SCOTT and FLITTNER: BEHAVIOR OF BLUEFIN TUNA SCHOOLS 



species. McHugh (1952) and Alverson (1963) 

 found differences in the stomach contents of al- 

 bacore, yellowfin, and skipjack tuna in different 

 geographical areas. McHugh noted that red 

 crabs were the dominant food item for albacore 

 off the Baja Cahfornia coast, while anchovies 

 and other baitfish dominated in fish stomachs 

 taken from southern California waters. Blunt 

 (1958) noted that of 168 T. thynnus taken off 

 California in 1957, 70% had been feeding on 

 anchovies. Longhurst (1967) observed that 

 during normal years the distribution of P. plan- 

 ipes reaches from the area of Cedros Island 

 south, while the range extends farther north 

 during warm water years. This distributional 

 pattern includes the entire area from which 

 jumpers and boilers were almost entirely absent. 

 While baitfish, like anchovies, are found in the 

 southern area, previously cited studies indicate 

 that red crabs may be the preferred food item. 

 Since P. plmiipes is a relatively weak swimmer 

 and occurs in very dense concentrations off south- 

 ern Baja California, a modified filter feeding 

 such as Sette (1950) described for Atlantic 

 mackerel (Scomber scombrus) might be em- 

 ployed by T. thynnus while feeding on red crabs. 

 On the other hand, the vigorous pursuit by scom- 

 broids feeding on forage fish is well known 

 (Magnuson, 1963; Whitney, 1969). 



The lack of jumping and boiling schools in 

 the southern area also may be due to differences 

 in the behavior of T. thynmts feeding on P. plan- 

 ipes and baitfish and a preference for the former 

 when available. Comparisons of stomach con- 

 tents of T. thynnus from the three school types 

 would provide the necessary test of this hypoth- 

 esis, but such information is not available. How- 

 ever, analyses of stomach contents for yellowfin 

 from an area in which P. planipes was the dom- 

 inant food item and from a second area in which 

 fish were the dominant item have been made 

 (Alverson, 1963). Yellowfin sets, which were 

 from these same two areas (but at different 

 times in 1965) and which were identified as to 

 school type, showed significant differences (chi 

 square 17.27, P<0.01) in the occurrence of 

 jumping, boiling, and breezing schools (Table 4) . 

 Greater numbers of jumping and boiling schools 

 and reduced numbers of breezers occurred in the 



Table 4. — Comparison of breezing, jumping, and boiling 

 yellowfin schools from the Gulf of Guayaquil and Baja 

 California.' 



1 Schooling data are for 1965 (Inter-American Tropical Tuna Commission, 



unpublished data). 



- Stomach contents: fish, 19.4%; red crabs, 78.1% (Alverson, 1963) 

 ^ Stomach contents: fish, 76.9%; red crabs, 0.0% (Alverson, 1963) 



red crabs do not occur in the Gulf of Guayaquil. 



area in which fish were the dominant food item, 

 whereas the opposite was true in the area where 

 red crabs were the dominant item. This lends 

 support to our hypothesis that these school types 

 reflect behavioral differences in the feeding pat- 

 terns of T. thynnus and other tunas on red crabs 

 and baitfish. 



If, in fact, many of the breezing schools in the 

 southern area of the fishery are feeding upon red 

 crabs, one would expect to find a greater per- 

 centage of successful sets on breezing schools 

 in this area when compared with breezing 

 schools taken in the northern area of the fishery. 

 However the observed dififerences are not sig- 

 nificant (chi square 2.18, P>0.20) . It might be 

 that if feeding breezers and nonfeeding breezers 

 were compared that differences in percentage 

 success between the two areas would be found. 



The greater number of fireball schools ob- 

 served in the northern area of the fishery may 

 reflect a difference in the distribution of biolu- 

 minescent organisms. However, persistent 

 stratus overcast in the southern California off- 

 shore zone during summer results from upwelled 

 water coursing southward from Point Concep- 

 tion. As a result of this stratus overcast, the 

 fishermen's ability to see fireballs at night may 

 be significantly enhanced by eliminating back- 

 ground illumination from the moon and stars. 

 The phenomenon requires further study. 



VULNERABILITY TO CAPTURE 



There were significant differences in vulner- 

 ability to capture, as indicated by percentage 

 of successful sets (Table 2) , among the five most 

 common daytime schools (chi square 62.32, 



919 



