Since 2016, clam farmers in Willapa Bay Washington have been reporting declines in commercial Manila clam (Ruditapes philippinarum) harvests. They have also noted an absence of juvenile recruitments and very few young clams during harvest. An internal poll of commercial clam farmers in Willapa Bay reported that since 2013 average annual harvests are down 500,000 pounds. In response, several farmers have ceased operations, laid off clam harvesting crews, and subleased tidelands to other growers due to the inability to meet market demands. The causes for declining harvest were not immediately apparent. Potential causes for the decline included widespread mortality events, HAB’s, extreme heat or cold weather events and, more recently, the dramatic expanded range and increased abundance of the invasive European green crab (Carcinus maenas), which preys on clams. The dinoflagellate (Akashiwo sanquinea) which has been implicated as a producer of yessotoxin, deadly to clams, has been observed in nearby Washington Department of Fish and Wildlife (WDFW) monitoring sites. Predation pits have been observed in graveled clam ground along western Willapa Bay in previous studies by the Pacific Shellfish Institute (PSI) and the Willapa Grays Harbor Oyster Growers Association (WGHOGA), but video had not identified the source.
Funded by WDFW, in the spring of 2023, PSI and WGHOGA scaled up the effort to determine the factors causing the decline in commercial clam harvests in Willapa Bay. Research included: grower surveys, video surveys for potential predators, installation of predator nets at 4 locations followed by population surveys under and adjacent to the nets every 6 months, and summarization and correlation of nearby HAB and water quality data. Initial video surveys point to European green crab (EGC) excavation pits, where manila clams are dug up and exposed, as an explanation of part of the decline (Figure 1). Increased EGC trapping has occurred in these areas after exposure and possible predation was documented. The potentially harmful algae, Phaeocystis, has been noted in 2023-24 WDFW plankton surveys of nearby marine waters.