Estimated biomass of vertically migrating mesopelagic fish off New Zealand

Abstract

The regional distribution and abundance of mesopelagic fishes migrating from deeper waters into the epipelagic zone over the Chatham Rise, the Campbell Plateau and Snares Shelf to the east and south of New Zealand is unknown, despite mesopelagic fish being important prey for the hoki ( Macruronus novaezelandiae Hector 1871) that support New Zealand's largest fishery. We calculate the biomass of mesopelagic fish (primarily Maurolicus australis, Lampanyctodes hectoris and Symbolophorus boops) that migrate into the upper 200 m during the “night” using acoustic backscatter and new estimates of target strength. The Chatham Rise supported 665×10 3 t of mesopelagic fish in December 2000–January 2001, compared to 438×10 3 t of mesopelagic fish on the Campbell Plateau/Snares Shelf in November–December 2000 (the regional surveys are separated by ≈1 month). There were strong regional differences in biomass. Mesopelagic fish biomass was greater in the Mernoo Bank region of the western Chatham Rise (18.2 t km −2) than on the eastern Chatham Rise (0.4 t km −2) . Highest area backscattering coefficients for the subantarctic region occurred on the Snares Shelf (13.4 t km −2) rather than on the Campbell Plateau (0.3 t km −2) . Mesopelagic fish biomass on the eastern Chatham Rise and Campbell Plateau were comparable to acoustic estimates from the mid-slope south of Tasmania (0.7 t km −2) . The biomass densities of mesopelagic fish on the Snares Shelf off New Zealand were of magnitude similar to the highest acoustic estimates from the Arabian Sea (13.1– 13.8 t km −2 ). The biomass measured in the Mernoo region off New Zealand ≈1.3 times the highest acoustic estimates in the Arabian Sea. The regional distribution of high density patches of mesopelagic fish measured acoustically were spatially coincident with the largest trawl catch rates for hoki on the Chatham Rise. This suggests that hoki abundance is related to the spatial distribution of their food resources.