AHLSTROM: FISH LARVAE IN EASTERN TROPICAL PACIFIC 



Larvae of some families of fishes were sampled 

 almost as well in day hauls as in night hauls 

 — including Sternoptychidae, Bathylagidae, and 

 Melamphaidae. In contrast, less than one- 

 fourth as many gonostomatid larvae and one- 

 third as many myctophid larvae were taken in 

 day hauls, on the average, as in night hauls. 

 Catches of scombrid larvae were more variable 

 with regard to time of sampling — the night-day 

 ratio in the outer half of the EASTROPAC area 

 was only about 1.5 to 1, whereas the ratio 

 jumped to about 7.5 to 1 in the inner pattern 

 occupied by Alaminos. Larvae collected about 

 in equal amounts in day and night hauls were 

 those known to occur principally below the 

 thermocline. 



Despite the lower abundance of larvae in day 

 hauls as compared with night hauls, the per- 

 centage of hauls containing larvae of most fam- 

 ilies was only slightly lower (Table 5). The 

 most marked day/night difference in frequency 

 of occurrence was for scombrid larvae, these 



Table 5. — Percentage of hauls containing larvae of 

 the more abundant fish families on EASTROPAC I, 

 grouped by day, night and dawn or sunset. 



Family 



Day 

 hauls 



Night 

 hauls 



Dawn or 



sunset hauls 



(± 1 hr) 



All 

 hauls 



were taken in 45 % of night hauls, but in only 

 31 '/'c of day hauls. In the discussions that fol- 

 low I make use of all collection data, irrespective 

 of time of collection. 



NUMBERS OF FISH LARVAE 

 OBTAINED ON EASTROPAC I 



Fish larvae were obtained in 478 of 482 

 oblique plankton tows made with the 1-m plank- 

 ton net on EASTROPAC I. The number of 

 larvae per collection ranged from to 2,197, 

 averaging 197 larvae (actual counts). 



Differences in abundance of larvae with lat- 

 itude are summarized for the four series in Table 

 6. Fish larvae were obtained in largest num- 

 bers, on the average, in an equatorial band ex- 

 tending from about lat 10° N to 5° S. The least 

 productive waters for fish larvae were in the 

 central water mass of the South Pacific, espe- 

 cially between lat 15° and 20° S. 



Abundance of fish larvae also decreased off- 

 shore,' averaging only 130 larvae per haul in 

 the outer pattern, occupied by Argo, as com- 

 pared with 246 larvae per haul in the inner 

 pattern, occupied by Alaminos. 



Tropical waters and oceanic waters are usu- 

 ally considered to be relatively unproductive, 

 compared with temperate coastal regions such 

 as the California Current region (Ryther, 1969). 

 Hence, it is surprising to find that the average 

 number of fish larvae obtained per haul on 

 EASTROPAC I was larger than either on the 

 CalCOFI cruises from the California Current 

 region (Ahlstrom, 1969) or on NORPAC (un- 



Table 6. — Total catches of fish larvae (actual counts) taken by the four research vessels on EASTROPAC I, 



summarized by latitude. 



