Jim, very dumb question here, but how do males allow other males around but not smaller males or any females? We can't tell the difference reliably, especially with 'sneaker' or 'mimic' fish, how do they do it? lack of pheromones? I'm sure they don't inspect the size of the ear tab or other visible characteristics like we attempt to do?
Olfaction probably does not play a role in threat assessments made by parental males. Rather, it is likely the male keys in on what the target fish is looking at. Keep a bluegill an aquarium and you will see the BG eyes readily betray what a given fish is interested in. The parental male does not want someone that is interested in his brood.
Olfaction paper ewest refers to is evidence for the BG's ability to use scent but not likely in the context of assessing who is a threat. To my eyes scent is what maintains the male's broodiness.
