Just thought I would copy and paste this info as well! While we are on the subject of tuna/fish...

Raw Fish

Feeding raw fish regularly can result in thiamine (vitamin B1) deficiency due to the enzyme thiaminase in the fish. It results in loss of appetite, seizures and (in severe cases) death. Although an occasional treat of raw fish is generally not harmful, over-consumption will cause problems. Cooking the fish will destroy the enzyme.

Raw salmon in parts of North America may carry flukes and Neorickettsia helminthoeca. This can cause salmon poisoning disease in pets. It has been reported in dogs rather than cats, but results in lethary, high temperature, vomiting and bloody diarrhoea and may mimic infectious enteritis. The mortality rate is around 90% if untreated. It is preventable by cooking the salmon.

Some fish also contains environmental pollutants e.g. PCBs or mercury; these are not removed by cooking, but their levels may be higher in raw fish that have not been gutted (the cat may eat the fish organs as well as the flesh). Mercury poisoning has been reported in cats in a coastal region of Japan where fish was a main ingredient of the diet. If fed in excess, oily fish such as tuna can lead to steatitis (Yellow Fat Disease, pansteatitis).

Many feral cats near fishing ports supplement their diets by scavenging raw fish. As long as the raw fish was not eaten in excess and contained no harmful contaminants the cats did not appear to suffer ill-effects.