Synergistic interactions between environmental disease conditions of salmonids

With respect to the culture of salmonids, environmental diseases per se, and the specific role of environmental factors in the genesis of infectious disease conditions, are often trivialized and in many cases have not been explored beyond a perfunctory level. The present trend toward use of recirculation technology will usher in an era of fish health management which will - out of necessity - approach environmental diseases and environmental factors in disease propagation, as highly significant primary areas of research and awareness. This paper will focus on one particular scenario, specifically the synergistic interplay between two common water quality problems - nitrite and gas supersaturation - and their combined role in the genesis of gas bubble disease (GBD). The principal pathobiological effect of nitrite, an intermediate product produced during biofiltration, is through the transformation of haemoglobin into methaemoglobin. Nitrite is also a powerful vasodilator and inhibitor of the immune system. Gas bubble disease (GBD) which stems from elevated total gas pressure (TGP), develops most commonly in vascular beds and rete where blood pressure is diminished. In the presence of nitrite-induced vasodilation, GBD can develop at levels of TGP which are typically considered "safe"..