Effects of Storage Time and Thawing Method on Selected Nutrients in Whole Fish for Zoo Animal Nutrition

Simple Summary Fish-eating animals in human care receive whole fish that were frozen, stored and thawed before feeding. Nutrient losses have been documented, but exact changes are unknown. The aims of this study were to evaluate whether or not frozen fish lose vitamins and trace minerals during storage, and if different thawing methods have an influence on the degree of these losses. Whole herring, mackerel, and capelin were analyzed at four time points within a storage period of six months at −20 °C. Each time, three thawing methods were tested: refrigerator, room temperature and running water. The following nutrients were analyzed: vitamin A, B1, D3 and E, iron, copper, zinc, and selenium. Copper was below detection limits in all samples, vitamin B1 in most herring (44/48) and capelin samples (25/36), and vitamin D3 in half of the capelin samples (18/36). Significant decreases of vitamin A, D3 and E concentrations were observed during a storage period of six months. Thawing fish with different methods resulted in a significant change of concentration of vitamin A. It is essential to supplement vitamin B1 and E in diets containing whole fish, and it should not be stored longer than 6 months, due to depletion of vitamin A, D3 and E. Abstract Piscivores in human care receive whole fish that were frozen, stored and thawed before feeding. Nutrient losses have been documented, but exact changes during storage and with different thawing methods are unknown. Primarily, it was hypothesized that frozen fish lose different vitamins and trace minerals during a storage period of six months. Secondly, that different thawing methods have a significant influence on the degree of vitamin loss. Three fish species, herring (Clupeus harengus), mackerel (Scomber scombrus) and capelin (Mallotus villosus) were analyzed at four time points within a storage period of 6 months at −20 °C. At each time point, three thawing methods were applied: thawing in a refrigerator (R), thawing at room temperature (RT), and thawing under running water (RW). The following nutrients were analyzed: vitamin A, B1, D3 and E, iron (Fe), copper (Cu), zinc (Zn) and selenium (Se). The statistical method used was a linear mixed effect model. Cu was below detection limits in all analyzed samples, vitamin B1 in most analyzed herring (44/48 samples) and capelin (in 25/36 samples), respectively. In addition, the vitamin D3 concentration was also below detection limits in half of the capelin samples (18/36). No concentration changes of Fe (p = 0.616), Zn (p = 0.686) or Se (p = 0.148) were observed during a storage period of six months, in contrast to a significant decrease in vitamin A (p = 0.019), D3 (p = 0.034) and E (p = 0.003) concentrations. Thawing fish with different thawing methods did not result in concentration changes of Fe (p = 0.821), Zn (p = 0.549) or Se (p = 0.633), but in a significant concentration change of vitamin A (p = 0.002). It is essential to supplement vitamins B1 and E in diets containing whole fish to avoid deficiencies in piscivorous species, and care should be taken not to store fish longer than six months, due to the depletion of vitamins A, D3 and E.