Abundance indices for black marlin (<i>Istiompax indica</i>) derived from 50 years of diarized daily catch-effort data from the charter fishery off the Great Barrier Reef, Australia

Abstract The capture of a 1000-pound black marlin off the northern Great Barrier Reef (GBR), Australia, in September 1966 initiated the rapid development of a recreational charter fishery that has operated every year since. The fishery has a distinct 2.5-month-long season, covers the same area off the outer edge of the GBR, specifically targets black marlin and has used essentially similar fishing methods through its history. For the first three decades of the fishery, there was no mandatory logbook requirement. Fortuitously, a study conducted in the late 1990s found that some charter captains and crew had kept detailed personal diaries or logs of daily catch and effort, including zero-catch days since the beginning of the fishery. The earlier study analysed CPUE data extracted from such records for the period 1970 through 1997. Here, we present an extension of this analysis with the addition of new daily records spanning 1998–2023. Nominal CPUE for the early period showed high but highly variable values at the beginning of the series, then a declining trend towards the late 1990s. Standardized CPUE since the early 2000s then showed a gradual increase to match that recorded in the early 1970s. Factors having a significant effect on CPUE were Southern Oscillation index, moon phase, meridional current velocity and direction, month, latitude, and captain. The results demonstrate that long-term personal, unbiased daily fishing records may be used to demonstrate fluctuations in stocks, in this case recording a decline and recovery of black marlin CPUE that may not have been possible from other data sources.