864 



Onset of association behavior in striped jack, 

 Pseudocaranx dentex, in relation to floating objects 



Reiji Masuda 



Fisheries Research Station 



Kyoto University 



Nagahama, Maizuru 



Kyoto 625-0086, Japan 



E-mail address; reijiiSkais.kyolo u.ac |p 



Katsumi Tsukamoto 



Ocean Research Institute 

 University of Tokyo. 

 1-15-1, Minamidai Nakano 

 Tokyo 164-8639, Japan 



Many marine and freshwater species 

 associate with floating objects ( flotsam ) 

 during some period of their hfe his- 

 tory (Senta, 1965; Hunter and Mitchell, 

 1967; Kingsford, 1993). This associa- 

 tion behavior is well known among 

 fishermen, who often choose flotsam- 

 associated schools (Yabe and Mori, 

 1950; Greenblatt, 1979). Among a wide 

 range of taxa collected in association 

 with flotsam, the Carangidae is one of 

 the most frequently observed groups 

 (Hunter and Mitchell, 1967; Kings- 

 ford, 1993; Clarke and Aeby, 1998). 

 The association behavior of carangid 

 fishes has been used by fishermen in 

 Asia. "Payaw" in the Philippines is a 

 flotsam made of bamboo and attracts 

 many species of Carangidae (Ibrahim 

 et al., 1990). Juvenile yellowtail, Seri- 

 ola qiiinqueradiota, are collected by 

 Japanese fishermen by surrounding 

 drift algae with a small scoop net. 

 These fish are then raised in cages 

 as a net-pen culture (Senta, 1965; 

 Sakakura and Tsukamoto. 1997 1. Pres- 

 ently, a new sea ranching project is 

 being developed, in which artificially 

 reared juvenile striped jack, Pseudo- 

 caranx dentex, are released at a floating 

 platform where they naturally aggre- 

 gate. There they are fed and harvested 

 after growth (Masuda and Tsukamoto, 

 1998a). 



The importance of this association 

 behavior from the perspective of fishery 

 ecology has generated a great deal of 

 field research and experimental work. 

 Hunter and Mitchell (1967) showed 



that large flotsam attracts more fish 

 than smaller flotsam, and fish associ- 

 ating with flotsam tend to have darker 

 body color, compared with the silvery 

 color of those that are not attracted 

 (Hunter and Mitchell, 1968). Using tag 

 and release techniques, Ibrahim et al. 

 (1990) demonstrated that fish released 

 as far as 180 m from flotsam will swim 

 back to their place of capture, sug- 

 gesting that these fish can learn the 

 topography of the region surrounding 

 the float. Much ecological research has 

 been conducted with the assumption 

 that floating objects, especially drift 

 algae, are an iinportant habitat for juve- 

 niles (reviewed by Ivingsford, 1993). 

 Although a considerable amount of 

 field-based work exists, little research 

 has been conducted on the develop- 

 mental aspects of association behavior 

 in the laboratory. 



The ontogenetic changes in associa- 

 tion behavior in striped jack, Pseudo- 

 caranx dentex, were therefore studied 

 with hatchery-raised larvae and juve- 

 niles. Possible sensory mechanisms 

 underlying association behavior were 

 examined by comparing four different 

 types of flotsam conditions I transpar- 

 ent flotsam, gray flotsam, shadow flot- 

 sam, and no flotsam [control] ). 



Materials and methods 



Two batches of fish were used in our 

 experiments: one hatched on 22 Feb- 

 ruary 1994, the other on 28 March 



1994, both at the Komame Branch 

 of the Japan Sea-Farming Association 

 (JASFA). These batches of fish were 

 transferred to the Kamiura Branch 

 of JASFA on the next day. Fish were 

 reared in 150-m'' concrete tanks on a 

 routine diet of rotifers and Artemia 

 nauplii, and formula diet (Masuda 

 and Tsukamoto, 1996). Eight different 

 stages of fish were used in the experi- 

 ments ranging from 5.5 mm (20 days) 

 to 28 mm (56 days) in mean total length 

 (Table 1). Because large variations in 

 growth occurred in the hatchery, the 

 medium-size fish were selected for each 

 age group, except for the 21-mm fish. 

 The 21-mm fish (54 days) were the 

 smallest group of those hatched on 22 

 February 



Four different types of flotsam were 

 created in 30-liter polycarbonate tanks 

 with water depths of 25 cm (ca. 26 

 liters). These were 



1 Transparent flotsam: a transpar- 

 ent acrylic pipe (60 mm in outer 

 diameter, 5 mm thickness) cut to 

 250 mm in length and covered by 

 a transparent circular plate (5 mm 

 thickness ) on the bottom. This was 

 filled with water and suspended 

 from the ceiling by fishing line, the 

 height adjusted so that 150 mm 

 of the pipe was in the water (Fig. 

 lA). The experimental tank was 

 marked into four sections, and the 

 flotsam was located at the center 

 of one randomly chosen section. 



2 Gray flotsam: an opaque gray PVC 

 pipe and plate of identical size 

 to that used in type 1 and sus- 

 pended in the same way (Fig. IB). 

 This arrangement provided stron- 

 ger visual stimulus than that pro- 

 vided in type 1. 



3 Shadow flotsain: a gray circular 

 opaque PVC plate (60 mm in diam- 

 eter, 5 mm thickness) was sus- 

 pended from the ceiling, so that 

 the bottom of the plate was about 5 

 mm above the water surface. This 

 created a shadow on the water sur- 

 face, but not on the bottom of the 

 tank (Fig. IC). 



4 No flotsam (control) (Fig. ID). 



Manu.script accepted 27 April 2000. 

 P'ish. Bull 98:864-869 (20001. 



