Gary C. Fry

Reducing impacts of trawling on protected sea snakes: by-catch reduction devices improve escapement and survival

Sea snakes (Hydrophidae) are by-catch of prawn trawling throughout the tropical Indo-western Pacific. We tested the effectiveness of three by-catch reduction device (BRD) types set at different distances from the codend in Australia’s Northern Prawn Fishery (NPF). Trained crew-member observers compared the numbers of sea snakes caught in paired Control and Treatment nets in 1365 trawls. Catches of sea snakes were reduced by 43% on those vessels where a Fisheye BRD was positioned less than 70 meshes from the codend. A separate study with a scientific observer undertook trials with a ‘popeye’ Fishbox BRD. This BRD reduced sea snake catch by 85% and small fish by-catch by 48%. Catches of target prawn were similar for all nets in both studies (difference <2%). Adoption of the more effective BRDs (‘popeye’ Fishbox or Fisheye BRDs) by the NPF and locating them within 70 meshes of the codend can potentially reduce sea snake catch and thus their mortality from ~7000 in the 2007 fishing season to as few as 1500 snakes. Our study shows that the use of BRDs in tropical coastal demersal fisheries and positioning them closer to the codend will greatly reduce the catch of vulnerable sea snakes.

Response of barramundi, Lates calcarifer, populations in the Fly River, Papua New Guinea to mining, fishing and climate-related perturbation

Migrating barramundi (Perciformes : Centropomidae) spawners support a valuable artisanal fishery on the coast of south-western Papua New Guinea. This fishery declined dramatically during the 1990s shortly after the large Ok Tedi copper mine began in the headwaters of the large Fly River nearby. In order to understand the factors causing the decline, populations were sampled quarterly with gill-nets at over 20 sites throughout the Fly River from 1987 to 2001. Barramundi were most abundant in the middle and upper reaches of the Fly River. No evidence was found that output from the large Ok Tedi mine was negatively impacting on barramundi catch rates. However, the commercial fishery in the middle Fly River was found to have a negative impact on the weight of barramundi in monitoring catches in that region. Additionally, catch rates of juvenile barramundi (1 year olds) in the Fly River were negatively correlated with the amount of rainfall on the breeding grounds during the previous monsoon. This suggests that the reduced catch rates in the coastal commercial fishery in the late 1980s and early 1990s may have been affected by both the riverine commercial fishery and the El Nino (ENSO) that occurred at that time.