The numbers of North Atlantic harbour porpoise (Phocoena phocoena) within the Bay of Fundy/Gulf of Maine were considered to be declining as a consequence of bycatch in the sink gillnet fisheries of both Canada and the United States. Evidence for a decline was based upon decreased sightings and a diminishing bycatch in areas considered to be a traditional habitat. Observed changes in the individual growth rates of porpoise, were also attributed to decreased density. By taking into consideration the dynamics of the habitat, as well as the possible effects of species interactions, an alternative interpretation is suggested, one which seems equally consistent with present observations: that changes in distributions and the growth response of porpoise reflects the strong coupling of this species to the distribution of herring (Clupea harengus). Furthermore, distribution of marine production affects the duration of the presence of herring and associated porpoise within the vicinity of sink gillnet fisheries. Recent surveys of porpoise numbers and distribution tend to support this hypothesis. Fluctuations in both the availability of herring and their energy content may also be a confounding factor affecting condition, previously used to argue for rapidly declining porpoise numbers. The probable reduction of shark predators through directed catches and bycatches in commercial fisheries over recent decades is considered here as a factor which may have enhanced the survival rate of this population of harbour porpoise, and that this larger population has been subjected to bycatch in an expanded gillnet fishery.