No abstract

Vertical behavior of cultured chub mackerel, Scomber japonicus, in a net-cage was investigated using depth data logger and infrared video camera footage during the spawning and non-spawning seasons. Marked differences in the vertical distributions of spawning and non-spawning mackerel were observed. Non-spawning mackerel swam near the bottom of the net-cage during the daytime, coming up to shallow waters during night-time. However, spawning mackerel remained near the bottom of the net at all times except for brief, night-time visits to the surface for gamete release. Spawning events started with the school of fish whirling near the surface, before two or three fish, possibly the female followed by the males, darted upward and away from the school at high speed. Once away from the school, the fish swam in a tight circle two or three times before releasing their eggs and sperm. Estimation of the timing of spawning events based on back-calculations of egg development and then correlating these data with changes in vertical movements revealed that there were 26 events over a three-day period, and that most events occurred between sunset and midnight. Differences in the depth distribution between the spawning season and the non-spawning season could potentially be used as new indicator for the spawning period in chub mackerel reared under artificial conditions. These results could help us to better understand the spawning season in mackerel and allow us to increase the sustainability of the natural reproductive process in captivity.

The characteristics of dietary utilization, energy conversion efficiency, metabolic rate and energy partitioning were measured for cultured Pacific bluefin tuna (PBT) juveniles fed on an artificial diet. Thirtyone juveniles (1.1 AE 0.3 g BW) were stocked into each of two 2500 L tanks to measure oxygen consumption (ḾO 2 ), swimming speed, digestibility and growth performance. ḾO 2 elevated until 2.5 AE 0.3 times of pre-feeding level within 1.5 h after feeding, except for the first feeding, and returned to pre-feeding level within 3 h. Swimming speed fluctuation was corresponded with the ḾO 2 fluctuation, and both parameters were stable from 02:00 to 06:00 and also during the whole day for starved fish. These indicate that feeding has strong influence on their metabolic rate. Energy partitioning for faecal, urinary and branchial, heat increment and voluntary activity, standard metabolism, and retained energy were calculated to be 17.2%, 5.9%, 14.9%, 41.3% and 20.7% of total ingested energy, respectively. The results indicate that, unlike other fish, juvenile PBT distribute large amount of energy for maintenance, which allows only a little proportion of ingested energy available for growth.