Cross reference: Human Dominance Hierarchies
Breeding Behaviour of Brook Lampreys Lampetra planeri: Experiments on Mate Choice (Goog)
"Aggression was observed mainly at the beginning of the experiment between nest-building males and intruding males. In spite of widespread promiscuity there was a significant positive correlation in size between the partners, indicating an assortative mating. Males competed for matings and it is suggested that the most alert males are the ones most likely to mate with the females."
This is quite clearly an example of territoriality.
See: The Territorial Imperative
excerpted in: Dominance Hierarchies in General
"Above all, there was territory. ..."