FISHERY BULLETIN: VOL. 76, NO. 3 



most abundant species in the collection. The peak 

 in the 150- to 200-m zone was largely due to 

 Pyroteuthis addolux and Heteroteuthis hawaiien- 

 sis; young Histioteuthis dofleini also contributed 

 considerably. YoungEledonella were also found in 

 this zone although they were excluded from the 

 figures as "larvae." 



The zone between 200 and 700 m was sparsely 

 inhabited at night. The peak between 700 and 

 1,000 m represented the deep nonmigrating popu- 

 lation. The capture rate in this region was almost 

 identical to the day capture rate at the same 

 depths: deep-living migrators were few. 



The total rate of capture for the water column 

 during the day was 459 specimens/1,000 min of 

 trawling. Surprisingly, the total capture rate at 

 night was only 309 specimens/1,000 min. This dif- 

 ference was largely due to smaller-than-expected 

 catches at night in the upper 200 m of the few most 

 abundant species. The reason for the low night 

 catches is unknown. Another estimate of the 

 number of animals in the upper 250 m at night was 

 obtained by assuming that the day peak from 300 

 to 700 m (minus the night catch at these depths) 

 shifted into this upper zone at night (see below). 

 On this basis, nearly 807c of the individuals occur- 

 red in the upper 250 m at night. If one considers 

 also the abundant ommastrephids which avoided 

 midwater trawls but occurred in near-surface 

 waters at night, then only a small percent of the 

 total number of individuals would remain below 

 250 m at night. 



In many species, most of the population shifted 

 upward at night. Such day-night differences 

 existed in at least 25 of the 47 species examined, 

 based on present data and literature records. 

 Adequate data were lacking for ommastrephids 

 and Neoteuthis sp. Therefore, where the vertical 

 ranges are known, nearly 60% of the species ex- 

 hibited diel shifts in habitat. Species not exhibit- 

 ing diel migrations belonged primarily to the 

 Cranchiidae ( seven species ) and the Octopoda ( five 

 species). 



At least 18 of the 25 species that exhibited diel 

 migrations occurred almost exclusively in the 

 upper 250 m at night. These included all 11 enop- 

 loteuthids, Liocranchia reinhardti, Phasmatopsis 

 fisheri, Ctenopteryx siculus, Octopoteuthis 

 neilseni, Brachioteuthis sp., Chiroteuthis sp., 

 Onychoteuthis compacta, and young Heteroteuthis 

 hawaiiensis. Two species for which the data were 

 incomplete (Cycloteuthis serventyi and 

 Chiroteuthis picteti) probably belonged to this 



category as well. Therefore, at least 80% of the 

 migratory species occurred in the upper 250 m at 

 night. 



Amesbury ( 1975) examined vertical zonation of 

 midwater fishes during the day off Hawaii. He 

 concluded that the water column could be divided 

 into three regions: epipelagic, mesopelagic, and 

 bathypelagic zones. The boundary between the 

 epipelagic and mesopelagic zones occurred at 

 about 400 m and was marked by a sharp increase 

 in the numbers of individuals. This boundary ap- 

 peared to apply equally well to cephalopods. The 

 boundary between the mesopelagic and 

 bathypelagic zones occurred at about 1,200 m. 

 This boundary was marked by a noticeable de- 

 crease in fish numbers and represented the 

 greatest day depths of vertically migrating fish. 

 This lower boundary was not applicable to 

 cephalopods; there was no comparable decrease in 

 numbers of individuals; and this depth seemed to 

 be well below the range of vertical migrators. 

 Amesbury further divided the mesopelagic zone 

 into upper and lower zones with the boundary at 

 about 650 to 700 m. Cephalopods exhibited 

 maximum species turnover at about this depth, as 

 well as changes in light-related adaptations in 

 some species (Young 1975d). Although fish and 

 cephalopod distributions differed with respect to 

 the lower boundary, the distribution of 

 cephalopods generally supported Amesbury's zo- 

 nation. 



In spite of the rather small size of the collection, 

 some evidence of vertical habitat separation 

 among closely related species emerged. Three of 

 the more abundant species belong to the subfamily 

 Pyroteuthinae: Pyroteuthis addolux, Pterygio- 

 teuthis microlampas. and P. giardi. In general 

 body proportions and armature, P. microlampas 

 was more similar to Pyroteuthis addolux than to 

 its congener. During the day, these two species 

 occupied the same depths around 500 m. At night, 

 their populations peaked at distinctly different 

 depths: Pterygioteuthis microlampas occurred 

 primarily between 50 and 100 m, while 

 Pyroteuthis addolux occurred primarily between 

 150 and 200 m. Although the data were less clear 

 for Pterygioteuthis giardi, this species seemed to 

 center around 400 m during the day and in the 

 upper 100 m at night with about half of the popula- 

 tion in the upper 50 m. Thus this species was 

 shallower than its two relatives during the day 

 and tended to be shallower at night, although 

 broad overlap occurred with its congener. 



610 



