The use of selected Baltic coastal organisms as potential alternative feed material in the formulation of rainbow trout diets was studied. German coastal water organisms (Delesseria sanguinea, seaweed red algae (A); Mytilus spp., Baltic blue mussel (M); Alitta virens, king ragworm (W)) were analyzed for nutrient, amino acid and mineral composition, and tested in comparative feeding trials. Five dietary treatments were supplied to a total of 165 juvenile rainbow trout (778 ± 111 g) for 75 days,

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... llotted in 15 special brackish (3–5 practical salinity units (PSU)) water basins consisting of 11 fish each (3 fish tanks (300 L) at 12 °C per feeding group). The fish were fed as follows: C group, 100% basic diet (control); A group, 10% red algae in C diet; M group, 10% mussel in C diet; W group, 35% ragworm in C diet; AW group, 10% algae + 30% ragworm in C diet. Feed provision was performed manually, once a day, with the feed offer adjusted to 1.8% of fish weight for the respective tank. The fish weight gain (WG) and feed conversion ratio (FCR) were recorded. In the proximate analysis of the different coastal organisms, the lowest crude protein content in dry matter (DM) was found in blue mussels (10.9%), whereas it was almost doubled in algae (21.8%), with the highest being found in the ragworm (63.1%). By contrast, the crude ash content was the highest in the mussel (84.4%, mostly due to CaCO3 from the shell), much less in the red algae (28.1%) and the lowest in the ragworms (20.1%). The gross energy (GE) concentration was the highest in the ragworm (18.8 MJ × kg−1), 12.1 MJ × kg−1 in the algae and the lowest in the blue mussel (2.93 MJ × kg−1). The final weight of the fish ranged between 1780 and 2310 g at the end of the feeding trial, being the lowest for the fish fed the diet combined with red algae (A diet group) and the highest for the fish fed the control diet. No differences in FCR were found for the fish fed the five dietary treatments (p > 0.05), except for the W diet group (king ragworm has a lower FCR than that of the A group red algae, p < 0.05). The results from this trial suggest that at the tested amounts, both king ragworm and blue mussels are promising alternative feed material for rationing the rainbow trout diet, but not red algae, unless combined with ragworms.