BROOD
STOCK MANAGEMENT OF MARINE
ORNAMENTAL
FISHES
Introduction
The absence of sexual dimorphism,
the complex patterns of sex change in certain groups and the problems of larval
rearing can be considered as the major reasons for the slow progress in the
culture of marine ornamental fishes. The most important aspect is the lack of
understanding on reproductive strategies and the difficulties of creating the
pelagic environment essential for larval survival. The concepts of breeding and
larval rearing of freshwater ornamental fishes are mostly not applicable in the
case of Marine ornamental fishes. Marine fish in general do not care their
young ones after they have hatched. Since most fish spend their larval period
as part of the plankton, plankton feeding fish quickly eat their own larvae, if
they happened to drift nearby. During their larval stage, fish are totally
dependent on planktonic microorganisms for food and their major movement is
dependent on ocean currents during their early larval life.
Reproduction
of marine fish can be categorized in 4 basic patterns:
1. The most common
is release of tiny, transparent, free-floating eggs with complete absence of
parental care. Angel fishes, butterfly fishes, tangs, groupers, snappers, wrasses
and parrot fishes are among those with this type of reproductive style.
2. The second most
common mode is attachment of the eggs to a secure substrate, usually near the
bottom, with nesting behavior. These are termed as demersal eggs and the
resulting larvae may be large as in the case of clownfish or quite small as with
damsel fish. Gobies, blennies, damselfish and clownfish are the common nest building
marine tropical fishes.
3. Some groups
exhibit oral incubation of eggs or mouth brooding. Instead of attaching the
eggs to the bottom, the male retains them in his oral cavity during the period
of incubation. Cardinal fishes are examples of this category.
4. A very few
marine species such as sea horses give birth to well developed young ones. Mouth
brooders and those fish that lay demersal eggs produce far fewer eggs than fish
that spawn pelagic eggs. These species aerate and protect their eggs from
predators during early development. Nesting and mouth brooding species incubate
their eggs from 3-10 days depending on the species, and the larvae are hatched
with a small residual yolk sac, fully developed eyes and mouth parts and the
ability to swim with purpose and direction. The spawn of species that protect
their eggs varies in number from a low of 50-100 to a high of 10,000-15,000,
whereas those species that spawn pelagic eggs can produce more than 2, 00,000 eggs or more per spawn and may
spawn almost daily during the spawning season. Some species such as Pygmy angel
fish spawn only fewer eggs, (300-500 per spawn) on daily basis. The successful rearing
of marine fish larvae requires consistent care, daily observation, basic
knowledge and a good deal of experience and dedication. Two key bottlenecks
currently limit the expansion of marine ornamental fish Aquaculture. First is the control of maturation and
spawning and the second is the identification of appropriate live food items
for larval first feeding.
Reproduction
One
of the cornerstones to success in marine ornamental fish culture is the
controlled reproduction of brood stock animals to ensure a constant
supply of seed stock throughout the year. Food fish farmers need to concentrate
on one or a few species, whereas farmers engaged in ornamental aquaculture must
maintain a variety of species to provide the product diversity corresponding to
market demands. Currently, about 800 species are actively marketed in the
marine ornamental fish trade, of which only about 30 species (less than 5%) are
bred in captivity. The resulting challenge faced by the marine ornamental
farmer is to determine the appropriate methods to control reproductive processes
in a wide variety of targeted species to allow sufficient market penetration. The
challenge is complicated by the fact that many reef species have complex reproductive
strategies that include the formation of social hierarchies, hermaphroditism and
sex change. Consequently factors that regulate social structure and sex of the
brood stock must be considered carefully when setting up brood stock
population.
Reproductive
development can be divided into two distinct sequential processes:
1. Gonadal growth
and development
2. Final
maturation and spawning
Many species undergo relatively normal
gonadal development in captivity when provided with suitable husbandry and appropriate
environmental parameters. Rearing tank size and shape are important elements to
reproductive success. Tanks that have proven adequate for growth may be
inadequate for captive reproduction. Many ornamental reef species such as Pigmy
angel fish and wrasses display complex and ritualized reproductive behavior
that require deeper tanks and specific reef substrates. Considerable success in
spawning of ornamental fishes is reported through the introduction of live
rocks and other natural substrates in the brood stock tank.
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