History of Human Impacts on the Harbor Porpoise

Many marine mammals have experienced substantial population declines over the last several hundred years. Large cetacean populations were decimated to the point of near extinction from whaling. Smaller marine mammals like seals, sea otter were heavily hunted for pelts. Others were killed because they were thought to be competing with human fishing harvests. From 1947 to 1960 in Washington a bounty was placed on seals because it was believed they ate significant amounts of commercially valuable fish. During that time it is estimated 17,000 seals were killed. (WDFW 2016). The Federal Marine Mammal Protection Act of 1972 now prohibits hunting, killing harassing or disturbing marine mammals. As a result, many of the marine species that have been listed as endangered, threatened or of-concern are recovering.

In addition to direct hunting, pollution, loss of habitat, bycatch, noise and human disturbance substantially reduced many marine mammal populations. The harbor porpoise has been affected by all of these factors.

The harbor porpoise population had experienced a significant decline in range and population from historic levels over the time interval 1940 to 1990. In the 1940s and 50s, it was abundant in south Puget Sound (Scheffer et. al. 1948). By the 1990s, its numbers and range had been reduced to the point that it was extirpated from south Puget Sound and rarely seen south of Admiralty Inlet (Carretta et. al. 2011, Chandler et. al., 2003). Beginning in 2007, there were anecdotal indications that the population was starting to recover. In 2015, Evenson (Evenson 2016), summarized and reported on the Audubon data from waterfowl surveys, which incidentally included data for harbor porpoise and Dall’s porpoise sightings. This single flyover, aerial survey was conducted in the seasonal interval from December through February from 1995 to 2014 and shows that the presence of the harbor porpoise in the south Puget Sound had increased from 1995 to 2014. The porpoise’s distribution has changed from a concentration of animals in the Strait of Juan de Fuca and San Juan Islands, and spread into the south Puget Sound area. Surveys have recently been done by the Navy beginning in August 2013 and ending in March 2015, but the information has not been publically available, and it is not in a form that abundance estimates can be compared to earlier work.

A significant reason for harbor porpoise decline into the 1980s is reported to have been entrapment in gill nets (Gearin 1998, Jefferson 1994, Flaherty 1982, Baird 1994a). Currently, habitat loss, decline of prey fish species, pollution and boat noise are thought to be the major contributing factors that keep the population low (Bossart 2011, Evenson 2016, Osmek 1996, NOAA 2011, NOAA 2003). In Canada, the harbor porpoise has been found to be sensitive to loss of forage fish, bycatch from gill netting, noise, human disturbance, pollution, human infectious disease and habitat loss (Baird 2003a, b). Its population trend is an indication of the health of the marine ecosystem because its population is affected by all of these environmental factors.

At present, the harbor porpoise is listed as a Candidate Species of Concern in Washington (WDFW, 2013) and a Species of Special Concern in British Columbia (Baird, 2003a). Questions have been raised as to whether gill net fishing is becoming a problem again (PBI workshop 2013). Scientists have reported a dramatically higher number of harbor porpoise strandings in recent years (Huggins 2015). Increased vessel noise from shipping (Viers 2015) and anecdotal reports of increased presence of transient orca by Orca Network (Orca Network 2016) could indicate factors that impact the harbor porpoise negatively.

NOAA has a mandate to conduct harbor porpoise stock assessments every 3 to 5 years under the Marine Mammal Protection Act, 1972, but the most recent aerial surveys are over 10 years old. WDFW has identified the harbor porpoise as a Candidate Species of Concern, but has not had the funding or information to complete the listing with a status review. The Navy initiated a set of aerial surveys beginning in August, 2013, but this information as of fall 2017 was not publically available. Smultea Sciences (Smultea, private communication 2015) reported on densities in 8 regions of the Puget Sound, found from her aerial surveys for the Navy, but the earlier NOAA surveys did not give densities and cannot be compared to investigate population trends. The surveys described above by Evenson are the best indication of population trends. While the harbor porpoise population has increased in the south Puget Sound, it is level to declining in the San Juan Islands and Rosario Strait. Biota Maxima’s Harbor Porpoise Project is using acoustic monitors to assess the trend of the harbor porpoise population in the Salish Sea.

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