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

Abstract

The Murray cod is artificially propagated by harvesting naturally spawned fertilised eggs laid in nest boxes by captive, pond-reared broodfish, in mid to late southern spring and early summer. From 1988–1993, a condition referred to as `swollen yolk-sac syndrome' (SYSS) accounted for increasingly significant mortalities of Murray cod eggs and newly hatched larvae, often resulting in total mortality of eggs and larvae during incubation and larval rearing stages. SYSS appears to be similar to the well known `blue-sac disease' (BSD) in salmonids but without the `bluish tinge' coloration of the serous fluid in the yolk-sac attributed to BSD. Early investigations ruled out that SYSS condition was caused by a pathogen, viral, bacterial or a parasitic protozoan, and this led to the subsequent investigations on the possible nutritionally related causes of the syndrome, which are reported here. The eggs of normal and SYSS spawns differed in many respects; the latter were lighter and had less moisture ( P<0.05), but did not differ in the amount of protein and/or lipid content per egg. The free amino acid (FAA) content 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±1.04; 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 the egg types were relatively minor. It was also observed that broodstock, the spawns of which manifested SYSS, when allowed to forage in a relatively large pond, with ample natural food sources, for a period of time, their subsequent spawns tended to be normal. In eggs and larvae of spawns of such rehabilitated females, the amount of individual amino acids of the FAA pool was intermediate to that of normal and SYSS. All evidence appears to indicate that the SYSS syndrome in Murray cod is related to broodstock nutrition, possibly a result of cumulative effects of dietary deficiencies over time.