The red alga Porphyra dioica as a fish-feed ingredient for rainbow trout (Oncorhynchus mykiss): effects on growth, feed efficiency, and carcass composition

Porphyra dioica meal was added at levels of 5, 10 and 15% to a diet for rainbow trout formulated to be isonitrogenous and isolipidic. The control diet was a commercial trout diet without seaweed meal. The experimental groups were fed in triplicate for 12.5 weeks, during which fish weight increased on average from 107–261 g. Seaweed meal inclusion did not affect significantly weight gain (WG), specific growth rate (SGR), feed efficiency (FE), protein efficiency ratio (PER) and apparent digestibility coefficient of the dry matter (ADCdm) for any of the diets. Voluntary feed intake (VFI) increased for all seaweed diets compared to the control diet but not significantly (P>0.05). Final weight (FW) was significantly smaller for the 15% P. dioica inclusion and hepatosomatic index (HSI) for the 10% and 15% inclusion. Carcass protein content increased for all three experimental diets, and was significantly higher for the diet with 10% seaweed inclusion. Rainbow trout fed with Porphyra meal presented a dark orange pigmentation of the flesh at the end of the trial, compared to the whitish color from the control fish. These results suggest that P. dioica can effectively be included in diets for rainbow trout up to 10% without significant negative effects on weight gain and growth performance. The pigmentation effect of the fish flesh by adding P. dioica meal to the feed is of a considerable interest to the organic salmon-farming industry.