|Overturf, Kenneth - Ken|
|GAYLORD, GIBSON - Us Fish And Wildlife Service|
|SEALEY, WENDY - Us Fish And Wildlife Service|
Submitted to: Aquaculture Nutrition
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/12/2014
Publication Date: 2/2/2015
Citation: Welker, T.L., Barrows, F., Overturf, K.E., Gaylord, G., Sealey, W. 2015. Optimizing zinc supplementation levels of rainbow trout (Oncorhynchus mykiss) fed practical type fish meal and plant based diets. Aquaculture Nutrition. 21:1-17.
Interpretive Summary: Fish meal (FM) is the primary protein source used in commercial rainbow trout diets. It is a complete protein source that is easily digested by trout. However, demand for FM has increased in recent years due to expansion and intensification of aquaculture production worldwide. This increased production has continued to place high demand on FM for use in fish feeds, and the price of FM has nearly tripled since 2000. Therefore, trout producers and feed manufacturers have looked to increase incorporation of low-cost alternative protein sources in feeds for trout. The majority of alternative protein sources is derived from plants. Soybean meal has been the most widely used alternative protein source and successfully incorporated into diets of rainbow trout and other fish species. However, recent findings suggest that many plant protein sources may be low in some vitamins and minerals compared to FM. Zinc is a mineral essential to the diet of trout, but the dietary zinc requirement for rainbow trout was established as 15-30 mg/kg over 30 years ago using diets containing animal protein sources, but little work has been conducted to determine dietary zinc requirements for rainbow trout using plant-based practical diets. Movement away from traditional protein sources, such as FM, and increased use of plant meals which are lower in zinc and high in phytic acid, may increase the likelihood of inadequate dietary zinc levels and zinc deficiency in trout fed diets high in plant products. The purpose of this study was to determine the optimal supplementation level for zinc requirement for juvenile rainbow trout fed practical, all-plant diets. Rainbow trout were fed either a FM or plant-based diet supplemented with various levels of zinc (0, 15, 30, 60, or 120 mg kg-1) for 12 weeks. Trout fed the FM diet had significantly higher weight gain than the plant-based diet and suggests that the FM does not require additional zinc supplementation. Trout fed the plant-based diet containing no zinc exhibited severe growth retardation, presence of cataracts, elevated levels of stress, and disruption in physiological functions. Addition of 30 mg kg-1 or higher dietary zinc produced normal growth and alleviated signs of deficiency. We concluded that that the dietary supplementation level of zinc needed to prevent deficiency and promote adequate growth in rainbow trout fed the plant-based diet in this study was 30.1 mg kg-1. This level is higher than the current dietary recommendation of 15 mg kg-1 for rainbow trout provided by the National Research Council (NRC). Following the NRC recommendation could lead to zinc deficiency in rainbow trout fed a plant-based diet.
Technical Abstract: Fish meal (FM) is the primary protein source used in commercial feeds for rainbow trout. However, FM is a finite resource and availability is increasingly limited due to continued expansion of fish culture worldwide. Limited availability has caused a significant increase in the price of FM. Therefore, trout producers have sought to increase the use of low-cost, alternative protein sources, most of which are derived from plants. Recent evidence suggests that many of these plant protein sources may be deficient in certain vitamins and minerals. Zinc is an essential mineral required in the diet of rainbow trout, but the dietary requirements for trout were established over 30 years ago with diets containing animal protein sources. There is a need to re-examine the dietary zinc requirement for rainbow trout fed plant-based diets. We investigated the zinc requirements of rainbow trout fed either a FM diet or a plant-based diet supplemented with various levels of zinc (0, 15, 30, 60, or 120 mg kg-1) for 12 weeks. Trout fed the FM diet had significantly higher weight gain than those fed the plant-based diet. Zinc supplementation in the FM diet had no effect on growth performance, suggesting that additional dietary supplementation of zinc is not required. However, in trout fed the plant-based diet, growth increased significantly up to 30 mg kg-1 zinc after which growth was not affected. Trout fed the plant-based diet containing no zinc exhibited severe growth retardation, and fish fed the 0 and 15 mg kg-1 zinc diets developed cataracts. Whole body zinc levels increased significantly as zinc increased in the diet for the fish fed both FM and plant-based diets. Using broken-line quadratic modeling we concluded that at least 30.1 mg kg-1 zinc was necessary to promote adequate growth in rainbow trout fed the plant-based diet. This amount is higher than the NRC (2011) dietary recommended level of 15 mg kg-1 and suggests that following the NRC recommendation for dietary zinc could lead to deficiency and reduced physiological performance in rainbow trout fed plant-based diets.