Informing the vulnerability of species to spawning aggregation fishing using commercial catch data

Fish spawning aggregations (FSAs) present a unique challenge for fishers and managers as aggregation fishing can be economically rewarding yet lead to overfishing and population collapse. Assessing the vulnerability of FSAs to fishing is often problematic due to a paucity of data. We present a tiered analysis for interrogating fishery logbook data for signals that fishing targets FSAs. We use two line fisheries that operate on Australia's Great Barrier Reef as models. The Reef Line Fishery (RLF) and Pelagic Line Fishery (PLF) operate across 9° of latitude, fishing occurs year round and the RLF has a seasonal spawning closure. A recent review reduced temporal closure days from 27 days to 10 days per year, though in the absence of any quantitative assessment of the vulnerability of targeted fish to increased harvest and hence risk. These fisheries target three reef fish groups (common coral trout Plectropomus leopardus, redthroat emperor Lethrinus miniatus, and a “other species” group mostly comprised of lethrinids and lutjanids) and a pelagic Scomberomorus commerson (narrow-barred Spanish mackerel). The tiered analyses captured the complete annual cycle of fish and fisher interaction across all latitudes and explored a number of catch metrics (landed catch, catch-per-unit-effort, and timing and size of large catch anomalies). No clear evidence of increased vulnerability of any of the three coral reef fish groups to fishing during spawning was found. Conversely the pelagic S. commerson is clearly more vulnerable to fishing during spawning. The tiered analytical approach is recommended as increased vulnerability of FSAs to fishing may not be identified by single analyses and without relative comparisons among species, the most vulnerable may be overlooked.