Pseudo-replication confounds the assessment of long-distance detection of gillnets by porpoises: Comment on Nielsen et al. (2012)

The harbour porpoise Phocoena phocoena is one of the small cetacean species most frequently caught in gillnets. Understanding how this occurs is important to devising effective mitigation strategies. To assess the distance at which harbour porpoises can detect and avoid gillnets, Nielsen et al. (2012; Mar Ecol Prog Ser 453: 241-248) quantified the movement of porpoises through a study area when a gillnet was present, and when it was not. They claimed to provide evidence that porpoises detected gillnets at distances >80 m, much farther than was thought possible. We show, however, that their results are undermined by pseudo-replication, and hence that their conclusion is unreliable. Mixed-effects modelling (van de Pol & Wright 2009; Anim Behav 77: 753-758) can be used to avoid this problem.