Gladstone Fish Health Investigation 2011 - 2012 - Western Basin ...

Size structure of mud crabs observed during fishery independent sampling2000 - 2009.Legal Male Crabs Legal Female Crabs Undersize Crabs90%80%70%60%Percentage50%40%30%20%10%0%Gulf(n = 1635)North QLD(n = 4371)Gladstone(n = 1492)Hervey Bay(n = 1898)Moreton Bay(n = 2884)LocationFigure 20. The size structure of mud crabs observed at sites monitored during a fishery independentsampling program conducted along the Queensland coastline 2000 - 2009. Note, (n) refers to the totalnumber of crabs examined at each site.As the size structure from this investigation is similar to historical data for Gladstone there isno evidence to suggest issues with local recruitment.Shell disease and abnormalitiesIn late 2011, commercial fishers were reporting that the occurrence of shell (carapace)abnormalities within the Gladstone mud crab fishery was increasing. These concerns wereinvestigated, as they relate to the health of crab stocks and the viability of commercialcrabbing in the region.Symptoms reported in the abnormal mud crabs were similar to those documented byAndersen (2000) for the description of rust spot shell disease. Lesions on the shell, called‘rust spots’, due to their orange colour, were described by Andersen (2000) along with theassociated histopathology.Rust spot shell disease was first reported by commercial crab fishers in 1994 in mud crabsfrom the Port Curtis area. Investigations into the prevalence of rust spot disease in the PortCurtis area across three sampling occasions between 1998 and 2001 determined prevalencehad declined from 18.3% during 1998/99 to 14.5% in 1999/2000 and dropped further to10.2% by the 2000/2001 sampling event (Andersen and Norton 2001). Although the authorsacknowledge that prevalences reported for 1998/1999 may have been inflated slightly due tothe recording of lesions caused by other factors. The average prevalence across all samplingevents was 14.3%. Andersen and Norton (2001) restricted their analysis to observations oflesions on the dorsal carapace. This information was used to provide a baseline for thepresent survey program.It should be noted that Fisheries Queensland figures include all shell abnormalities, as it wasnot possible to definitively identify rust spot shell disease in the field. Fisheries Queenslandalso documented shell abnormalities regardless of where they occurred on the crab shell(e.g. legs, claws, carapace). Examples of shell abnormalities observed in the field are shownin Figure 21.41