Salinity acclimation and free amino acid enrichment of copepod nauplii for first-feeding of larval marine fish

Abstract

Live foods used in a marine fish hatchery must provide the nutrients needed for the rapidly developing larvae. Marine pelagic fish eggs can contain up to 50% of their total amino acid pool as free amino acids (FAA), of which most is used up shortly after the onset of exogenous feeding as a source of energy. Copepod nauplii are richer in FAA than many traditional foods given to marine fish larvae. However, copepod nauplii can be difficult to produce in mass quantities in the hatchery and are more readily available by capture from the wild. The process of capture and acclimation may impact the nutrient quality of such nauplii. This study describes the FAA profile of nauplii of Apocyclops panamensis collected from low salinity fertilized brackish-water ponds, the effects of acclimation from 6.6‰ to 30‰ seawater on the FAA profile, and the feasibility of direct enrichment of nauplii with isoleucine, leucine, lysine and valine. Nauplii were exposed to four treatment protocols over a 3 h period and sampled for FAA profiles, 1) no salinity change, 2) direct transfer to 30‰, 3) acclimated to 30‰, and 4) acclimated + FAA enrichment. Nauplii collected from 6.6‰ salinity ponds had a mean total FAA content of 27.22 ± 5.85 nmol · mg −1 w.w. nauplii at the time of collection. Forty-two percent of the total was comprised of essential amino acids of which arginine, lysine, leucine and valine were the most prevalent, and 55% were non-essential amino acids, primarily glutamic acid, alanine, proline, asparagine and glycine. A direct transfer of nauplii from a salinity of 6.6‰ to 30‰ resulted in a significant reduction in total FAA (TFAA) particularly in essential FAA (EAA). Acclimation of nauplii from a salinity of 6.6‰ to 30‰ over 3 h resulted in a significant increase in TFAA and particularly in non-essential FAA (NEAA). The addition of isoleucine, leucine, lysine and valine to the holding water during acclimation further increased their concentrations in the nauplii after a 3 h exposure.