Quality of fish farm waste determines carbon turnover in Ophryotrocha craigsmithi

Polychaetes are promising candidates for recycling organic waste from fish farming. In Norway, the polychaete Ophryotrocha craigsmithi Wiklund, Glover & Dahlgren, 2009 is currently being evaluated as an extractive species in integrated multitrophic aquaculture (IMTA), as preliminary research suggests high potential for carbon turnover. Yet knowledge gaps remain regarding dietary requirements and carbon turnover rates across their lifespan, which are crucial for estimating carbon turnover by populations. We aimed to estimate carbon turnover of O. craigsmithi feeding on different waste fractions: sludge, fish pellets, or bacteria cultivated with fish pellets. Respired carbon and carbon gained in tissues were measured for individuals, and their sum was used as a proxy for carbon turnover. No differences in respiration rates were found between diet treatments; hence, growth primarily determined the carbon turnover rates. Slow growth in polychaetes fed sludge (0.15% d^-1) resulted in low carbon turnover, whereas a 6-fold higher turnover was found for individuals fed fish pellets. Allometric scaling coefficients for respiration rates were defined as log a = 0.76 and b = 0.80. Fatty acids from the bacteria-enhanced diet were assimilated by the polychaetes, though this did not improve growth or survival over the high-quality fish pellet diet. These findings provide insights into carbon turnover of fish farm waste by O. craigsmithi, and its dietary requirements alongside allometric scaling coefficients for estimating carbon turnover by populations. Our results highlight both the potential and limitations of O. craigsmithi as a waste recycling species in IMTA systems.