Acoustic estimates of mesopelagic fish: as clear as day and night?

O'Driscoll, R. L., Gauthier, S., and Devine, J. A. 2009. Acoustic estimates of mesopelagic fish: as clear as day and night? – ICES Journal of Marine Science, 66: 1310–1317. The pelagic ecosystem on the Chatham Rise east of New Zealand has been monitored annually using acoustic surveys since 2001. Most of the acoustic backscatter comes from diurnally migrating mesopelagic fish, which are the major prey of hoki (Macruronus novaezelandiae) and other valuable commercial species. Mesopelagic schools and layers typically occur at 100–400 m depth during the day and migrate above 200 m depth at night. Previous acoustic estimates of the biomass of mesopelagic fish on the Chatham Rise were based only on night-time estimates from the upper 200 m and may significantly underestimate actual biomass. Paired day–night comparisons found that an estimated 20% of the total daytime backscatter migrates to depths of 0–14 m at night, where it is too shallow to be detected by hull-mounted acoustic instruments. In contrast, there is a positive bias associated with acoustic estimates of mesopelagic organisms during the day because they can occur close to the bottom mixed with demersal fish. Two alternative approaches for estimating biomass of diurnally migrating mesopelagic species are suggested. The first applies a correction based on the estimated proportion of backscatter migrating into the surface deadzone to night-time estimates of backscatter in the upper 200 m. The second uses night-time estimates of demersal backscatter, which remains deeper than 200 m at night, to correct daytime estimates of total backscatter. Both methods gave similar estimates of abundance and demonstrated that there was no statistically significant trend in mesopelagic fish biomass on the Chatham Rise over the past seven years.