Grenadiers (Macrourus spp.) are the main bycatch species in the exploratory longline fishery for toothfish in the Ross Sea. Previous studies concluded that acoustics methods could be used to index the relative abundance of grenadiers and be useful for exploring spatial distribution. Automated acoustic methods were developed to estimate grenadier distribution and abundance based on echo counting. These methods can be applied to large volumes of relatively low quality, opportunistically collected acoustic data. Trials using data from SSRU 88.1I showed positive correlations between acoustic targets and longline catches of grenadiers and toothfish. Single targets were most abundant within 250 m of the bottom at seabed depths of 700–1000 m, and revealed consistent spatial patterns, with higher numbers of targets on the eastern side of the Iselin Bank. The acoustic target strength distribution of single targets was similar to that predicted based on the expected size range of grenadiers. Variability in spatial coverage between years meant that it was not possible to obtain a consistent time-series of relative abundance estimates for grenadiers from acoustic data collected opportunistically by New Zealand vessels in SSRU 88.1I. The next step will be to apply to these methods to a wider set of data across the Ross Sea region.
Recommendation:
We recommend that other nations routinely collect acoustic data in the Ross Sea fishery and make this available for analysis. Collection of acoustic data from a larger pool of vessels would increase the available information, so that more consistent spatial coverage of grenadier distribution might be achieved.