What do you mean by "aggression" ? That can mean many things and can not be defined simply in an ethological context. This can cover the gambit of intra and extra-species conflict which can be further broken down in relation to predation / defense instinct, Circadian cycles or any other number of Genetic, environmental, chemical cues and influences. Rather than the term "Aggression", Conflict is actually a better term to use from a defined standpoint..
As for behavior research on Lepisosteids, yes there have been a few experiments that are unpublished, otherwise little has been done. The only key published behavioral paper I can think of regards Shortnosed gar and feeding on cicadas. Behavior coincidentally has also been my primary area of research in Lepisosteids.
In standardized experiment, it has been observed there are different levels of conflict and the causes for those conflicts within Lepisosteids. It has been difficult to determine what the conflict escalation / de-escalation sequence is in the fish, or the specific triggers to the behavior in many cases. What I can say is that there is an observed sequence that leads to extra-specific conflict (Much more defined in intra-specific as noted in a bit) and it is often not a quick process but rather one that is the culmination of repeated probing on behalf of the fish. These probing actions often result in the fish disengaging as it sees no threat from the object of the action or the object just does not respond in a manner that escalates the instinctual behavioral response.
As seen intra-specifically, you see a this-for-that type exchange which may or may not escalate depending on the contestants behavior. (Initiation of action> Nudging if not effective > turns to Tail chasing > and if not effective turns to physical biting). Other species do not respond to this sequence as they do not have the behavior in the first place. Why would they they are not Gars?
That said, Changes in environmental conditions definitely show a strong reaction on behalf of Atractosteus (lesser observed in Lepisosteus) to escalate a mild conventional level of conflict sequence to "Total war" much quicker. This is precisely what is occurring in Maddings tank. I'm unsure the specific triggers but for some reason the fish will drop the lower levels of conflict resolution or probing and become agitated to a much more "aggressive" response initially and repeatedly. I have some theory it may actually be a stress trigger that makes them skip the typical sequence of A>B>C>D and instead go directly to a C>D response.
Anyhow there is my take on making the complex sound simple.....
