284 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



Table 10. — Quantitative differences in the catch of day and night hauls with three types of trawls 



[Based on 40 hauls) 



1 Displacement volume. 

 ! Countercurrent. 



shortly after midnight. Although minor differ- 

 ences between these two stations in the physical 

 and chemical features of the environment might 

 have influenced the abundance of forage organ- 

 isms, we believe that the major differences in the 

 catch were related to the time of hauling. The 

 average volume of 11 hauls at station A was 1.8 

 ml. and the average number of organisms in the 

 catch was 47, as compared with an average voliune 

 of 9.0 ml. and an average number of 93 for 11 

 hauls at station D. The night/day ratio for vol- 

 umes was 4.90, and for numbers, 1.98. The aver- 

 age individual size of organisms was 0.04 ml. for 

 the day hauls and 0.10 ml. for the night hauls. 



At the start of Manning cruise 20, four test hauls 

 were made with the 6- foot Isaacs-Kidd trawl in 

 Hawaiian waters to determine the difference in 

 catch between hauls made in the morning about 

 2 hours after sunrise and at night about 1 hour 

 after sunset. For this series the average volume of 

 the two night samples was 22.8 ml. and the average 

 number of organisms was 257; for the two day 

 samples, the average volume was 5.8 ml. and the 

 average number of organisms was 106. The night/ 

 day ratio was 3.91 for volume and 2.42 for number 

 of organisms. Again the average size of organisms 

 was much larger in night than in day hauls (table 

 10). 



The main objective of Manning cruise 20 was 

 to conduct longline fishing in the e<:|uatorial Pa- 

 cific. During the first 4 days of operations in the 

 Countercurrent, a trawl haul was made each morn- 

 ing about 2 hours after sunrise, soon after the 

 longline had been set. Because of the poor catches 

 that were being made, the hauling time was 



changed on the 5th day to about 1 hour after sun- 

 set, just after the longline gear had been retrieved 

 for the day and the shijj was underway to the next 

 station. The two hauls made at this new time 

 were also in tlie Comitercurrent, and averaged 

 106.4 ml. in volume, as compared with an average 

 of 11.4 ml. for the four day hauls taken earlier. 

 The average numbers of organisms, night/day 

 ratios, and average size of the organisms are given 

 in table 10, along with similar data for the other 

 cruises mentioned. No day hauls were made with 

 the 10-foot Isaacs-Kidd trawl. 



Composition of the Catcli 



There were also marked diurnal differences in 

 the composition of the trawl catches. Lists of 

 organisms occurring in the day and night hauls 

 referred to above are given in tables 11 and 12 and 

 appendix table 6 and summarized by general cate- 

 gory in table 13. Coelenterates, certain molluscs, 

 and tunicates occurred in about equal frequency in 

 the day and night collections. Among the Crus- 

 tacea, the ampliipods and stomatopods were taken 

 in about equal numbers in day and night hauls, 

 whereas the decapods were taken principally at 

 night. Very few squids and fishes, except larval 

 and juvenile forms, were captured during day- 

 light. The main difference, therefore, between day 

 and night hauls was the capture at night of the 

 stronger swimming animals which were either ab- 

 sent from the upper layer during tlie day or were 

 able to dodge the net. We suspect that most of 

 these animals were capable of making extensive 

 vertical migrations during the day to depths below 

 those sampled by the trawls and became concen- 



