Master's Thesis - University of Rhode Island (1996)


Hundreds of harbor porpoise die each year as bycatch in the Gulf of Maine’s sink gillnet fishery. In recent years, concern has arisen whether the Gulf of Maine population of harbor porpoise can sustain this level of bycatch. It is believed that the population may have declined, or is declining as a result of the incidental mortalities. This paper reviews what is known about Gulf of Maine harbor porpoise demographics and how that information is incorporated into the management of the porpoise/fishery interaction. Included in this paper is a new study on the trend of relative abundance of harbor porpoise off mid-coast Maine 1982-1990. Sighting data from a commercial whalewatch was used to determine changes in the relative abundance of harbor porpoise over the time period. Relative abundance was calculated as the number of harbor porpoise sighted per nautical mile. Mean annual sighting rates south of Mount Desert Island, Maine increased from a low of 0.17 porpoise per nautical mile in 1984 to a high of 0.54 porpoise per nautical mile in 1990, the last year of this survey. In 1982. the first year of the survey, the mean was 0.30. The increasing trend may represent a redistribution of animals into the study area rather than a large-scale population phenomena. Harbor porpoise distribution may be related to environmental factors such as the density of prey species. To investigate this possibility, I compared the relative density of Atlantic herring, which increased during the survey period, to the relative abundance of harbor porpoise. Herring data was collected within a 30 mile radius of my study area. A positive correlation was found (r = .96). This positive correlation indicates that the increase in the relative abundance of harbor porpoise may be related to the increase in herring, the porpoise’s favorite prey in this region.