Acoustic evidence for unusual diel behaviour of a mesopelagic fish (Vinciguerria nimbaria) exploited by tuna

Abstract

For 15 years, there has been a seasonal tuna fishery in the open equatorial Atlantic Ocean. There are no good reasons for explaining this concentration of fish, except the possible abundance of micronekton. However, micronekton are only found during the night in the upper layers, whereas tuna feed during the day, according to current assumptions. An acoustic cruise carried out in November 1992 clearly showed the presence in this area of a large schooling biomass of a small mesopelagic fish, Vinciguerria nimbaria (Photichtyidae). Instead of diving to a great depth during the day as usual, they remained in the upper layers and tuna (Katsuwonus pelamis and Thunnus albacares) were found to feed on them. The atypical behaviour of Vinciguerria involved two processes: reverse phototaxis and a break in the diel vertical migrations. A review of the literature shows that this fish: (1) is a common item in tuna stomach contents; and (2) is never caught in the upper layers during the day. This apparent discrepancy has been resolved by our results: schooling behaviour during the day prevents Vinciguerria from being sampled adequately by the usual systematic net tows. The need to use acoustics for successful sampling is obvious. It is likely that this behaviour occurs in other areas and could explain the local concentration of tuna.