Acoustic assessment of potential bias in abundance estimates of mackerel icefish from trawl surveys

In 2000, CCAMLR asked the Working Group on Fish Stock Assessment (WG-FSA), whether trawl surveys are the most reliable and appropriate method to survey stocks of mackerel icefish (Champsocephalus gunnari). Recent acoustic data from the South Atlantic suggested that mackerel icefish may not always be distributed closely on the sea floor during the day as conventional wisdom would have it, but that a significant proportion of the stock occurred in midwater. If this is the case, then bottom trawl surveys may underestimate the biomass of mackerel icefish. This paper addresses this question by comparing trawl and acoustic data taken contemporaneously during a survey for mackerel icefish in the Heard Island region (CCAMLR division 58.5.2) in May 2000. The relationship between mackerel icefish catches and the acoustic signal are discussed. The vertical distribution of mackerel icefish in relation to the diel light signal and the potential for biased estimates of abundance from bottom trawl surveys are also investigated.