Abstract: A failure to account for changes in fish spatial behaviour as populations declined due to fishing precipitated the collapse of some of the world’s most important fisheries, notably north Atlantic cod. The mechanism underlying many of these collapses was density-dependent catchability, where vulnerability to capture increases as populations decline. Density-dependent catchability often occurs in species that aggregate but has rarely been demonstrated in coral reef fisheries, even though schooling and the formation of spawning aggregations are common. However, density change may not serve as an ideal proxy for vulnerability to fishing since several other factors, such as gear saturation, are known to influence catchability. Moreover, density-dependent catchability leading to hyperstability in catch-per-unit-effort (CPUE) requires that fishers are able to locate remaining areas of high density as populations decline. In reef fisheries, where fishers lack sophisticated technology such as sonar to locate fish, fisher perceptions of the predictability of aggregations will be therefore play an important role in their vulnerability to fishing. My PhD aims to disentangle the mechanisms and factors that influence catchability in reef fishes that aggregate. This aim will be achieved through four research questions. Firstly, the PhD will examine how fisher perceptions on the predictability of reef fish aggregations in Papua New Guinea (PNG) relate to other socio-economic factors that influence vulnerability to fishing, such as gear selectivity. Secondly, the PhD will determine the role of inter-specific differences in competition for baited gear and depth in modifying density-dependent catchability at a multispecies grouper spawning aggregation site, also in PNG. Thirdly, research at a rabbitfish spawning aggregation site in Seychelles will determine the relative importance of density-dependence and gear saturation (effort-dependence) on catchability. Fourthly, a framework for predicting the vulnerability of aggregative spawners to fishing will be developed and tested based on a fuller understanding of the factors that influence catchability. Understanding catchability is important for managing reef fisheries since resource users and managers often perceive resource status from CPUE. Density-dependent catchability can cause ‘an illusion of plenty’ and result in fishing effort remaining high or even increasing as populations decline, rapidly forcing the population towards critical thresholds of collapse.

Biography: Jan grew up in Manchester but moved to the UK coast for his BSc (Marine Biology, University College of Swansea) and MSc (Applied Marine Science, University of Plymouth) degrees. After a start in pollution ecology with Plymouth Marine Laboratory, His head was turned by the allure of tropical coasts and he headed out to Seychelles with the Royal Geographical Society’s Shoals of Capricorn programme in 1998. He never left Seychelles, becoming Manager of Fisheries Research for Seychelles Fishing Authority (SFA) and developing a particular interest in how reef fish spatial behaviour influences their assessment and management. He has led and participated in regional research and fisheries management programmes funded by the Western Indian Ocean Marine Science Association, the World Bank and FAO, as well as initiating GEF-funded co-management initiatives for coastal fisheries in Seychelles. Seychelles is an economy founded on large pelagic fish rather than coral reef fish and Jan chaired the Indian Ocean Tuna Commission (IOTC) Working Party on Billfish from 2008 to 2011. Since 2011, he has been Vice-Chair of the Scientific Committee of IOTC. After realising that he was increasing tied to his desk and had little time to develop research interests, he decided the time was right to study for a PhD.