Oregon Sea Grant

To determine the efficacy of different methods of oyster habitat restoration methods in enhancing juvenile life-history traits, this study compared the settlement, growth and mortality of wild juvenile Olympia oysters among three shell treatments: (1) bagged shells and (2) loose shells, which are commonly used methods of oyster habitat restoration, and (3) rafted-line shells, which is a method used by the Oregon Oyster Farm of Yaquina Bay to commercially grow their Pacific oysters.

The bagged and rafted-line shells were deployed in July 2017, and were sampled monthly until June 2018; the loose shells were deployed by the Confederated Tribes of Siletz Indians (CTSI) in April 2017 and were sampled twice during the study period. This study also quantified the size-specific fecundity and sex ratios of adult Olympia oysters by sampling 30 to 31 oysters every two weeks between April and June 2018 from a subtidal site 1 km downriver from the shell treatment sites.

Per 100 shells, the bagged shells captured one to two orders of magnitude more juvenile Olympia oysters than either the loose or rafted-line shells. There were no significant differences in growth or mortality among the three shell treatments, but the significantly lower abundance on the loose and rafted-line shells may affect the population structure in later life stages in future generations of Olympia oysters.

Brooders were first found in late May, with average brood sizes of 339,000. Brood size was significantly positively correlated to shell height (R2 = 0.719), suggesting that Olympia oyster fecundity increases with body size. The male ratio increased over the course of the spawning season, which may be related to the depletion of lipid and metabolic reserves required for oogenesis after the initial spawn of the season (Abad et al., 1995; Joyce et al., 2013).

The results of this study may inform future restoration efforts in Yaquina Bay, and fill regional knowledge gaps of the current status of remnant Olympia oyster populations. Continued research on Olympia oyster abundances, geographic extents, environmental stressors, space
competitors and interspecific interactions with non-native Pacific oysters may further our understanding of the variables that could affect the success of future Olympia oyster restoration projects.