This paper describes a new method for estimating illegal, unregulated and unreported (IUU) catches of fish and by-catch of birds. It utilises high quality, well-documented fisheries protection vessel (FPV) cruise data. The method takes explicit account of both ‘seen’ and ‘unseen’ IUU fishing by using a simulation model to arrive at statistically rigorous estimates of, and confidence intervals for, fish and bird catches by IUU vessels. In order to estimate an IUU catch there must have been at least one encounter with an FPV in a year; no encounters in a year are interpreted as zero IUU fishing. For each IUU incident detected by the FPV, a theoretical maximum time over which this IUU activity could have occurred was calculated. This was then converted to estimated actual IUU fishing time, using a model simulating IUU vessel and FPV behaviour. IUU activity was considered to have been observed when the IUU vessel and the FPV vessel were in the same place at the same time. When this occurred, the FPV was assumed to detect IUU activity according to an ‘encounter probability’. The encounter probability was estimated from the known encounters of FPVs with licensed vessels. The total annual IUU catch of toothfish and birds was calculated using a second simulation model. Subarea 48.3 was divided into six areas for the purposes of the estimation of fish and bird catch associated with IUU fishing. The mean and variance of fish catch rate was calculated for each area and each year using reported catch and effort data. The catch rate of birds was estimated separately for summer and winter by bootstrapping previously published CCAMLR observer data weighted by the number of hooks observed. These data were obtained in the early licensed fishery (1997) when few vessels used mitigation measures.

Three years were analysed: 1998/99, 1999/2000 and 2000/01. Each year fully covered the period 1 October to 30 September, thus including one summer and one winter period. The estimated mean toothfish catches attributable to IUU fishing were 776, 1 019 and 198 tonnes in 1998/99, 1999/2000 and 2000/01 respectively (a total over the three years of 1 994 tonnes). The distribution of estimated total bird catch was highly skewed, with medians in the three years of 621, 2 852 and 550 birds respectively. 95% confidence limits were calculated to be 54–2 017, 468–1 779 and 22–487 tonnes respectively for fish and 162–7 752, 1 040–23 361, 140–9 023 respectively for birds. Despite continuing high levels of FPV coverage from 2001 to 2004, no further IUU activity has been detected, leading to estimates of zero fish and bird catches for the years 2001/02, 2002/03 and 2003/04.