FISHERY BULLETIN: VOL. 70, NO. 3 



The primary purpose of the present study is 

 to evaluate the extent that a large grab sampler 

 (miniature purse seine) can generate samples 

 of fish larvae that are less biased than towed nets 

 with respect to avoidance. The catches of an 

 anchovy larva (Stolephorus purpureus) were 

 enumerated from the samples. These were 

 thought to be the most informative of the or- 

 ganisms captured by the net in part because they 

 were present consistently in the samples, in part 

 because they provide a spectrum of small to large 

 zooplankton essentially constant in body form, 

 and in part because there is a wealth of literature 

 on the sampling of fish larvae, in particular, an- 

 chovy larvae. 



DESIGN AND OPERATION OF 



THE NET 



The net employed was patterned exactly, in- 

 sofar as operational considerations are con- 

 cerned, after that described in Hunter, Aasted, 

 and Mitchell (1966), excepting that the body 

 of the net was constructed of 333 ^t Nitex.' The 

 length of the Nitex section was 100 ft and the 

 depth 21 ft. A small cod end was placed near 

 the bitter end of the net. 



The net was usually set in the form of an in- 

 complete circle from a platform mounted on a 

 16-ft Boston Whaler, so that the net set off the 

 port side. Closure of the circle was effected 

 during the first stages of hauling. The operation 

 from starting the set to pursing generally took 

 less than 5 min. Hauling up the net required 

 15 to 20 min with two men at work. Figure 1 

 shows the net in the water at the start of pursing. 



All of the data considered in this report were 

 taken in Kaneohe Bay, Oahu, where swell is 

 negligible and seas are generally less than 1 ft. 

 It was tested in open ocean swell and in seas up 

 to 4 ft. The operation was not affected by swell 

 and seemed to be effective in seas of short period 

 waves, although the effect of waves splashing 

 over the float line of the net was not evaluated. 



* Reference to trade names in the publication does not 

 imply endorsement of commercial products by the Na- 

 tional Marine Fisheries Service. 



One operational problem that we anticipated 

 never materialized. We had feared that signifi- 

 cant amounts of zooplankton would catch on the 

 net as it was being hauled up. This did not 

 happen even in the instance of chaetognaths, ap- 

 parently because the plankton, being alive, avoid- 

 ed the mesh. During the final stages, it was 

 necessary to carefully wash the catch toward, 

 and finally into, the cod end. 



FIELD AND LABORATORY METHODS 



The primary objective of the study, compar- 

 ison of the catches by the purse seine with catch- 

 es by a meter net, dictated the field sampling 

 pattern. Two skiflfs were employed, one with 

 the seine and the other with the meter net. As 

 soon as the seine was set, the second skiff began 

 towing the meter net as close to the seine as pos- 

 sible. The meter net was raised and lowered so 

 as to proportionally sample the same water col- 

 umn fished by the seine. The plankton net was 

 fitted with a Rigosha flow meter. Tows were 

 generally 10 min in duration, but sometimes they 

 were reduced to 5 min when clogging was a 

 problem. 



In the laboratory the total wet volume of the 

 samples was measured. The entire sample was 

 scanned for fish larger than 10 mm. Then either 

 the entire sample or an appropriate aliquot was 

 examined in detail, enumerating and measuring 

 all anchovy larvae {Stolephorus purpureus) , and 

 enumerating anchovy eggs. The entire sample 

 was then reconstructed by appropriate linear ad- 

 justments. Finally, all results were adjusted to 

 numbers per 300 m\ This value was selected 

 because it was intermediate between the actual 

 typical volumes of water strained by the two 

 samplers, giving the advantage that the numbers 

 to be dealt with are roughly the same as the ac- 

 tual numbers of organisms captured. 



In all, there were 44 day stations and 10 night 

 stations from Kaneohe Bay. The exact locations 

 of the stations are not relevant to this study and 

 so are not given. Most were made at Tester's 

 stations 1, 2, 5, and 10 (Tester, 1951). Sam- 

 ples were roughly evenly spaced from November 

 29, 1966, to August 25, 1967. 



790 



