Conspecific cueing in the sea lamprey: do reproductive migrations consistently follow the most intense larval odour?

Conspecific cueing occurs when an organism receives information about habitat quality from the distribution of conspecifics. Consequently, animals should prefer to settle in habitats with high occupancy when conspecific cueing is used. Unlike salmonid fishes, which often acquire chemical recognition of natal streams in early life, sea lamprey, Petromyzon marinus, larvae produce a remarkably potent pheromone that migrating adults use to locate spawning streams. The size of the spawning migration in tributaries to the Great Lakes is correlated with the number of larvae resident to the stream, and therefore the amount of pheromone discharging into the lake. By releasing controlled amounts of larval odour into a lamprey-less stream, we tested the hypotheses that migrating sea lampreys ( 1) avoid swimming in waters that lack larval odour and ( 2) actively choose to swim in waters activated with the highest concentration of larval odour. Migrating sea lamprey clearly showed. ne-scale movements (<3 m) in streams to avoid swimming in waters that lacked larval odour. However, migrants only preferred intense odours after the difference in pheromone concentration reached an order of magnitude. These results suggest that the migratory pheromone functions primarily to establish the eligibility of a river for larval rearing and as a source of public information to indicate large differences in reproductive habitat quality to migrating adults. (C) 2009 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.