of spleen from 983 fishes representing seven species from 266 st ations scattered across coastal estuaries of the Gulf of Mexico. At 16 stat ions, at least one fish exhibited a high density of MAs (> 40 MAs/mm(2)). D ensities of MAs that exceeded 40/mm(2) correlated with exposure to either h ypoxic conditions or sediment contamination. Fisher's exact test showed tha t the observed frequencies of joint occurrence between high numbers of MAs and both high sediment contaminants and low dissolved oxygen were significa ntly greater than the expected background frequencies. For all 16 sites whe re MAs were greater than 40/mm(2), sediments displayed at least one contami nant at a concentration in the highest 5% of those observed for all Gulf of Mexico stations. Additionally, comparison of subjective visual analyses wi th the image analysis measurements showed a strong correlation, indicating that similar analyses can be performed without computer image analysis. Thi s study demonstrates that splenic MAs are effective biotic indicators for d iscriminating between fish exposed to degraded and nondegraded environments .