Movement of sea mullet Mugil cephalus as indicated by a parasite

Most sea mullet in SE Queensland have numerous brown patches over the surface of the intestine. These are the remains of eggs of the blood fluke Plethorchis acanthus. Adult parasites are rarely found and when they occur are not within blood vessels but on the surface of the viscera over which they crawl as they lay their eggs. Infections apparently originate when fish are juveniles in fresh water. The abundance of the parasite in SE Queensland and the absence of the parasite from mullet further south indicates that the wave of fish that appears to move up the coast in the autumn may be an illusion. The parasite data suggest that large numbers of fish are not moving from northern NSW into SE Queensland. It is more likely that the apparent seasonal wave is caused by fish sequentially leaving local estuaries for the sea as the temperature drops in autumn.