Sampling fish by trapping can lead to biased conclusions about a population. We used catch data to assess differences between two types of traps for adult sea lamprey (Petromyzon marinus), submerged-funnel traps and studded-tile traps, which are angled ramps with trickle flow leading out of the water. The studded-tile trap at one river caught about 50% more females than the funnel trap. It caught males that had a smaller body size and females with a lower gonado-somatic index (GSI). The likelihood of catching lamprey in the studded-tile trap increased after they had been caught once. This was not the case for the funnel traps, which are used for mark–recapture-based population assessment of invasive sea lamprey in the Great Lakes. The apparent trap response caused by studded-tile traps may have been caused by a behavioral bias, i.e., the traps consistently attracting a subset of the population. Use of the studded-tile trap for population assessment should only be considered after more is known about its recapture bias. The differences between lamprey caught in the two trap types suggests that a variety of trapping methods needs to be employed in order to get a representative sample from a fish population.