Approaches to monitoring and assessing the abundance of rattails (Macrourus spp.) and skates in the Ross Sea

This report presents results from a desktop study to consider approaches to monitoring and assessing rattails and skates, which are major bycatch species in the exploratory fishery for toothfish in the Ross Sea. We review standardised CPUE analysis, quantitative research longline surveys, experimental manipulation of fishing effort, catch-curve analysis, tagging programmes, bottom trawl surveys, and acoustic surveys.
We recommend that a random bottom trawl survey would be the best approach towards obtaining abundance estimates for rattails and skates in the Ross Sea. The major advantage of this approach is that preliminary stock assessments could be carried out for both species groups after only one successful trawl survey. Simulations have indicated that only 35–40 trawls would be required in the depth range 600–1500 m to obtain a precise estimate of bycatch abundance in the area of highest densities (SSRUs 881E, G, H, I, J, and K). A trawl survey could also be used in conjunction with other methods of monitoring abundance, e.g., skates caught during the trawl survey could be tagged and released, rattails could be aged for catch-curve analysis. The main limitations of this approach is the variable ice cover in the Ross Sea, which may restrict access to some areas, the rough bottom topography, and concerns about the environmental impact of bottom trawling on benthic communities. Tag-recapture experiments for skates and experimental manipulation of fishing effort are alternative methods which show some promise for monitoring abundance.