What’s been causing mass shellfish die-offs around Puget Sound?

Since early July, WDFW has received reports of large-scale shellfish mortality events that appeared to follow the first heat wave in June. The precise causes are still being explored and it is possible that a variety of factors are behind these shellfish die-offs — including high water and air temperatures (especially coinciding with low tide events), low sediment or water oxygen levels, nutrition-related stress, or an over-abundance of harmful algae affecting feeding shellfish. The possible role of shellfish disease is also being considered, although no disease issues were confirmed in sampling following similar 2018 events and none have been implicated in to date.

Similar shellfish die-off events were documented in Willapa Bay, South Puget Sound, Hood Canal, and elsewhere in Puget Sound in 2018 — far exceeding the “summer mortality” that is sometimes reported by shellfish growers.

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An example of oyster mortality. Photo credit: Doug Rogers/WDFW

Current hot-spots for clam, oysters, and mussel mortality:

· Discovery Bay on the Strait of Juan de Fuca and Similk Bay in the northern Whidbey Basin. — Pacific oysters on farm sites in these areas are reportedly experiencing mortality rates reaching 80–90% per oyster bag (e.g. 5–10% is more typical) in Discovery Bay on the Strait of Juan de Fuca and the northern Whidbey Basin. Growers have reported that oysters appear to be incapable of handling stressors (handling, crowding) that they are typically more resilient to. Mass mortality is affecting oysters of all age classes.

· South Sound — Large-scale cultured and wild-stock clam die-offs have been reported in South Sound — including Rocky Bay, Vaughn Bay and North Bay (all located in northern Case Inlet) and affecting a variety of clam species — including Manila, butter, cockles, native littleneck, varnish, and Macoma clams.

In addition to the potential challenges of temperature and localized hypoxic conditions (low oxygen levels in seawater or sediment) and changes in food resources, shellfish in South Sound (as well as other regions of Puget Sound) may be affected by abundance of certain phytoplankton (microscopic marine algae) — including Protoceratium reticulatum. This algae species has been shown to be toxic to shellfish in other regions due to the production of yessotoxins.

Washington Sea Grant and the Northwest Fisheries Science Center’s SoundToxins program has been gathering water and shellfish tissue samples at mortality sites to examine for the presence of P. reticulatum and yessotoxins.

An abundance of this species are being reported by SoundToxins in Case Inlet, Carr Inlet, Pickering Pass, Peale Pass, Nisqually Reach, and in rising numbers in Oakland Bay, Hammersley Inlet, and Tramp Harbor. Whether P. reticulatum is the cause of the mortality in Rocky Bay and other bays within Puget Sound this year and in past years is a point of active research by the SoundToxins team, which in collaboration with WDFW Coast Shellfish Biologist Zach Forster, submitted a grant proposal to NOAA earlier this year for further study.

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P. reticulatum cysts and cells from Rocky Bay marine water samples. Photo credit: T.King, Washington SeaGrant, SoundToxins

· Hood Canal — In addition to potential challenges of temperature and hypoxic conditions that could contribute to any of the mortality events, SoundToxins, identified a bloom of Emiliania huxleyi, another type of phytoplankton that has exhibited intense summer blooms in Hood Canal annually since 2016 , beginning this year on June 24 in Quilcene Bay. It has since expanded southward into the main basin of Hood Canal, appearing most brilliantly in Lynch Cove. SoundToxins also reports a Phaeocystis bloom was observed in Discovery Bay on July 3; Phaeocystis has been suspected in shellfish mortalities in the summer of 2018 in Hood Canal and Discovery Bay.

· Willapa Bay — WDFW has received reports of widespread die-offs of wild and cultured Pacific oysters in Willapa Bay, especially bottom-cultured oysters.

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Please report if you see large areas of dead clams, mussels or oysters.

WDFW is collecting and summarizing reports of shellfish mortality events, which can be reported to deadshellfish@dfw.wa.gov. WDFW is also evaluating impacts to shellfish resources on WDFW-owned and other public tidelands.

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The Washington Department of Fish and Wildlife is dedicated to preserving, protecting and perpetuating the state’s fish and wildlife resources.

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