WICKHAM and RUSSELL: EVALUATION OF MID-WATER ARTIFICIAL STRUCTURES 



Figure 7. — Underwater photograph of a mixed school of round scad (Decapicnis punctaius) 

 and Spanish sardine (Scudinclla anchovia) swimming past an artificial structure. 



A different pattern of behavior was observed 

 by the divers when the bait-fish schools were 

 threatened by the presence of feeding predators, 

 i.e., Spanish mackerel (Scomberoniorus macu- 

 latus), king mackerel (S. cavalla), little tunny 

 (E. alletteixitus). and bluefish {Poniatomus salt- 

 atrix). On these occasions, relatively small bait- 

 fish schools, i.e., 100 kg (220 lb) or less, would 

 form a milling ring with the structure in the 

 center or swirl in a tight group in quick passes 

 close to the structure as the predators made 

 darting attacks on the school. Larger schools 

 would usually be split by the attacking predators 

 with one group of bait fish moving to the struc- 

 ture and circling it as described above while the 

 remaining fish moved off in tight, fast-darting 

 groups. 



Behavioral Mechanisms 



Different sizes and species of fish apparently 

 associate with objects in the sea for different 

 reasons involving different behavioral mechan- 

 isms. Hypotheses advanced to explain the asso- 

 ciation of fishes with floating objects were 

 reviewed by Gooding and Magnuson (1967). The 



initial attraction of pelagic fishes to objects 

 probably results from their visually detecting 

 the object in the optical void of the pelagic en- 

 vironment, since fish beyond the visual range of 

 a structure or structure-attracted fish school are 

 not attracted (Hunter and Mitchell, 1967). Sig- 

 nificantly improving the visual characteristics of 

 an object apparently increases the rate and num- 

 ber of fish it attracts (Hunter and Mitchell, 

 1967; Klima and Wickham, 1971). Objects, how- 

 ever, must serve a meaningful function beyond 

 that involved in the ;initial visual attraction in 

 order for pelagic fish to remain in association 

 with them. To tentatively explain this behavior 

 in mixed schools of round scad (D. puuctatus) 

 and Spanish sardine (S. anchovia) around 

 artificial structures, Klima and Wickham (1971) 

 proposed the hypothesis: "Floating objects and 

 underwater structures provide spatial references 

 around which fish can orient in the otherwise 

 unstructured jielagic environment." This tenta- 

 tive hypothesis was given some support by our 

 study, but it must be modified and expanded to 

 account for our additional behavioral observa- 

 tions. Our studies indicate that although coastal 



189 



