Fish meal replacement by plant protein substitution and guar gum addition in trout feed, Part I: Effects on feed utilization and fish quality

Abstract In a two-factorial experiment the effects of three rainbow trout diets were tested with respect to growth and health of fish stock, product quality and sustainability. The diets, comprising widely available and affordable raw ingredients, were iso-energetic and iso-carbohydrated, but contained protein of different provenances as follows: 100% of protein from fishmeal (FI diet), 50% of protein from fishmeal and 50% from plant sources (FP diet), and 100% plant protein supplemented with methionine/lysine (PL) diet. Three further diets were formulated, replicating FI, FP and PL, but enriched with 0.3% guar gum binder to counteract possible negative effects of excreted waste characteristics. Fish performed well on all diets. The observed significant differences were attributable almost entirely to protein provenance rather than to the presence or absence of binder. The post mortem data on gross body composition revealed that fillets from fish fed the PL diet were significantly leaner, with a greater percentage protein content than those from individuals raised on the FI or FP diets. Pathological alterations observed in the livers of fish fed the FI diet were in the expected range for fish reared on modern high energy diets. However, pathology was significantly reduced in fish receiving plant protein in their feed. Individuals raised on the PL diet exhibited almost completely healthy livers. The main shortcoming of the plant protein diets was a reduction in lipid digestibility, leading to slight depression in growth and feed conversion rates. The potential value of plant proteins as components in feeds to reduce eutrophication by fish farm wastes was confirmed. Their low intrinsic phosphorus content allows dietary levels to be easily adjusted to meet, but not exceed, the physiological needs of the fish, thus minimizing the excretion of excess phosphorus. A strong effect on hepatosomatic index (HSI), was observed, but was ascribed to the greater availability of carbohydrate in plant diets rather than to the origin of protein. Assessment of flesh quality parameters revealed slight differences between treatments, although these were not pronounced enough to be reflected in the results of organoleptic trials by a sensory test panel. In conclusion, even a 100% substitution of fishmeal proteins by affordable plant protein provides a competitive feed. Furthermore, some surprising health benefits for fish stock were also noted. The next challenge in developing a successful plant-based diet is to understand and counteract the effects on lipid digestibility and to identify the functional component that may be affecting fish health.