After reading up for like 5 hours in our university scientific articles online database, my brain is in a world of hurt.
Basically,
This is what I have learned. The short story from well over 100 pages of text from like 20 or so articles (forgive me if I don't reference them... I pieced these thoughts together from a lot of fragmented studies...).
As far as the comparison of commercial fish feed to live forage.
There is evidence that there is a difference between commercial feeds and live forage (theoretical, unmeasured).
It has to do with the nutrients that make up those food sources.
Protein levels, for example, have been correlated with growth hormone levels. And other micro-nutrients and amino acids have had an elevating effect on hormones such as T4, and the deionization to T3.
I know I am being vague as far as specifics of which hormones specifically, and in what species of fish. Right now, that isn't important (and is extremely boring).
What is important is that studies are showing that levels of specific nutrient(s) components (such as protein, carbohydrates, lipids), and their ratios, may have an effect on circulating hormone levels (Growth Hormone Specifically).
So what does that mean?
This tells us that fish nutritionists could potentially formulate diets that will have an effect on the endocrine system, and increase circulating hormone levels that are directly correlated with growth.
Instead of looking at what feeds convert better, we could be looking at what feed compositions actually increase feed conversion on the physiological level. This goes beyond digestibility as it pertains to particle size of ingesta, surface area, etc. This is analogous to increasing growth via exogenous (injected) hormones like we do with cattle, except the hormones are endogenous (naturally occurring) through nutritional manipulations.
There are also a lot of confounding factors (problems) that need to be addressed.
There are boatloads of environmental factors that have been shown to affect circulating hormone levels, growth, and feed conversion in fish.
The first is the time in which the fish are fed, and the frequency.
In one study looking at trout in particular, those fish fed at night had higher growth hormone levels than those that were fed during the day at a frequency of one feeding every 24 hours.
In other species, it was totally backwards, and in others there was no difference at all. This makes this information not so useful at this point in time if we are talking about managing the growth of a variety of species.
The only important tid-bit that came out of this portion of the reading was that fish that are fed more than once a day had a higher circulating level of Growth Hormone (pretty much observed in most studied species). Basically, to maximize growth feeding more than once a day is ideal (theoretically... Real world results are what count!)
Whether or not those gains are really worth the extra hassle, I couldn't really say. I have no experience in the fish management arena, and the studies just stated that there were statistically significant differences, rather than stating weight gains (It was a review article talking about other primary research articles).
Another confounding factor is that for every hormone like Growth Hormone, there needs to be an active hormone receptor, like "Growth Hormone Receptor" (for our purposes).
Even if there is a change in endocrine function and level of Growth Hormone and other Insulin Growth Factors (Other junk that contributes to actual growth), that doesn't mean that it will be put to use.
Think of it this way. Lets say I have 5 basketballs. Lets also say that I have 3 hoops.
In this example...
Basketballs = Growth Hormone Circulating In The Fish.
Hoops = Growth Hormone Receptors (Where the points are scored, and actual change, or growth, is initiated).
If you shoot all 5 basketballs at the exact same time, then you will only score 3 baskets. You had an excess of basketballs (growth hormone), and not enough hoops (growth hormone receptor sites).
In this example, the fish's body doesn't react to having a high level of growth hormone. It ultimately only really matters how many receptors there are. That is the limiting factor.
Basically this means that even if nutrition can change circulating growth hormone levels in a fish, it may not even cause them to grow. Or maybe it will... Who knows... (Remember, hormones are not given to cattle because they don't work... They seem to have more than enough hormone receptors to make use of the excess hormone and grow).
There is a lot of other stuff, but my brain started hurting so I stopped.
So, if you want skip all that junk, here is the conclusion I have come to..
Commercial fish feeds have the potentially to overtake live fish and other forages as far as measurable growth is concerned. The keyword is potential. From what I've read there isn't really an indication that today, this is the case.
There is no reason to discount live forage. The main advantage of pelleted feeds when compared to an equally nutritious live fish prey item is that the pelleted feeds are designed so that an increased surface area of nutrients comes in contact with the digestive tract. This theoretically would increase feed conversion efficiency.
Commercial fish farming is really starting to take off, and I am confident that there is going to be a lot more research published in the next 10-15 years than there has ever been on freshwater fish. The quality of nutrition really increases exponentially when that happens.
I think its kind of funny how we know more about cattle, and take care of them better nutritionally than we do horses. A lot of people treat their horses better than members of their own family, yet nutritionists still don't know where horses are absorbing their vitamin K from. Its pretty important, and no one really has a clue, just theories. No one is really investigating it because no one is really throwing money at them, horse farming as far as growth is concerned isn't a substantial industry.
Anyhow, back on track.
This has got me curious. I am going to look into seeing if I can get an experiment going injecting some trout, bass, or bluegill with amino acids in an effort to see if any of them have an marked increase in circulating hormone levels.
