When it comes to growth, I think the primary contributor is the consumption of foods. Anything consumed above maintenance can be assimilated to flesh and bone thus increasing the weight of fish. So at the very least we know the reason for the noted growth is that the YP were eating more than in the standard treatments.
As for whether the reason for increased consumption per fish was because of pheromones ... this may stand as a hypothesis ... but to be reduced to an indication of fact we maintain adequate control so that its effect is adequately isolated. It is well known that YP stunt in the absence of predators due to competition. The increased consumption per YP is the key element and so was this because the predators reduced the numbers of YP?
In order to answer that question the authors introduced FHM giving the predators something additional to eat. Even so, there were probably some YP consumed but the authors seemed to omit this even happened. In any event, how many YP there were and how much they ate are essential knowledge needed to isolate the effect of the pheromones (assuming there was an effect).
The FHM introduction casts more doubt because in an otherwise equal experiment it introduces more food. Now one may say FHM were only food for the predators ... but can we be sure that YP did not consume predator wastes? My grandpa once told me that one could finish 2 pigs by feeding only one where the second pig had access to only the wastes of the fed pig. He claimed that the second pig would gain a major portion of the gain of the fed pig. Notwithstanding such here say, I have no doubt that young YP would consume predator FHM waste
IF they might gain from it.
For me the litmus test for testing the hypothesis lies not in the gain. Rather it lies in the consumption. So the authors tested for and noted an increase in the concentration of growth hormone. This should result in a propensity to consume more food. I think perhaps a better test would be to provide the test treatment with pheromone laced water free of food. In other words, water that passes the pheromone but where all food is filtered out so that the only food consumed is that introduced by feeding. Then the satiation for the test and control treatments could be determined. If the YP in pheromone laced water showed a higher propensity to consume (as evidenced by a greater feed proportion of their body weight at satiation) then the effect would appear to be evident.
Some might argue that the discovery of the increased growth hormone is sufficient evidence. I consider this to be very strong evidence of increased consumption as this hormone is a precursor to growth and the consumption required to support growth. Even so, it is wise to remember that fish cannot grow on hormone alone. Deeper questions might be whether high levels of hormone increase/decrease the efficiency of assimilation. It's really difficult to anticipate how such might pan out but clear evidence would be lower maintenance rate and increased intrinsic conversion. Even so, my intuition suspects that maintenance increases when there are higher levels of growth hormone. This is not to say that feed would have lower feed conversion efficiency when growth hormones are higher. Only that the assimilation efficiency needn't be benefited in order to improve feed conversion. This is because the way to maximize conversion efficiency is to achieve the optimum consumption rate. Higher levels of growth hormone can facilitate or make possible the optimum consumption rate.
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