6 
Fishery Bulletin 1 1 1 (1) 
5 mm 
5 mm 5 mm 
Figure 4 
Examples of close-up, in situ images of different lar- 
val fish taxa imaged with the In Situ Ichthyoplankton 
Imaging System (ISIIS). (A) Paralichthys dentatus (4 
mm); ( B) Gobiidae (8 mm); (C) Gadidae (32 mm); (D) 
Clupeidae (21 mm); (E) Merluccius spp. (14 mm); (F) 
unknown (preflexion stage) (3.2 mm). 
see Fig. 3), and larval fishes were relatively rare, the 
imagery provided a relative measure of abundance of 
different plankters. In most cases when fish larvae 
were encountered, the imagery was sufficient to dis- 
cern characteristics valuable for identification at the 
family or genus level (e.g., shape, number and location 
of fins, overall body shape, fish size, and, in some cases, 
certain skeletal features; see Fig. 4). 
The 2 sampling methods allowed us to detect com- 
parable quantities of larval fishes. ISIIS imaged a total 
of 409 larvae, and the bongo tows collected a total of 
359 larvae. When standardized for the volume of wa- 
ter actually sampled, ISIIS estimated -0.18 fish larvae 
(±0.015 standard error of the mean [SE] nr 3 ), a value 
that was not significantly different from the estimate 
from the bongo tows (0.24 ±0.037 SE nr 3 ; P=0.074). 
Similarly, within gears, there were no differences in 
larval fish concentrations between transects. 
The estimates of larval abundance, however, were 
made on the basis of the 2 gears sampling different 
portions of the water column. The bongo net sampled 
all depths equally as it was towed from depth to the 
surface, but ISIIS spent less time at depths >40 m 
than at depths near the surface (Fig. 5A). This sam- 
pling effect is evident in the difference in measured 
fish abundance by depth (Fig. 5B), where the apparent 
pattern was for a continual increase in fish abundance 
with depth from the surface down to 40 m and then 
a decrease in abundance by depth beyond 40 m. This 
decrease was directly coincident with the drop-off in 
sampling time with depth by ISIIS. When an adjusted 
abundance was estimated by computing depth-specific 
concentrations (Fig. 50, then with the assumption 
of equal sampling effort per depth as with the bongo 
tows, an adjusted mean ISIIS fish concentration was 
0.22 fish larvae rm 3 , which is very close to the bongo 
estimate. 
The taxonomic diversity collected by each gear also 
was similar; both collected larval fishes representing 
the same 7 families (Table 1), although bongo samples 
were typically identifiable to lower levels (genus and 
species) than those in ISIIS samples. Images of fish 
larvae from ISIIS were identifiable to at least the ge- 
nus level for -35% of larvae (143 out of 409). On the 
other hand, larvae were unidentifiable in 60 fish im- 
ages and most of these unidentifiable fishes were in the 
early preflexion stages (-15%); in contrast, all bongo 
tow larvae were identified at least to the family level. 
Comparison of the relative proportions of taxa between 
the 2 sampling methods indicates that they were simi- 
lar. There were a few notable exceptions: ISIIS under- 
estimated paralichthyids and scopthalmids and esti- 
mated relatively greater proportions of phycids and 
ophidiids than the bongo sampler. The total number 
of larvae sampled was similar, but it is not known if 
the “unknown” category would have evened these dis- 
crepancies or added further differences among certain 
taxa. 
Size distributions of larvae differed considerably be- 
tween the 2 sampling methods. ISIIS imaged a larger 
size range and larger mean size of fish larvae than the 
bongo sampler (Fig. 6, Table 2). This sampling gear 
pattern was evident across several individual taxa, no- 
tably the gadiform fishes, Phycidae and Gadidae, with 
the latter mean size from ISIIS samples being more 
than 3 times the mean size of this family from bongo 
samples (Table 2). There was also a significant differ- 
ence between gear types with respect to size of Para- 
lichthyidae, although this very small difference (0.103 
mm) may not be biologically meaningful and likely was 
significant only because of the rank nature of the Krus- 
kal-Wallis test. There was a significant difference in 
overall larval size between transects for the ISIIS sam- 
ples, but there was no significant difference in overall 
larval size for the bongo tows between the 2 transects 
or for any taxonomic group between transect within 
gear type (Fig. 6, Table 2). Therefore, most of the dif- 
ferences in size were attributed to sampling gear. 
