Substrate selection for oyster reef restoration: Differences in oyster settlement and recruitment on fresh and old oyster shells

Abstract

The lab and field experiments of 2019 examined the effects of oyster shell type and biofilm on oyster settlement and recruitment. Results from the lab experiment indicated that both shell type and biofilm significantly affected the densities of settled oysters. There were higher settlement and recruitment of oyster spats on the fresh oyster shell with biofilm than on the old oyster shell with biofilm (<italic>P</italic><0.05), but differences in the densities of settled spat were not evident between these two shells without biofilm (<italic>P</italic>>0.05). Biofilm significantly increased the attractiveness of the fresh shell to oyster larvae (<italic>P</italic><0.05), but had little influence on the old shell (<italic>P</italic>>0.05). The field experiment of 2019 found the contrast patterns. Oyster shell type had no influence on the densities of settled oysters, while biofilm significantly increased the biological acceptability of the old shell. The field experiment of 2020 examined the effects of oyster shell and tidal zone on the densities of settled oysters and barnacles. Two-way ANOVA found that shell types significantly affected the densities of settled oyster spats, while tidal zone had little effect on oyster settlement. At each of the two tidal levels (0.6 m and 1.1 m), there were consistently greater densities of settled oyster spats on the fresh shell than on the old shell on the 15 d, 45 d and 70 d(<italic>P</italic><0.05). Results from two-way ANOVA demonstrated that neither shell types or biofilm had significant effect on the shell height of settled oysters. Settled spats measured on 45 d and 70 d had significantly greater size than those on 15 d(<italic>P</italic><0.05). Results from two-way ANOVA found that oyster shell had no influence on barnacle settlement, while tidal zone significantly affected barnacle settlement. Higher densities of settled barnacles were found on the fresh shell than on the old shell in the 0.6 m tidal zone on 15 d(<italic>P</italic><0.05), whereas the difference disappeared at each of the two tidal levels on the 45 d and 70 d (<italic>P</italic>>0.05). The study indicated that there were generally higher oyster settlement and recruitment on the fresh shell than on the old shell, and the evident differences extended to the recruitment season. Therefore, it concluded that the fresh shell were more preferable substrate for oyster reef restoration than the old shell.