Hello, MiddleChild has asked me to come on this thread as the two of us are forever tangoing on the issue on that "other forum".
First things first, I absolutely agree with you that a fish-only diet is not the best diet for a cat (but than again I don't think that a diet of any one protein source is a balanced diet or indeed a diet from just one brand) and that a diet that contains a high proportion of "raw" or human-grade "cooked" fish is an absolutely no-no for many of the reasons listed here (but there are also others). With high proportion I personally mean it being fed more often than once a week at the very most. Because it is so strongly flavoured, I also agree that tuna-based food in particular can create fussy eaters.
But there are a few reasons why I personally think there is some misinformation about commercially produced cat food that is fish-based. But please keep in mind that I can only talk about the foods that are available to us in the UK and Continental Europe
* The fish used in canned pet foods usually includes bones, and are high in phosphorus and magnesium, which can be an issue in cats with a history of urinary tract disorders or kidney disease. In practice I have seen quite a few cats develop urinary tract infections and blockages if they eat much fish–even boneless fish.
It is not just fish-based cat food that contains bones. Meat-based food can contain chicken carcasses and other bone sources. If they don't use them then they either use a mineral complex that contains calcium or they use calcium carbonate or eggshells.
Fish-based cat food doesn’t, as a rule, contain more calcium, more phosphorus or more magnesium than meat-based cat food – at least not in the foods that I have looked at. That holds true for both wet food as well as dry food (a few of us have compared over 150 dry foods available in the UK and the ones with the highest mineral load aren’t fish-based ones).
It may be worth keeping in mind that many of the foods that are marketed as fish-flavoured food are just that; fish-flavoured. The actual fish content within that food is quite low and typically the food is packed with other (cheaper) protein-sources.
Personally, I think there is more variation amongst food brands in the mineral load of the food then there is variation between fish and meat foods within the same brand. So, if you are worried about FLUTDs etc it is worth looking at the labels, converting the wet food analysis to a dry matter analysis and choosing the one that has the lowest but still sustainable mineral load.
In terms of dry food, the crude ash content can vary from 4% to a whopping 10.9% (incidentally, as I have said before, the foods with the highest mineral content are a chicken and venison food).
* Many cats are sensitive or even allergic to fish; it is one of the top 3 most common food allergens.
Now that is interesting. Is there a reference to this somewhere? On that other forum we often get cats with dietary sensitivities – and just going by the rather unrepresentative sample of forum users, this seems to be on the increase. However, when I looked for a breakdown of the most common allergens I couldn’t find one that is relevant to cats. So, I would be grateful if anyone could push such a list my way.
Incidentally, I don’t think we have had a cat with a fish-allergy yet but plenty with grain and chicken sensitivities