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

Of the 58 crabs observed to have shell abnormalities in the field, 9 (15.5%) had lesionslocated on areas of the shell other than the dorsal carapace (legs, claws, abdomen andthorax).Lesion grade ratings recorded by Fisheries Queensland field staff and those recorded byBiosecurity Queensland from June/July sampling showed a significant (P < 0.001) degree ofassociation, with the counts listed in Table 5. There was agreement between field andlaboratory assessments in 79.71% of these cases. Interestingly, the field assessment washigher than the laboratory assessment in 14.49% of these cases, whereas the reverse onlyoccurred in 5.8% of the crabs. These results provide confidence for the field assessmentsmade by Fisheries Queensland staff when recording the presence and severity of lesions inthe field for both Phase 1 and Phase 2 of the program.Table 5. Highest lesion grade for each crab as assessed by Fisheries Queensland in the field andBiosecurity Queensland in the laboratory. The grey numbers indicate agreement between the twoassessments.LaboratoryAssessmentLesion Grade(0 – 5)Field AssessmentLesion Grade (0 – 5)0 1 2 3 4 50 47 0 2 4 1 01 2 1 2 0 0 02 0 1 2 0 0 13 0 0 0 1 0 04 0 0 1 0 1 15 0 0 0 0 0 3Considering the prevalence of rust spot shell disease documented by Andersen and Norton(2001), the potential bias by Fisheries Queensland staff to overestimate the prevalence ofshell lesions during field observations, and that observations of shell lesions were notrestricted to the dorsal carapace during this program, these results of 3.9% shell lesionssuggest that the prevalence of rust spot shell disease has decreased from levels of 10.2%previously recorded by Andersen and Norton (2001).It was assumed that the number of crabs with shell lesions that were reported in Phase 1were not under reported, as the above statistical analysis of Phase 2 for mud crabs did notidentify under reporting, and the staff and field methods were the same between Phases 1and 2. The combined results of Phase 1 and 2 do not indicate changes in the frequencies ofoccurrence over the sampling period or sampling location in the Harbour. The level of shelllesions observed during Phase 1 and 2 was significantly lower than the 37.8% reported formud crabs from Gladstone by Landos (2012), and lower than Andersen and Norton (2001).Andersen and Norton (2001) found the highest average percentage in Gladstone of 18.3% in1998/99, which reduced to 10.2% in 2001.Laboratory findings Phase 2NecropsyThere was no significant difference (P = 0.68) between the hepato-somatic index ofapparently normal and abnormal crabs (0.057 ± 0.002 vs. 0.054 ± 0.002). Hepato-somaticindex (HSI) is defined as the ratio of liver weight to body weight. HSI is generally used as anindication of energy stores, and these results would suggest that the presence of shelllesions is not having a significant effect on feeding.44