Incidence and composition of calculi in the urinary bladder of intensively reared marine fish larvae

Calculi were observed in the urinary tracts of a wide range of intensively reared marine fish larvae at the Port Stephens Research Centre. Data are presented for three species: snapper Pagrus auratus, mulloway Argyrosomus hololepidotus, and Australian bass Macquaria novemaculeata. Calculi first appeared in P. auratus on Day 5 after hatching, in M. novemaculeata on Day 11, and in A. hololepidotus on Day 12. Around 80% of larvae typically had calculi after two weeks of intensive rearing. No relationship could be found between the absence of the functional swim bladder and the presence of calculi in all species studied. The presence of calculi did not stop the larvae of any species from actively feeding on live prey. Calculi from M. novemaculeata and P. auratus were analysed with an electron probe microanalyser. Phosphorus and calcium were detected in all calculi, usually accompanied by magnesium. Potential causes for the formation of calculi are discussed.