*Article* **The Negative Relationship between Fouling Organisms and the Content of Eicosapentaenoic Acid and Docosahexaenoic Acid in Cultivated Pacific Oysters,** *Crassostrea gigas*

**Megumu Fujibayashi 1,\*, Osamu Nishimura <sup>2</sup> and Takashi Sakamaki <sup>2</sup>**


**Abstract:** Bivalves serve as an important aquaculture product, as they are the source of essential fatty acids, such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), in our diet. However, their cultivation in the wild can be affected by fouling organisms that, in turn, affect their EPA and DHA content. The effects of fouling organisms on the EPA and DHA contents of cultivated bivalves have not been well documented. We examined the effects of fouling organisms on the EPA and DHA contents and condition index of cultured oysters, *Crassostrea gigas*, in an aquaculture system. We sampled two-year-old oysters from five sites in Shizugawa Bay, Japan, in August 2014. Most of the fouling organisms were sponges, macroalgae, and *Mytilus galloprovincialis*. A significant negative relationship existed between the DHA content in *C. gigas* and the presence of sponges and macroalgae. A lower *C. gigas* EPA content corresponded to a higher *M. galloprovincialis* fouling mass and a lower *C. gigas* condition index. This can be explained by dietary competition between *C. gigas* and *M. galloprovincialis* for diatoms, which were the main producer of EPA in our study sites. Our findings indicate that fouling organisms likely reduce the EPA and DHA content in cultivated oysters. Therefore, our results suggest that the current efforts to remove fouling organisms from oyster clusters is an effective strategy to enhance the content of EPA and DHA in oysters.

**Keywords:** dietary resource; *Mytilus galloprovincialis*; *Crassostrea gigas*; diatom; competition; biofouling; EPA; DHA
