Doyle et al.: Neustonic ichthyoplankton in the western Gulf of Alaska 



239 



the day (Fig. 3A). This pattern was not apparent for 

 the bongo catches taken from the same stations (Fig. 

 3B). Some of the highest catches of larvae in the 

 bongo samples were taken during daylight. The ra- 

 tio of night:day catches for the neuston was 9.1:1, 

 whereas it was 1.6:1 for the bongo tows. The high 

 ratio of night:day catches in the neuston may be at- 

 tributed to two factors: 1) vertical migration of lar- 

 vae into the neuston at night and 2) enhanced avoid- 

 ance of the neuston sampler during daylight. One or 

 both of these factors may operate among the species 

 of larvae in the neuston. 



Diel variation in catches among all the dominant 

 taxa of neustonic larvae suggested a daytime de- 

 crease in density in the neuston. All larvae, except 

 Hexagrammos decagrammus and Theragra chalco- 



Total Fish Larvae m Neuston Samples 



B 



3 5 7 9 11 13 15 17 19 21 23 



Local Time (Hours) 



Total Fish Larvae in Bongo Samples 



° 2000 



llllllllllllllllllll 



1 3 5 7 9 11 13 15 17 19 21 23 



Local Time (Hours) 



Figure 3 



Diel variation in density and occurrence of total 

 fish larvae in (A) neuston samples and (B) bongo 

 samples for all cruises combined (1981-86). Bars 

 represent mean density offish larvae; dashed line 

 represents total number of collections; solid line 

 represents the number of hauls that collected fish 



gramma, had lowest occurrences in neuston samples 

 during the day (Fig. 4). Sampler avoidance by the 

 larvae during daylight probably contributed signifi- 

 cantly to this pattern. The taxa Hemilepidotus 

 jordani, H. spinosus, Myoxocephalus spp., Bathy- 

 master spp., and Zaprora silenus were absent from 

 neuston samples during most daylight hours but 

 were relatively abundant in twilight or nighttime 

 samples. Because the latter three of these taxa were 

 relatively common in bongo samples (Table 4), indi- 

 cating a facultative association with the neuston, 

 their scarcity in the neuston during the day may have 

 been at least in part due to a diel pattern of vertical 

 migration with larvae moving toward the surface 

 zone at night. This may have also been true for T. 

 chalcogramma and Ammodytes hexapterus whose 

 larvae were extremely abundant in the bongo net 

 samples (Table 4) but abundant in the neuston 

 samples only at night (Fig. 4). Mallotus villosus lar- 

 vae were also relatively common in bongo samples 

 (Table 4), although they were most abundant in the 

 neuston at night (Fig. 4). 



As expected, catches of T. chalcogramma eggs 

 showed no discernable diel variation in either den- 

 sity or frequency of positive hauls. The large mean 

 densities during two periods of the day are most likely 

 due to spatial variation of egg densities rather than 

 to any biological factor. 



Length distributions of dominant neustonic 

 taxa 



Standardized length distributions were plotted for 

 the dominant neustonic taxa (Fig. 5). Comparisons 

 were made with the corresponding length distribu- 

 tions of larvae in the subsurface zone for six of these 

 taxa that were sufficiently represented in the bongo 

 samples. For all these six taxa, greater median 

 lengths were documented for larvae in the neuston 

 than in the bongo hauls, especially in the case of 

 Mallotus villosus and Ammodytes hexapterus. 

 Mallotus villosus seemed unusual in that all larvae 

 caught in both neuston and bongo net samples were 

 >25 mm SL indicating a predominance of postflexion 

 larvae. This is most likely due to species identifica- 

 tion capabilities, as it is not possible to identify small 

 osmerids to species until the pectoral fin rays are 

 completely developed. With the exception of 

 Bathymaster spp., the larvae caught in the neuston 

 were also significantly larger than those caught in 

 the bongo collections (Kolmogorov-Smirnov (K-S) 2- 

 sample tests; allP<0.01). For A. hexapterus, it seemed 

 that only the large postflexion larvae and early ju- 

 veniles (mostly >20 mm SL) migrated into the neus- 

 ton, mainly at night; most A. hexapterus larvae 



