The question is why and what caused the bluegill to stay and forage in the plants even when that was not optimum for growth and energy usage (energetics).
There is substantial evidence in the literature that habitat complexity increases standing weights. Generally, it is thought that this arises from an increase in available foods where plants, brush, etc. provide substrate for food organisms. This finding seems to challenge those findings. One possible explanation deals with the environmental factor of space and perhaps even the perception of space. Perhaps the BG were exhibiting territorial behavior and the presence of structure may have reduced the limiting territorial requirement of space and allowed more individuals to survive.
More measures beyond length might be instructive. For example, standing weights, relative weight, mortality rate, etc. It is complex.
One area I wish the study would have addressed in more detail is phenotypic plasticity. That is the ability of an individual or population to change due to environmental influences. Can environmental conditions during early development shape individuals’ phenotypes so they become more adaptive to the conditions they encounter? Were the long bluegill that fed in open water that way because longer fish can swim better in open water and were the shorter bluegill that way because being short allows them to maneuver around the weeds better? Plasticity has been shown to effect sunfish (Lepomis) shape, feeding and behavior in some cases.
Good points. I wonder also whether different prey types and their corresponding nutritional value may play a role. For example, do the open water organisms provide more of nutrition needed for bone growth?
