Report of the International Whaling Commission (1996)


Effects of Beaufort sea state heterogeneities on density estimates of harbor porpoises from data collected during ‘independent’ team line
transect sighting surveys are investigated. The surveys were conducted in the Gulf of Maine/Bay of Fundy region during 1991 and 1992.
It is shown that as Beaufort sea state increased (weather worsened), sighting rates (n/L) and the resulting density estimates markedly
declined. At the same time, estimated probabilities of detecting the harbor porpoises [1/f(0) and g(0)] are reduced, but to a much lesser
degree. Such results are expected if there is heterogeneity due to Beaufort sea state and the teams are conditionally independent. The
relationship between Beaufort and density cannot be fully explained by small-scale spatial correlations between Beaufort sea state and
estimated harbor porpoise density or by a team- or area-heterogeneity. The data could be interpreted that the Beaufort heterogeneity
affected both the efficiency of the observers and the behavior of the harbor porpoises. Ways to compensate for the bias in this data set
are discussed.