Temporal changes in the suitability of claywater as a greenwater substitute for rearing larval sablefish (Anoplopoma fimbria)

For some species, marine aquaculture facilities increase turbidity to improve larval feeding, growth, and survival, typically by mixing algae with seawater to make “greenwater.” Clay is a less expensive and inorganic potential algae substitute that has been shown to reduce bacterial levels relative to algae and promote equal or better growth and survival in some species. Sablefish (Anoplopoma fimbria) is a prime candidate species for aquaculture but the rearing of sablefish larvae has not yet been experimentally tested with clay. This study tested whether clay is a viable algae substitute for rearing sablefish, and whether the relative performance of clay versus algae varies as a function of time. In the first week of larval rearing, algae (Nannochloropsis) produced more than three times greater survival than clay (Kentucky Ball Clay OM4). However, switching from algae to clay at the beginning of the second week led to 1.5 times greater larval growth compared to tanks where algae was used in both weeks. The performance difference between clay and algae, despite equal turbidity, suggests that clay and algae have potentially harmful and beneficial effects in addition to turbidity effects, and that these effects change through time. However, because the first experiment was a replacement study, it was not possible to know whether algae produced better survival in week one because 1) clay, which might be harmful, was not present, 2) algae, which might be beneficial, was present, or 3) both. A further additive study was conducted to test the second possibility. The experiment found that adding algae to clay in the first week of larval rearing leads to greater growth and survival, suggesting that algae has beneficial effects beyond turbidity. We discuss some possible mechanisms for potential harmful and beneficial effects of algae and clay.