Climate change is negatively impacting shellfish aquaculture operations. Ocean acidification (OA) increases shell dissolution rates and decreases calcification rates, which ultimately negatively affects overall growth and development. This study addressed the negative impacts of OA on red abalone (Haliotis rufescens), through Integrated Multi-Trophic Aquaculture (IMTA) with the red seaweed Dulse (Devaleraea mollis). We tested the hypothesis that IMTA can mitigate the impacts of acidified water by raising seawater pH through photosynthesis. The current study also tested improvements in IMTA system design, by reducing the set-up from two integrated culture tanks to a single co-culture tank with a surplus of algal productivity.
A 5-month experiment was conducted to determine the benefits of IMTA on abalone growth, shell composition, and morphometrics using 4 different treatments: IMTA, Ambient, Sub-IMTA, OA with average pH values of 8.0, 7.9, 7.7, and 7.6, respectively (4 240gal tanks per treatment). To simulate OA and Sub-IMTA conditions, the ambient and IMTA seawater pH readings were reduced by 0.3 pH units by bubbling in CO2. The upper bound of the pH in each of the treatments was controlled by additional oxygen released from seaweed photosynthesis. A subsample of abalone were measured monthly in each tank and seaweed weight was recorded biweekly to track growth over the course of the study.
After 5 months, a two-way ANOVA, comparing dulse buffering and CO2 bubbling in each treatment, showed abalone raised in ambient and IMTA treatments had an increase in shell length and density and demonstrated less variation than those raised in low pH treatments (OA and sub-OA). Interestingly, the IMTA treatment had the highest total mass per square centimeter (Fig. 1A). Preliminary analyses of shell structure and function suggest that these growth differences between OA and ambient treatments may translate to differences in shell properties as well. Seaweed from the OA treatment grew faster than those grown in the ambient seawater and IMTA treatments, suggesting that elevated pCO2 levels may increase seaweed production. Overall, seaweed buffering of acidified water may allow abalone aquaculture to persist in the face of an acidifying ocean, and as an added benefit, it will require less space to yield a better overall product.