I would sure like to see the original paper and where it was published.
removal of organics includes pathogens. the more fish, the more pathogens are being
released. each pathogen challenges the immune system individually and as the number
of challenges increases the antibody response per immunogen decreases. one reason we
dont get a flu shot if we are sick. it doesnt "take".
This is also the reason why introducing new fish into existing pond or tank without
quarantine is dangerous. New stressed fish are shedding cooties that they are more
or less immune to but the existing fish arent. so the older fish get sick and/or
die.

real proof of pheromones is to isolate the compound in a pure form, then reintroduce
it at different doses and see the effect on antibody levels after challenge.

Of course, lower stocking levels is just good common sense. Lower levels of wastes
(ammonia has been shown to directly cause stress) and more safety margin for when
things go wrong. Ingrid