Using an echosounder system to study the vertical movements of captive blue fin tuna (Thunnus thynnus) in floating cages

Abstract

The vertical migrations of bluefin tuna (27 individuals averaging 100 kg body weight) held in a 25 m diameter x 30 m deep "Polar Cycle" floating cage has been monitored using a SIMRAD EY 500 Echosounder. The system was composed by a 10x20 38 K cycles ceramic transducer attach to a platform floating in the center of the cage and connect to a transceiver. Data were stocked in a laptop. Energy for all the system was provide by two batteries. All these devices were located into an hermetic stainless steel box fixed on the external ring of the cage. Daily, in the morning, the system was stopped and data were downloaded The monitoring period was between February and April 2005. Tunas were fed with raw fish once a day, over 10:00 in the morning. The mean water temperature was 13.3 ºC. No termocline was observed in this period. Binary data provided by the transceiver, in daily files were visualized with the Sonar Data Echoview® (SonarData Pty Ltd, GPO Box 1387 Hobart, Tasmania, Australia