Factors preventing the recovery of a historically overexploited shellfish species, Ostrea conchaphila

Authors: TRIMBLE, ALAN - University Of Washington; RUESINK, JENNIFER - University Of Washington; Dumbauld, Brett

Submitted to: Journal of Shellfish Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/28/2009
Publication Date: 3/31/2009
Citation: Trimble, A.C., Ruesink, J.L., Dumbauld, B.R. 2009. Factors preventing the recovery of a historically overexploited shellfish species, Ostrea conchaphila. Journal of Shellfish Research. 28(1):97-106.

Interpretive Summary: The native oyster, Ostrea conchaphila, which occurs in estuaries along the Pacific Northwest coast of North America was overharvested in the late 1800’s and though still present became commercially extinct before 1930. Washington State set aside marine reserves in 1897 to protect young oysters and preserve the industry. Despite this protection and no commercial harvest for the past 80 years, native oysters remain rare. We examined the factors influencing this failed recovery in Willapa Bay, Washington once the home of the West coast’s largest native oyster fishery. Failed recovery could be due to either reproductive failure and no recruitment or to poor post settlement survival. Reproductive failure was found to be unlikely in Willapa Bay because both historical (1947-1983) and modern (2002-2006) records revealed five fold greater annual spatfall (settlement of oyster larvae) of native oysters than the introduced Pacific oyster (Crassostrea gigas) which is abundant in this estuary and supports a viable “natural” fishery. Settlement was evaluated by placing shell strings (made from Pacific oyster shell) in various habitats (open mud, beds of eelgrass, and Pacific oyster reef) at two intertidal elevations. Settlement of native oysters was significantly higher to shells in Pacific oyster reefs at both intertidal elevations. Post settlement survival was evaluated by outplanting juvenile native oysters (on plates) at three tidal elevations and five sites. Fouling organisms (e.g. other oysters, barnacles and tunicates) were removed from some plates to evaluate their effect on survival. Short emersion time was found to significantly reduce survival (80%) but not growth of native oysters. Fouling organisms (mostly introduced tunicates) were found to depress survival by 50% and growth by 20%. In a final experiment in which juvenile native oysters were outplanted directly on the bottom on various substrates, we found that thin unconsolidated layers of natives on Pacific oyster shell cultch were easily moved or buried. Thus recovery of native oysters in Willapa Bay was found to be hampered by the lack of suitable shell settlement substrate at appropriate low tidal height (shell and oysters removed by original harvesters in the 1800’s), by direct competition from introduced fouling organisms, and by the presence of large quantities of good settlement substrate (Pacific oysters) at an inappropriate tidal height for native oysters to survive.

Technical Abstract: The native oyster in estuaries along the Pacific coast of North America, Ostrea conchaphila (prev. Ostrea lurida, Olympia oyster), experienced overexploitation throughout its range in the late 1800’s, resulting in commercial extinction before 1930. Significant harvest restrictions and marine reserves were established in Washington State by 1897 to protect new recruits, and harvest pressure has been negligible for the past 80 years. Nevertheless, O. conchaphila remains locally rare. This study focuses on the contemporary dynamics of the remnant population of O. conchaphila in Willapa Bay, Washington, historically home to the largest native oyster fishery on the coast, with a broad focus on factors preventing recovery. Failed recovery could be due to reproductive limitation, or to poor post-recruitment performance. In this case, reproductive limitation appears unlikely, because historical (1947-1983) and modern (2002-2006) records reveal five-fold higher annual spatfall for O. conchaphila than introduced Pacific oysters (Crassostrea gigas.) However, O.conchaphila remains rare and C. gigas is commercially exploited from natural recruitment. To evaluate the effects of abundant C. gigas in intertidal areas on O. conchaphila settlement patterns, strings of C. gigas shell were placed at two tidal elevations in three habitat types - open mud, eelgrass beds of Zostera marina, and C. gigas reefs. Settlement of O. conchaphila was significantly higher on the shell strings placed in the C. gigas reefs at both tidal heights. To evaluate post-recruitment demography, juvenile O. conchaphila were outplanted at three tidal elevations at five sites, and fouling organisms were manipulated to test for competition. Short emersion times (8% greater exposure) reduced survival by 80% relative to subtidal treatments, but did not affect growth rates of survivors. Naturally-setting competitors, mostly nonindigenous, depressed survival by 50% and growth by 20%. In a third experiment, manipulating the density and stability of shell substrate, O. conchaphila was easily moved or buried when outplanted in a thin, unconsolidated layer. These results indicate that recovery has been hampered by the removal of dense subtidal native oyster shell accumulations during exploitation, by direct competition from exotic species, and by the appearance of novel introduced oyster shell settlement substrate in the intertidal zone. This altered web of interactions influencing O. conchaphila serves as a model for beginning to explore the failed recovery of overfished species in rapidly changing coastal systems.