KENDALL and NAPLIN; DIEL-DEPTH DISTRIBUTION OF ICHTHYOPLANKTON 



Gulf Stream flounder spawned in the morning 

 and the eggs hatched around noon. The progres- 

 sion from precell to hatching stages in all of these 

 species allowed construction of detailed develop- 

 mental timetables. Ahlstrom (1943) found spawn- 

 ing to occur during only a few hours of the day, and 

 that plankton samples contained discrete batches 

 of eggs spawned on different days. He also deter- 

 mined developmental times from this informa- 

 tion. Ferraro (1980) reported on daily spawning 

 times of a number of species. All fish investigated 

 spawn during limited times of day, mainly at some 

 time between noon and midnight. 



For species that spawn on a regular diel basis, 

 the number of predominant stages at any time 

 about equals the number of days the eggs require 

 to hatch (Table 14). Therefore, longer incubation 

 times mean that more days of spawning are 

 represented at any one time, making the separa- 

 tion of eggs into discrete spawning batches more 

 difficult. Atlantic whiting eggs, with a short 

 incubation time and spavined in the upper 10 m 

 of water, were readily separated into discrete 

 batches. Theoretically, snake eel eggs with a 

 development time of 96 h would be more difficult 

 to separate into batches than Gulf Stream floun- 

 der eggs with a development time of only 72 h. 

 This was not the case, probably because of the 

 depth stratification of the Gulf Stream flounder 

 eggs and their longer daily spawning period com- 

 pared with the homogeneous depth distribution 

 of snake eel eggs and their very short daily 

 spawning period. 



Among the eight most abundant types of larvae 

 collected during this study, all were more abun- 

 dant closer to the sea surface at night. Two general 

 patterns were seen: bluefish and frigate mackerel 

 larvae were mostly above the thermocline (most 

 abundant at 6 m) and some migrated to the surface 

 at night; the other six species were most abundant 

 below the thermocline at 15 m during the day but 

 some larvae of each species migrated to waters 

 above the thermocline at night. Within these two 

 patterns, specific variations occurred. No two 

 species showed the same combination of signifi- 

 cant F-values for the factors tested and their 

 interactions. We remained in a body of water 

 where factors affecting larval abundance were so 

 constant for three of the species that no significant 

 differences were seen in the catches over the 3-d 

 study. The two species most closely associated 

 with the surface (bluefish and frigate mackerel) 

 showed the greatest fluctuations in abundance 



over the 3 d, indicating that in following the 

 drogue, we moved to areas where variation in 

 factors affected larval abundance occurred. In 

 such areas where water movement varies consid- 

 erably with depth, a drogue can be expected to 

 track only uniform patches of tho.se organisms 

 whose depth ranges are similar to that of the 

 drogue. Four species were collected in signifi- 

 cantly larger numbers at night than during the 

 day. This may have resulted from increased avoid- 

 ance by larvae during daylight. However, signifi- 

 cantly more Atlantic whiting larvae were taken 

 during day than at night. Some of these differ- 

 ences may thus reflect differences in vertical 

 distribution on a diel cycle. If the larvae were more 

 concentrated during day than at night at one or 

 more of the levels sampled, they would appear 

 more abundant during the day, although their 

 overall abundance in the water column would not 

 actually be different. Significant portions of the 

 larval population of species that were abundant in 

 our deeper nets may have occurred below the 

 depths we sampled. 



This study, as have others (e.g.. Miller et al. 

 1963), points out the necessity of sampling the 

 entire water column, at least over continental 

 shelves, during ichthyoplankton surveys. This is 

 accomplished in the many recent surveys which 

 employ oblique or vertical plankton tows. In other 

 sampling designs significant portions of the popu- 

 lations can be either undersampled or oversam- 

 pled depending on their depth distribution and the 

 time of day of sampling. The implications of this 

 study regarding effects of time of day on numbers 

 of fish eggs in the water column due to diel 

 spawning and embryonic developmental cycles 

 need to be considered in analyzing results of 

 ichthyoplankton surveys. 



ACKNOWLEDGMENTS 



We extend our appreciation to the many mem- 

 bers of the staff of Sandy Hook Laboratory for 

 their help at sea and in the laboratory in gather- 

 ing and processing the samples on which this 

 paper is based. Special thanks are due to W. G. 

 Smith and Jack Colton who reviewed drafts of 

 the manuscript. 



LITERATURE CITED 



AHLSTROM, E. H. 



1943 . Studies on the Pacific pilchard or sardine ( Sardinops 



725 



