NOTE Masuda and Tsukamoto: Association behavior of Pseudocaranx dentex in relation to floating ob|ects 



865 



200 mm 



Figure 1 



Schematic drawings of experimental tanlis with each type of flotsam. (A) trans- 

 parent flotsam, (Bl gray flotsam. (C) shadow, and ID) control. 



Carangid fish juveniles are commonly seen associated 

 with floating and underwater objects, such as air tubes 

 and vinyl bags, both in hatchery tanks and natural waters 

 (Tachihara et al. 1993). Therefore acrylic and PVC pipes 

 were suitable representative materials for use in examin- 

 ing the developmental changes of association behavior. We 

 observed only horizontal distributions as an index of asso- 

 ciation behavior because in hatchery tanks, larvae and 

 early juveniles of striped jack usually occur in the upper 

 30 cm of the water column. 



All treatments were triplicated (twelve 30-hter tanks 

 were used in total) and the location of the same type of 

 flotsam was changed among triplicates. Experiments were 

 conducted in an indoor facility and all tanks were placed in 

 a water bath that was strongly aerated. Water temperature 

 in each tank was maintained at hatchery levels (22.5°C). A 

 photoperiod of 8 h light and 16 h dark was provided with 

 12 halogen illumination lights set above the tanks (light: 

 08:30-16:30 h i. With this lighting, the light intensity at the 

 surface of tanks was 0.01-0.05 lux at 20:00 h, lower than 

 0.01 lux between 00:00 and 04:00 h, 260-1000 lux at 08:00 

 h, and 17,000-24,000 lux at 12:00 and 16:00 h. 



At 20:00 h. fish were transferred from the hatchery tank 

 to the 12 experimental tanks and ten fish were introduced 

 into each tank (120 fish in total). Observations were con- 



