
True, although with the amount of solanine in a tin of 125g of sardines once or twice a week, I doubt if it would cause much of a problem, and there are, as you've pointed out, other tinned options, including spring water, where the useful fish oils can be obtained.
As with any food it's a case of everything in moderation, and it also has it's benefits, the red pigment lycopene is an anti-oxidant which is thought to neutralise the effects of free radicals, twice as effectively as betacarotene; studies have also shown that it can substantially cut the risk of developing some cancers in humans. The advice to avoid solanine in plants of the nightshade family, is to avoid the green parts of potatoes, green tomato pulp or skin, and use red capsicum instead of green. So in this instance, being green isn't all that good!!
My bitch Indie has arthritis in one of her knee joints and her elbows, but I still give a tin or two per week of fish in tomato juice, for me, the benefits from the oil and lycopene make it worth while.