1. Introduction
Ž .
The long-chain highly unsaturated fatty acids HUFAs , particularly eicosapentaenoic Ž
. Ž
. acid EPA, 20:5n y 3 and docosahexaenoic acid DHA, 22:6 n y 3 , are important in
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the nutrition of young marine fish Kanazawa, 1985; Watanabe et al., 1989 . Various investigators have used DHArEPA ratios as an index of the optimal level required for
Ž normal growth and development in fish larvae Koven et al., 1993; Mourente et al.,
1993; Rainuzzo et al., 1994; Reitan et al., 1994; Tocher et al., 1997; Rodriguez et al., .
1998 . This is based on the proposition that optimum DHA and EPA levels are Ž
. determined not on total amounts per se as excess can be harmful but rather on the
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Ž relative proportions of these essential fatty acids EFAs in the diet Watanabe and
. Kiron, 1994; Rodriguez et al., 1997, 1998 . Apart from DHA and EPA, arachidonic acid
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Ž ARA, 20:4 n y 6 has also been recognized as essential for marine fish Castell et al.,
. 1994 . ARA is the main precursor of eicosanoids responsible for osmoregulation,
Ž cardiovascular functions, neural control and reproduction
Mustafa and Srivastava, .
Ž .
1989 . Sargent et al. 1997, 1999 have suggested that desirable ratios of 22:6 n y 3r20:5n y 3r20:4 n y 6 can be useful in determining optimal requirements in fish
larval nutrition. Ž
. Opercular abnormalities in fish affect its morphology Koumoundouros et al., 1997
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and biological performance Andrades et al., 1996; Sumagaysay et al., 1999 . Shortened operculum and distortion of the support cartilage have been described to be signs of
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nutritional deficiency e.g., vitamin C in fish caused by impaired collagen formation Ž
. and support cartilage formation Halver et al., 1975 . Some researchers have theorized
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that opercular deformities e.g., milkfish are caused by mechanical stress, especially Ž
during egg collection, transport or routine hatchery operations Toledo et al., 1996; .
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Hilomen-Garcia, 1997, 1998 . Toledo et al. 1996 even recommended transporting milkfish eggs at C-shaped embryonic stage to improve viability. Other causes such as
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genetic variations or factors, however, cannot be discounted Sargent, 1995 . Recently, Ž
. Gapasin et al.
1998 observed that the incidence of opercular deformities among hatchery-reared milkfish larvae could be alleviated by feeding them live food supple-
mented with EFAs and vitamin C. The DHArEPA ratios reported in that study ranged from 0.33–0.74 for HUFA-enriched rotifers and Artemia nauplii while values for the
unenriched live food were very low ranging from 0.01–0.04.
This follow-up study was therefore conducted to determine whether increasing the Ž
. DHA levels and the corresponding DHArEPA ratio to a value of at least G 1.0 in live
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food organisms using commercially available enrichers could further improve milkfish larvae performance.
2. Materials and methods
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