FISHERY BULLETIN: VOL. 72. NO. 1 



a 600 

 bj 



9 



-2600 

 - 2000 



DAY SET 

 I NIGHT SET 

 ] DIVERS' AVERAGE ESTIMATE 



M 



I 



- 2600 



- 2000 



i 



1 



Figure 6. — Average diver estimates and day and night purse seine catch weights for each structure on (A) August 20, and 

 (B) August 21, 1970. No diver observations or purse seine sets were made at structure three on August 21 due to an after- 

 noon squall. 



not designed to evaluate the effects of structure 

 placement on fish attraction and further studies 

 would be necessary to meaningfully evaluate 

 these effects. 



Responses to Moving Structures 



A bait fish school was observed by Klima and 

 Wickham (1971) to have remained with a free 

 drifting artificial structure moving slower than 

 the current. One of our structures (Structure 

 No. 8. 19 August 1970). with a school of bait fi.sh 

 in attendance, was also observed dragging its 

 anchor and moving slowly with the current. This 

 structure was towed for 20 min at a speed of 

 ai)proximately 2 knots against a 0.5 knot current 

 for a distance of ai)proximately 0.8 km (0.5 mile) 

 in order to return it to its experimental mooring 

 location. The structure moved up to the surface 

 while being towed, but the fish swam along with 

 it, trailing out behind when the towing speed 

 was increased. After the structure was re- 

 anchored in position, the fish school began 

 swimming around it in the usual manner. Divers 



estimated that over half the original number of 

 fish remained around the structure after towing. 

 A purse seine set made on this structure after 

 repositioning produced 545 kg (1,200 lb) offish. 



Behavior Observations at Structures 



Our observation of bait-fish-school behavior 

 at the structures is in general accordance with 

 the behavior described by Klima and Wickham 

 (1971). The bait-fish schools normally main- 

 tained a position up-current from the structures 

 and were observed continuously feeding on crab 

 larvae and other particulate material in the 

 water. During very slow or zero current condi- 

 tions, the bait fish would often mill about in a 

 loose aggregation (Figure 7) or form long 

 streaming schools making large looping passes 

 out and around the structures in all directions. 

 The schools would frequently swim beyond the 

 divers' range of visibility, remaining out of sight 

 for periods up to 3 min or longer before stream- 

 ing back in and around the structures from a 

 different direction. 



188 



