Characterisation of 'swollen yolk-sac syndrome' in the Australian freshwater fish Murray cod, Maccullochella peelii peelii, and associated nutritional implications for large scale aquaculture

The Murray cod is artificially propagated by harvesting naturally spawned! fertilised eggs laid in nest boxes by captive, pond-reared broodfish, in mi d to late southern spring and early summer. From 1988-1993, a condition ref erred to as 'swollen yolk-sac syndrome' (SYSS) accounted for increasingly s ignificant mortalities of Murray cod eggs and newly hatched larvae, often r esulting in total mortality of eggs and larvae during incubation and larval rearing stages. SYSS appears to be similar to the well known 'blue-sac dis ease' (BSD) in salmonids but without the 'bluish tinge' coloration of the s erous fluid in the yolk-sac attributed to BSD. Early investigations ruled o ut that SYSS condition was caused by a pathogen, viral, bacterial or a para sitic protozoan, and this led to the subsequent investigations on the possi ble nutritionally related causes of the syndrome, which are reported here. The eggs of normal and SYSS spawns differed in many respects; the latter we re lighter and had less moisture (P < 0.05), but did not differ in the amou nt of protein and/or lipid content per egg. The free amino acid (FAA) conte nt of the two types of eggs were different however, when all the essential amino acids (EAA) and non-essential amino acids (NEAA) of the FAA pool were found in significantly lower amounts (nmol/egg) in SYSS eggs (EAA-11.84 +/ - 0.0.90; NEAA-10.68 +/- 0.66) compared to normal eggs (EAA-21.67 +/- 1.M; NEAA-16.64 +/- 1.26). On the other hand, the differences in the total amino acid (TAA = protein bound + free) content and the fatty acid content of th e egg types were relatively minor. It was also observed that broodstock, th e spawns of which manifested SYSS, when allowed to forage in a relatively l arge pond, with ample natural food sources, for a period of time, their sub sequent spawns tended to be normal. In eggs and larvae of spawns of such re habilitated females, the amount of individual amino acids of the FAA pool w as intermediate to that of normal and SYSS. All evidence appears to indicat e that the SYSS syndrome in Murray cod is related to broodstock nutrition, possibly a result of cumulative effects of dietary deficiencies over time. (C) 1998 Elsevier Science B.V. All rights reserved.