Shelter Selection by European River Lamprey Lampetra fluviatilis (Petromyzontidae) Smolts during their Seaward Migration

The behavior of migrating smolts of the European river lamprey Lampetra fluviatilis associated with the choice of shelters was studied experimentally. The experiments were conducted in a water stream under both day and night illumination conditions as smolts migrate at night and hide during the day. It has been shown that during daylight hours, smolts preferred to hide in shelters under the following conditions: absence of current, presence of substrate suitable for burrowing, and, additionally, shading. The smolts spent up to 94.4% of the experiment time in these shelters. The lampreys did not use shelters with any one of these conditions and spent more than half of the experiment time in the current, attached to the walls or bottom of the experimental installation. Under night illumination, the smolts did not use shelters. In the four series of experiments, they spent on average 49.8% of the time swimming downstream, periodically resting (27.6% of the time) by sucking onto walls or bottom of the installation, briefly visiting the limnozone (11.9%), and swimming against the current (10.7%), choosing the fastest jets of the flow. The active nature of smolt migration and their preference for shelters that provide rest and protection from predators during the daytime are adaptations aimed at increasing the survival rate of specimens during the transition between habitats.