First Report on the Use of the Larval Catcher Type Trawl Net for the Efficient Sampling of Juveniles of Two Tuna Species (Thunnus orientalis and T. albacares) in the Offshore Waters of the Nansei Islands, Japan

Abstract

Establishing sampling techniques for juveniles of tuna species is essential for understanding their ecology and population dynamics because only individuals showing rapid growth during larval and juvenile stages are able to survive. This study is the first report on the successful sampling of juveniles of tuna species using the larva catcher (LC)-type trawl net, a pelagic trawl gear with a 10 × 10-m mouth opening, 40-m total length, and 5 × 5-mm mesh size. It was used in a research cruise in May-June 2009. The LC net was horizontally towed for 30 min at depths of 0 m-10 m at speeds of about 1.8 m/s. Overall, 66 and 163 juveniles of Pacific bluefin tuna (PBT) and yellowfin tuna (YFT) were collected; the standard length (SL) of the PBT and YFT ranged from 15.0 to 29.7 mm and 11.7 to 34.8 mm, respectively. The total operation time for the trawls of the LC-type net was much shorter than that of the usual trawls; therefore, a larger number of operations can be conducted in a day. We concluded that the LC-type trawl net can be used for collecting tuna juveniles because of its catchability and ease of operation.