Choat et al. Comparison of ichthyoplankton sampling methods 



205 



POMACENTRIDAE 



MULLIDAE 



2 3 

 Sompling Dote 



4 5 6 7 

 - December 1986 



2 3 4 5 6 7 

 Sampling Date — December 1986 



Figure 7 



Changes in mean density i±SE) of large !>6mml pelagic pomacentrids and mullids 

 sampled by six methods over six nights, 2-7 December 1986 off Lizard Island, 

 Great Barrier Reef. Density estimates for the aggregation devices are not ad- 

 justed for volume sampled. Some methods did not collect large pomacentrids or 

 mullids. 



The aggregation devices indicated that large mullids 

 were rare or absent until the 5th, and increased greatly 

 in density on the 7th (Fig. 7). This trend was not present 

 in data from the nets. Only the neuston net caught 

 large mullids, but in low and variable numbers. 



Discussion 



The taxonomic composition obtained when sampling 

 for larval and pelagic fishes is highly method- 

 dependent. The bongo net captured the largest num- 

 ber of families, many of which were rare in the samples. 



Among abundant taxa, the four nets 

 provided similar estimates of taxo- 

 nomic composition. The light-trap, 

 however, was more selective, and its 

 catch differed in composition from that 

 of the nets. Taxonomic composition of 

 the light-seine samples was interme- 

 diate between the trap and nets, an 

 expected result given its mode of op- 

 eration. 



Our results suggest that capture by 

 the light-trap is dependent on fish 

 size: larger pelagic stages are more 

 likely to be attracted to the light and 

 to swim into the trap than are small 

 stages. However, trap performance 

 may also be time-dependent. For ex- 

 ample, apogonids, carangids, lutjanids, 

 and scarids, which were rare or ab- 

 sent in light-trap catches during this 

 study, have been captured during ex- 

 tended light-trap sampling around Liz- 

 ard Island (M. Milicich, Griffith Univ., 

 Nathan, Queensland, pers. commun.). 

 The absence from light-traps at par- 

 ticular times may simply indicate that 

 large or well-developed individuals 

 of some families were not present at 

 that time. 



However, our study provides evi- 

 dence that pelagic stages of some 

 families may not be photopositive or 

 enter traps, thus indicating some se- 

 lectivity by the aggregation devices. 

 Schindleriids were present in the net 

 samples to adult size, yet were not 

 captured with either of the light- 

 aggregation methods. The net samples 

 may have included the largest pelagic 

 individuals of callionymids, and per- 

 haps platycephalids and bothids, because they leave 

 the pelagic environment (i.e., settle) at a relatively small 

 size (see Table 3). These families were not present in 

 the light-trap catches. 



The size-distribution and density estimates of pelagic 

 fishes captured also differ among nets. The bongo net, 

 neuston net, and purse-seine captured predominantly 

 smaller fishes. For abundant families, density estimates 

 by the bongo net and purse-seine were generally simi- 

 lar, neuston net estimates were somewhat lower, and 

 the Tucker trawl provided still lower estimates. The 

 bongo net provided the highest abundance estimates 

 for most sizes of most families. The Tucker trawl 



