PROGRESS TOWARD
DEVELOPMENT OF A MICRODIET TO REPLACE LIVE FOOD ORGANISMS IN MOLLUSKS AND
LARVAL FISH AND CRUSTACEAN CULTURE
J.D. Castell,
T.J. Blair
Abstract:
Unlike
salmonids first-feeding marine fish (haddock, halibut, flounder etc.) cannot
be reared with formulated feeds. There are a number of possible reasons for
this difference. Many marine fish larvae have a poorly developed
gastrointestinal tract consisting virtually of a tube from mouth to anus,
with slight enlargement where the stomach will develop. It takes a number of
days or weeks until the full compliment of digestive enzymes is in full
production. The movement of live food organisms may be required to stimulate
the feeding response of some fish larvae. Marine fish larvae are much
smaller than salmon hatchlings and the particle size must be very small 50
to 200 µm to be consumed. Because of the very small particle size required,
many water-soluble nutrients are leached from such food particles almost
instantly in water, leading to nutritional deficiencies in larvae feeding on
such small food particles. In spite of these obstacles, significant progress
has been made in recent years in developing microdiets for partially
replacing live food organisms in larval fish culture. The loss of
water-soluble nutrients can be reduced by microencapsulation on special
binders. There has been some success in producing lipid walled or lipid
spray-beads that contain water-soluble nutrients. Cross-linked
protein-walled microcapsules have been used to successfully deliver
carbohydrates, proteins and other macronutrients to larval fish. Another
approach is the production of liposomes to encapsulate nutrient supplements
for larval fish. It is probable that a microdiet combining two or more of
these different technologies will be required to provide a diet capable of
supplying all the nutritional needs of marine fish, crustacean and mollusks
in artificial or formulated feeds. While we will be able to reduce the
requirements for live foods; it will be some time before the requirement for
live feeds will be eliminated in larvae culture.
(Department of
Fisheries and Oceans, Biological Station, 531 Brandy Cove Road, St. Andrews,
NB E5B 2L9, Canada; e-mail: CastellJ@mar.dfo-mpo.gc.ca)