Eighteen Dead Harbor Porpoises

Dead porpoise. Photo: Beach Watch

By Nina Bubert

In the past two months, eighteen harbor porpoises have been found dead on beaches between Bodega Bay and Daly City.  According to the Marine Mammal Center, porpoises were found on the shores of Tennessee Valley, Abbotts Lagoon, Salmon Creek, Rodeo Beach and Thornton Beach.  This is an elevated rate for this area based on Beach Watch data.

The Farallones Sanctuary is working with NOAA Fisheries Marine Mammal Stranding Network, The Marine Mammal Center, California Academy of Sciences, and California Department of Public Health to determine the cause of the harbor porpoise strandings.  Most of the porpoises found initially were females with full term fetuses.

Domoic Acid (DA) poisoning is one potential cause. It is the result of an extreme proliferation of marine algae – a Harmful Algal Bloom (HAB). It can cause brain degeneration and sometimes death in marine mammals, seabirds and humans, so the sanctuary is working closely with the Department of Public Health.

Harbor porpoises are among the smallest cetaceans.  They reach an average length of five feet and weigh approximately 200 pounds, and the females are larger than the males.    They have a stocky midsection and a short rounded snout.  Their dorsal fin is triangular, unlike a dolphin’s which is falcate, and they have rounded pectoral fins. Harbor porpoises are dark grey with a white underside, as well as a light grey patch on their side. 

As their name indicates, the harbor porpoise inhabits inshore, shallow, coastal waters.  This is a possible reason why they strand frequently.  The harbor porpoise is listed by the IUCN as a threatened-vulnerable species.  They can become entangled in fishing nets, such as gill and trawl nets here in the Pacific.  The population has also declined due to pesticides, coastal development and vessel traffic.    

The Beach Watch data indicates that June and July are the peak months for harbor porpoise strandings in the Bay Area.  Since October 1993, surveyors have documented a total of 66 stranded harbor porpoises, 54% of which stranded during the months of June or July. 

It is a concern that eighteen porpoises have washed up in the past two months.