Effect of ranching time on melanomacrophage centres in anterior kidney and spleen of Southern bluefin tuna, Thunnus maccoyii

Melanomacrophage centres (MMCs) are common in the haemopoietic tissue of fish and are an important component of the humoral immune response. Increasingly they are being studied as a biomarker of environmental stress in both wild and aquaculture fish stocks. Computer image analysis of histological sections of the head kidney and spleen of ranched Southern bluefin tuna (SBT) was used to investigate changes in MMCs over various stages of the ranching cycle. Histochemical pigment differentiation indicated haemosiderin, lipofuscin and melanin were present in MMCs of both the kidney and spleen, with kidney MMCs being highly positive for lipofuscin and melanin and spleen having high levels of haemosiderin and a lower level of melanin than kidney. The MMC area in the kidney of wild sampled SBT ranged from 0.41 ± 0.13% to 2.95 ± 0.26% and that in the spleen ranged from 0.35 ± 0.09% to 2.75 ± 0.86%. The MMC area in the spleen of SBT increased over the latter stages of ranching and at harvest compared to SBT sampled in the wild and at the end of tow. There was no statistically significant difference in the MMC area in the kidney of ranched SBT sampled at various stages. SBT appear to have a higher MMC area in haemopoietic organs than numerous other fish species, perhaps providing a greater capacity to deal with 'wear and tear' and increased immune-competence.