400 MECHANICAL AND ACOUSTICAL SENSES 



Table 4. Number of sharks sighted and mean response intensities for three 

 25- to 100-Hz pulsed sounds. Based on 15 sound and 15 control periods 

 for each sound (Eniwetok).* 



*From Nelson and Johnson 1972. 



'Mean of the maximum observed response intensities for each sound period; values based 

 on an arbitrary eight-point scale. 



sensitivity of fishes appears to be affected not only by prevailing ambient 

 levels (Banner 1972, Buerkle 1968, Ha 1968, Tavolga 1967) but also by 

 high ambient levels experienced prior to tesing (Ha 1968, Popper and Clarke 

 1976). Many workers have nevertheless neglected ambient noise as a poten- 

 tial source of disturbance. When responsiveness has been considered in sharks 

 and teleosts, it often ceases when the signal level drops to a point between 

 15 and 25 dB above the prevailing spectrum-level ambient (e.g., Banner 

 1967, 1972; Buerkle 1969; Cahn et al. 1969; Myrberg et al. 1969; Nelson 

 1967). Thus, although a given signal may propagate quite far through the 

 medium before falling below the spectrum-level noise, it probably reaches 

 either an inaudible level or a level inadequate for response long before that 

 distance is attained. 



This illustrates one major difference between natural biological sounds 

 and the synthesized signals frequently used in field studies of shark attrac- 

 tion. Although such studies occurred where sharks were rare or variable 

 daily, an adequate sample size was, nevertheless, essential for meaningful 

 analyses. This requirement thus forced signal transmissions to be at levels 

 sufficient to provide reasonable areas of coverage. Such transmissions often 

 attained levels of between +37 and +55 dB//ibar re 1 m. With a relatively 

 smooth sea, such signals could reach many hundreds of meters from the 

 sources before reaching ambient levels. Only a few biological sounds reach 

 these rather high levels. Therefore, one can predict that most biological 

 sounds of interest to sharks probably are detected only at distances much 

 less than 100 m from the source. 



An extreme example may be the maximum detection distances that 

 Banner (1967) found for various prey sounds, using young lemon sharks in 

 very shallow water (~30 cm depth). Such detection distances never exceeded 



