FOOD OF ALBACORE IN THE PACIFIC 



471 



Figure 15. — Variation in stomach content volumes of 115 

 albacore caught by trolling in the temperate North 

 Pacific during summer, according to time of day when 

 captured. ( Numbers in parentheses refer to sample 

 sizes. ) 



Evaluating figure 15, one may say that while 

 feeding take.s place throughout the day, two gen- 

 eral feeding periods are indicated — one in the 

 early morning and another towards evening. The 

 evidence for such an interpretation gains addi- 

 tional weight if the high value recorded for the 

 1000-1159 period is considered a sampling artifact, 

 since one albacore accounts for 40 percent of the 

 total value shown for that period. 



Feeding periods have been reported by Uda 

 (1940) and Xakainura - for skipjack tuna {Katsu- 

 wonii.s pelamis). Uda states that off Japan skip- 

 jack feed most actively in the early morning, again 

 around noon, and presumably again near sunset. 

 Nakamura found skipjack caught near the 

 Marquesas Islands to be heavy feeders in the 

 morning around 0900 hours, with little feeding 

 around noon and another period of heavy feeding 

 before sunset. The data shown in figure 15 more 

 closely resemble conditions described by Naka- 

 mura for skipjack than those reported by Uda. 

 As Nakamura points out, tliis probably reflects the 

 lessened u\ailability of tuna forage due to tiie 

 downward daytime migration of zooplankton, the 

 prey of much tuna forage. 



The consumption of saury and .squid thiougiiout 

 the day was examined and tlie results are given 



in table 6, whicii allows a comparison with 

 McHugli's (1952) data on diurnal variation in al- 

 bacore food. For hourly periods when at least 10 

 stomachs were sampled, percentages were calcu- 

 lated when eitlier saury or squid were dominant 

 in the stomachs. The results generally agree with 

 McHugh's in that saury dominated frequently 

 throughout the day and .squid did not dominate as 

 frequently in tiie early morning and late afternoon 

 hours as they did during other times of the day. 



Circumstances of the catch of the four specimens 

 in the period from 2000 to 2035 hours (fig. 15), in 

 which lighting conditions were approaching total 

 darkness, lead to a discussion of whether or not 

 albacore feed at night, a question briefly alluded to 

 earlier in tliis report. Tliree of these four albacore 

 stomachs contained food in the following amounts : 

 40 cc, 15 cc, and 7 cc. 



Table 6. — Percentage of troll-caught albacore stomachs in 

 which either squid or saury was the dominant food 

 organism, acconling to time of capture 



(Data Riven only for hourly intervals when 10 or more stomachs were sampled) 



2 Nakiinuira, K. L., Fiiod and feedinp Itabits of .M:irtiiif.^;iii skip- 

 jack (KatKuwonu:* pelamis). MS , Bureau of Commerciai Fisli- 

 erics Itt((l()gical Laboratory, Honolulu. 



Stomach volume data (table 2) from gill netted 

 albacore show that there was, on the average, 

 much less food in such stomachs than in the 

 stomachs of longlined or troll-caught albacore. 

 Since the gill net was fislifd at night, this differ- 

 ence suggests that albacore may feed less during 

 the hours of darkness. By tlie nature of the gear, 

 however, longlines and trolling lures are probably 

 selective for actively feeding fish, while the passive 

 gill net would take albacore whicii were not feed- 

 ing. There is also a possibility that gill netted 

 albacore regurgitate their stomach content.s, but 

 the higli percentage of typically empty stomachs 

 (with a narrow hinien and deeply convoluted 

 I'ugae) in such fisii makes tliis seem unlikely. 



There is indirect evidence, discussed below, 

 which indicates that albacore probably do feed at 

 night. This conclusion is supported by the fact 

 that some food has been found in the stomachs of 

 albacore taken in night gill net fishing. The suc- 

 cess of nighttime as compared with daytime feed- 

 ing is (lillicult to estimate because of the selectivity 

 of the different (i.shing methods. 



