Estimating uncertainty associated with acoustic surveys of spawning hoki (Macruronus novaezelandiae) in Cook Strait, New Zealand

Eleven acoustic surveys carried out between 1991 and 2002 provided estimates of the relative abundance of spawning hoki in Cook Strait, New Zealand. The precision and bias of each survey were estimated using a new Monte Carlo simulation method, which combined uncertainties associated with survey timing, sampling error, detectability, species composition, target strength, calibration coefficients, and missing strata. Because hoki have a long spawning season (more than 2 months) with turnover of fish in the survey area, survey timing was the most important source of uncertainty. Uncertainty was reduced by having a number of sub-surveys (snapshots) over a 4–6 week period, centred on the middle date of the spawning season. The other major source of uncertainty was the occurrence of 40–70% of hoki in mixed species: “hoki fuzz” marks. The acoustic analysis assumed all acoustic backscatter from hoki marks was hoki, so the presence of other species caused a positive bias in relative-abundance indices. The magnitude of this bias differed between years because the proportion of hoki in the “fuzz” marks was variable. There was additional uncertainty from the variability in the species composition of mixed marks that affected survey precision. The abundance indices were corrected for estimated bias, and the estimate of precision, expressed as the coefficient of variation or c.v., was applied to weight the results from each acoustic survey in the assessment model used to set commercial-catch limits.