I've had a quick look at rapeseed mainly from a nutritional point of view, to try to determine the necessary preparation for fish feed/bait at a minimum.
I would conclude; edible, to some degree (with preparation) but not as good as first thought.
Unfortunately, though not surprising, there are lots of different varieties abound, and I found very little information of British varieties of rapeseed which is what were likely to encounter in the UK. That said; I would conclude that there are indeed some anti-nutrients present (all varieties and strains) which can be detrimental to the health of monogastric animals without proper preparation. I found a little but not much information on fish fed on rapeseed, most of the studies done are for ruminants.
Most notable are the anti-nutrients present which form complexes with proteins making them unavailable to the animal.
The specifics might bore the **** out of some of you but here's a few from some of the studies I've read, simplified for the sake of clarity.
Antinutritional factors such as glucosinolates, phenolics, phytates and hulls.
The content of phenolics in rapeseed flour is nearly 30 times higher than that of soybean. (if you consider that soya flour used in a 50:50 mix is bad?)
Phenolic compounds contribute to the dark colour, bitter taste and astringency of rapeseed meals. They may also interact with amino acids, enzymes and other food components, thus influencing the nutritional value of rapeseed meal and its products.
Therefore, phenolic compounds are important factors when considering rapeseed meal as a protein source in food formulations. Available literature data on phenolic compounds and tannins of rapeseed/canola are fragmentary and diverse. Developing a standardized method for analysis and quantitation of these compounds is warranted. Interaction of rapeseed phenolics/tannins with proteins and their effects on enzymes and other food components remain to be studied.
Enzymatic and nonenzymatic oxidation of phenolics may readily react with the e-NH2 group of lysine and CH3S group of methionine of enzymes and other proteins to form complexes, thus rendering them nutritionally unavailable. In other words, Methionine and Lysine being essential amino acids means that they will not therefore be assimilated and digested - fish cannot synthesise essential amino acids for themselves. *If fish require these particular nutrients, they are unlikely to feed solely on rapeseed for long periods.
Amino acid - responsible for smell/scent and taste.
Essential amino acid - one the animal can only acquire through their diet.
(Fish have a requirement for 10 different essential amino acids)
These are the 10 essential amino acids required by fish: Arginine, Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, and Valine. As an aside; I can tell you there is not one single food on the planet that contains all of them.
Tanins
Reduction of food intake and growth of experimental animals (rats and chicks), as well as a decrease in protein digestibility have been associated with the ingestion of polyphenols. Research also shows increased fecal calcium losses by rats fed with 3% tannic acid.
Tannin content (wherever encountered in food) as known, is highly correlated with a decrease in weight gain, decreased feed intake, decreased retention of protein and increased fat retention. It is thought this may be due to the binding of tannins to digestive enzymes and/or to dietary proteins. Condensed tannins may also bind to methionine.
Tannins may precipiate a wide range of essential minerals, thus lowering their bioavailability.
It was found that heating rapeseed (meal) for 0.5 h at 95°C substantially reduced the ability of starches to complex with either tannic acid or catechi (whatever that is?).
In other words soak (for some specified time) and boil the whole seed for at least 1/2 hour to lessen the effects of tanins on proteins.
Some studies also suggest Phenolic compounds have been identified as possible inhibitors of iron absorption.
Tannic acid has been found to precipitate vitamin B12 making it unavailable and contributing to anaemia.
"Phenolic compounds not only affect the taste of rapeseed protein products, but also lower their nutritional value However, the available information on the undesirable properties of rapeseed/canola phenolic compounds is still fragmentary. Therefore more detailed studies on the nutritional implications caused by interaction with phenolic compounds of rapeseed food components and their sensory effects are required. The variability of the reported results on tannin content in rapeseed/canola is caused by the existing differences in the solvent-extraction systems employed for their recovery and quantitation methods used.
The specificity of tannin-protein interactions has been well documented for a number of plant tannins. However, little is known about the tannin-protein interaction in rapeseed/canola."
Rapeseed meal has a reasonably well-balanced amino acid content and a favourable protein efficiency ratio.
However, use of rapeseed meal as a source of food-grade protein (for humans at least) is still thwarted by the presence of phytic acid and phenolic compounds.