AHLSTROM: FISH LARVAE IN EASTERN TROPICAL PACIFIC 



per mixed layer or in the upper portion of the 

 thermocline, between the surface and approxi- 

 mately 125 m. Of the 15 most common kinds 

 of fish larvae taken in vertical distribution ser- 

 ies, 12 were so distributed (ibid., p. 134). Two 

 of the kinds that occurred most commonly below 

 the thermocline were bathylagid smelts, closely 

 related to the two common bathylagid smelts 

 taken on EASTROPAC I. 



On the NORPAC Expedition of August 1955, 

 two depth strata were sampled at most stations ; 

 a closing net, fastened to the towing cable 200 m 

 below a standard open plankton net, sampled 

 the level between 262 and 131 m on the average, 

 while the upper net sampled from the surface 

 to approximately 131 m deep. Only about one- 

 ninth as many larvae were taken in the closing 

 net hauls as in the upper net hauls; fully half 

 of these were larvae of hatchetfish, family Ster- 

 noptychidae, largely absent from upper net 

 hauls. The two most abundant kinds of fish 

 larvae taken on EASTROPAC I were those of 

 the myctophid lanternfish, Diogenichthys kitem- 

 atics, and of the gonostomatid lightfish, Vinci- 

 guerria spp. In NORPAC collections, only 3 % 

 of the larvae of D. laternatus were taken in the 

 closing net hauls and only 2 % of the Vinciguer- 

 ria larvae. Among the kinds of larvae common 

 to both the NORPAC and EASTROPAC sur- 

 veys that occurred in significant numbers in the 

 deeper NORPAC collections were those of Chaul- 

 iodus (72 % taken in closing net hauls), Proto- 

 myctophum (48 %) and I diacanthus (32 %). 



Inasmuch as the vertical distribution studies in 

 the California Current region had pointed up the 



importance of the thermocline in the depth distri- 

 bution of larvae, the pattern of thermocline depth 

 was analyzed for EASTROPAC I (Table 3). 



Thermocline depth was invariably shallow in 

 the inner pattern occupied by Alaminos (data 

 not included in Table 3) ; the greatest depth 

 recorded was only 40 m, and the majority of 

 observations were at depths shallower than 20 

 m. Along the six station lines covered in Table 

 3, thermocline depths were shallowest near the 

 equator, and usually were deepest at the north- 

 ern (20-15° N) and southern (15-20° S) ends 

 of the lines. The thermocline also deepened off- 

 shore; approximately three-fourths of the rec- 

 ords of thermocline depths of 50 m or greater 

 were from the tw^o outer lines, occupied by Argo. 



Most oblique plankton hauls taken on EAS- 

 TROPAC I sampled to depths of 200 m or more 

 (Table 2), hence sampled considerably deeper 

 than the thermocline in all parts of the EAS- 

 TROPAC area. 



EFFECTIVENESS OF SAMPLING FISH 



LARVAE IN DAYLIGHT HAULS 

 AS COMPARED WITH NIGHT HAULS 



Fewer fish larvae were obtained in hauls made 

 during daylight hours than at night (Table 4). 

 Original (unstandardized) counts of larvae av- 

 eraged 2.76 times as many in night hauls as in 

 day hauls, 285 larvae per occupancy as compared 

 with 103 larvae. Hauls made within 1 hr of 

 sunrise or sunset contained intermediate num- 

 bers of larvae, averaging 217 larvae per oc- 

 cupancy. 



Table 3. — Summary of records of thermocline depths along six station lines occupied by the research vessels 

 Rockaway, David Starr Jordan, and Argo on EASTROPAC I. 



Station line 

 along longitude 



Range in depth of thermocline (m) at latitudes 



15-10° N 



5° N-0° 



0-5" S 



5-10° S 



10-15° S 



15-20° S 



All 

 latitudes 



