I recently read that cat's (and I'll bet all other carnivores') bodies don't
contain the enzyme needed to transform flax seed oil's alpha-linolenic
omega-3 fat to the useful forms of omega-3 : DHA and EPA.

If you wish to provide your cat with EPA/DHA, then feed it wild fish from
deep cold water. Sardines from nontropical oceans would be an OK source.
Canned mackerel would provide greater amounts of EPA/DHA than sardines, and
would cost less. Wild red salmon (pricey even when canned) provides still
greater amounts of EPA/DHA, but unless it is from Alaskan waters it will
also contain mercury.

Farm raised salmon is not such a great source of omega-3 fat as most
consumers have been led to believe.

"Vegetable" oil is actually soy oil. That cheap crap is unhealthful to
people and animals. Canola (rapeseed) oil is no better. Hexane or other
solvents are applied to help separate the oil from the rest of the grain,
leaving trace amounts of these toxins in the commercial product. It is a
paradigm that after hundreds of millions$ in marketing campaigns spanning 3
decades that both soy and canola are now generally perceived by the public
as health foods, when in fact these grains themselves are quite toxic and
inedible before industrial processing. Their processing almost always leaves
some original toxins and adds additional (residual) ones too.

Soy
http://www.shirleys-wellness-cafe.com/soy.htm
http://www.soyonlineservice.co.nz/files/Shadow.htm
http://www.mercola.com/2004/dec/4/soy_truth.htm
http://www.mercola.com/article/soy/avoid_soy.htm

Canola
http://www.westonaprice.org/know_your_fats/conola.html
http://www.luminet.net/~wenonah/new/canola.htm
http://www.breathing.com/articles/canola-oil.htm
http://www.sdadefend.com/canola.htm
http://www.rense.com/politics5/dare.htm