FISHERY BULLETIN: VOL. 69, NO. 3 



Table 1. — Length data for species groups. 



were between 0.5 and 0.6 mm in lenjrth. It i.s 

 possible that the decline in numbers below the 

 median length was partly the result of increasing- 

 escapement with diminishing size, but the escap- 

 ing fraction, known to be negligible for sizes 

 above 0.4 mm, is assumed to be relatively small 

 for sizes down to 0.2 mm (O'Connell and Leong, 

 1963) . On this assumption the length-frequency 

 distribution is considered representative for this 

 size range of copepods. 



The large-copepod group shows a modal shift 

 to smaller sizes at night, although the size range 

 and degree of skewness are not markedly dif- 

 ferent for the two periods. Organisms less than 

 1.5 mm in length were largely copepodites, while 



SMALL COPEPODS 



LARGE COPEPODS 



CHAETOGNATHS 



12 :4 16 



FiGliRK 2. — Length-frequency histograms of organisms 

 in four species groups, as determined from selected 

 samples. For large copepods and eu|)hausiids the wide 

 bars show day frequencies and the narrow Itars night fre- 

 quencies. 



those between 1.5 and 3.0 mm were adult Calanus 

 helgolandiciw and Pamcalanus sp., with Centro- 

 payes sp. also present in the night samples. 

 Those larger than 3.0 mm were largely Rhincal- 

 anus s\). in both day and night samples. 



The euphausiid group, which ai)peared to be 

 composed largely of Euphaiisia pacifica, though 

 Nyctiphanes simplex and Nematoscells dlfficlUs 

 were also in evidence, showed a marked shift 

 to larger sizes at night, obviously the result of 

 vertical migration. It is ai^ijarent from the sizes 

 involved that the day samples were composed 

 mostly of larval stages and the night samples 

 of .larval stages and juveniles, with few if any 

 adults. The largest individuals in the samples 

 were considerably smaller than the maximum 

 total length for the species, 25 mm (Boden, 

 .lohnson, and Brinton, 1955). 



The i)ump samjiles did show some evidence of 

 fragmentation of larger euphausiids, and for this 

 reason the size frequency distribution for the 

 night period might be slightly biased in favor 

 of the smaller sizes, and estimates of numbers 

 sampled might be a little low. 



Fragmentation probably involved far more 

 juveniles than adults. Samples from opening- 

 closing nets 1 m in mouth diameter taken in 

 sjiring and summer off southern California 

 (Brinton, 1962) showed adults to be scarce or 

 absent in the upper 10 m during the night as well 

 as the day. -Juveniles were predominant at this 

 stratum. Even at depths where adults were 

 most abundant at night — 40 m in one case and 

 140 to 280 m in another — they were only one-fifth 

 and one-tenth as numerous as juveniles. Night- 

 time obli(|ue hauls with nets 1 m in mouth di- 

 ameter off central Baja California indicated 

 essentially the same kind of vertical distribution 

 for euphausiid species in that area (Ahlstrom 

 and Thrailkill, 1963) . On the basis of such evi- 

 dence it seems probable that the size-frequency 

 distriljutions shown by the pump samples are 

 reasonably representative of the day and night 

 pojiulations near the surface, though certainly 

 not of the population in the entire water column. 



The chaetognath group was composed largely, 

 if not entirely, of Scu/lttu eiuierltlca. and the size 

 range is jiroljably representative for the near- 

 surface population samiilcd. The size range for 



684 



