Recreational fishing of sharks in the Great Barrier Reef World Heritage Area: species composition and incidental capture stress

Even though recreational fishing is a very popular activity in the Great Barrier Reef World Heritage Area (GBRWHA), with approximately 180,000 recreational fishers living in the area, catch composition data for sharks caught by this sector are very limited. Fisheries Queensland conducted diary and telephone surveys and estimated the recreational catch of sharks to be higher than the state's commercial catch; however such surveys have not provided information on the species composition of the catch. Despite the large number of sharks being caught by recreational fishers in the GBRWHA, the surveys conducted by Fisheries Queensland found that most of the sharks are released. While this may be positive, there is potential for high post-release mortality rates, which is likely to be the greatest effect of recreational fishing on sharks in the GBRWHA. Different shark species have distinctive life history characteristics which can influence their relative vulnerability to fishing pressure; therefore the effective assessment and management of shark fisheries require collection of catch data on a species-specific basis. Because previous research has shown that most recreational fishers cannot accurately identify their shark catch, collection of species composition information cannot rely solely on recreational fishers' knowledge. The aims of this project were to engage the recreational fishing community of the GBRWHA in shark research to describe the species composition of the recreational incidental catch of sharks, and assess capture and handling effects on their post-release survival to determine appropriate handling and release protocols. Catch composition data were collected through individual fishers, charter operators and fisheries-independent sampling. Individual fishers and charter operators provided photos and tissue samples of sharks they caught. Charter operators also allowed the researcher to go on board free of charge to collect data. A total of 209 sharks were identified during the study consisting of 17 species representing four families. The family Carcharhinidae was the most diverse, with 12 species, and the most abundant constituting 86.8% of the catch. The morphologically identical Australian blacktip shark Carcharhinus tilstoni and common blacktip shark Carcharhinus limbatus were the most abundant species. Size ranged from 420 mm for a milk shark Rhizoprionodon acutus caught inshore to 2200 mm for a tawny nurse shark Nebrius ferrugineus caught on a mid-shelf reef. The species composition of the recreational catch of sharks in the study area had an 80% overlap with the composition of the inshore commercial gill-net catch. The results demonstrate that recreational catch of sharks is very diverse in the GBRWHA waters and the overlap with the commercial catch composition means that both sectors need to be involved in the assessment and management of these species. Capture and handling physiological stress was measured using whole blood lactate concentration as an indicator of stress caused by angling duration and air exposure in five species of shark. Increased angling and air exposure durations caused a significant increase in whole blood lactate concentration in all five species of shark examined. However, the maximum whole blood lactate concentrations measured in line-caught sharks were lower than in individuals of the same species caught by other fishing methods such as gill-net and longline. There was some degree of individual variation in response to stressors within each species but there was limited variation between species and between sexes. Post-release monitoring, good release conditions and relatively low whole blood lactate concentrations suggested that all sharks caught in the study had a high likelihood of post-release survival. The results of this study demonstrated that the capture and handling practices of recreational fishers that do not target sharks have only limited impact on shark's postrelease survival. As such, their practices are unlikely to directly cause mortality of sharks however there is potential to sub lethal effects not measured in this study to have significant effects on shark populations. Nevertheless, until recreational catches are properly quantified and identified and are included in stock assessments together with other sources of mortality it is not possible to conclusively affirm that recreational fishing is not impacting on shark populations