Though the potential of polychaetes has not been fully investigated, they are known to function as organic extractive species in IMTA systems. Owing to the high levels of polyunsaturated fatty acids (PUFAs) found in several polychaetes species it is worthwhile to explore the practicability of employing polychaetes to convert fish farm waste into alternative resources for fish diet. Within this context, Nederlof and his team evaluate the potential of polychaetes Capitella sp. and Ophryotrocha craigsmithi to convert fish waste into valuable ingredients for fish feed formulation. The polychaetes were fed different forms of salmon feces (fresh, acid-preserved, or oven-dried) and their production rate as well as body composition, especially, fatty acid (FA) profiles were determined to evaluate their applicability in (de)coupled integrated multi-trophic aquaculture (IMTA) systems. Coupled production involves direct integration of polychaetes and fish within the same eco (system), whilst in decoupled production, the rearing units can be separated spatially or functionally. Preservation of fish waste is recommended in case of decouple system. The authors observed the highest growth rate in both species when fed with fresh feces, whilst a negative growth rate was observed in O. craigsmithi when fed with preserved diets. This in fact suggests the importance of microbial role in polychaetes’ diet. Despite relatively low content of PUFAs (5-9% of total FAs) in the diets, the fatty acid profile showed that both species were rich in PUFAs (>30% of total FAs). Moreover, all essential FAs needed for fish nutrition were also found. The FA profile was enriched in Capitella sp. when fed acid-preserved diets. The enriched levels of FAs found in both species might be accredited to the accumulation of PUFAs, de novo synthesis and/or transfer via bacterial biomass that could have led in the up-regulation of PUFA content. On the whole, the authors conclude that both species are highly valuable marine products that can be used as an alternative source for fish food formulations. Deducing from the recorded growth rates with different diets, O. craigsmithi seems more suitable for integration in coupled systems where fresh fish feces are continuously supplied, whilst Capitella sp. is interesting for both coupled and decoupled integrated systems as favourable growth was observed on all diets tested.
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