SUBSAMPLER FOR ESTIMATING THE NUMBER 



AND LENGTH FREQUENCY OF SMALL, 



PRESERVED NEKTONIC ORGANISMS' 



When many samples, containing large numbers of 

 organisms, must be processed it is often necessary 

 to take subsamples and assume that they are rep- 

 resentative of the total sample. Frequently sub- 

 samples are taken in some arbitrary fashion 

 which is described in such terms as "100 fish were 

 randomly selected." However, it is doubtful 

 whether any selection can be adequately random. 

 Therefore, numerous devices have been designed 

 in attempts to secure more representative sub- 

 samples and to increase the speed and efficiency of 

 subsampling. 



Most subsamplers have been designed for use 

 with plankton, small benthos, and invertebrate 

 drift samples and are generally unsuitable for 

 larger organisms. However, Lewis and Garriott 

 ( 1971) modified a Folsom plankton splitter for use 

 on meter net samples containing larval fish up to 

 19 mm long, and Hightower et al. ( 1976) described 

 a subsampler specifically designed for use with 

 nektonic organisms. 



In the present paper I describe the design, oper- 

 ation, and efficiency of a subsampler originally 

 built for research on estuarine nekton (Herke 

 1971). The subsampler proved to be useful for es- 

 timating the number and length frequencies of 

 small nektonic organisms such as the bay an- 

 chovy, Anchoa mitchilli, tidewater silverside, 

 Menidia heryllina, and brown shrimp, Penaeus az- 

 tecus, as well as young of larger species such as 

 gulf menhaden, jBreyoor^/apafronus, and Atlantic 

 croaker, Micropogon undulatus. Although differ- 

 ent from most subsamplers, the design is fairly 

 similar to that described by Hightower et al. 

 (1976); it bears some similarities to those de- 

 scribed by Hewitt and Burrows ( 1948) for subsam- 

 pling live hatchery fish, by Gushing (1961) for 

 plankton, and by Sodergren (1974) and Hickley 



(1975) for benthos. 



My sampler differs from that of Hightower et al. 



(1976) in at least four respects: 1) it has fewer 

 moving parts; 2) fewer water jets are required to 

 achieve through mixing of the sample; 3) a cen- 

 tral pillar or cylinder prevents organisms from 

 clumping in the center; and 4) the total sample is 



'Contribution no. 24 of the Louisiana Cooperative Fishery 

 Research Unit: Louisiana State University, Louisiana Wildhfe 

 and Fisheries Commission, and U.S. Fish and Wildhfe Service 

 cooperating. 



subdivided by raising vanes through the mixed 

 sample, rather than allowing the sample to settle 

 into baskets. Also, spin-dry weighing is required, 

 but it takes <1 min to complete the subsampling 

 process (after the organisms are placed in the sub- 

 sampler), rather than several minutes as required 

 for the subsampler described by Hightower et al. I 

 have made no comparative tests between the two 

 subsampler designs, however; individual cir- 

 cumstances may determine which would be most 

 practical in any given situation. 



Subsampler Construction 



My subsampler can be constructed of various 

 materials, and the same general design can be 

 used for large and small models. A small Plexi- 

 glas^ version ( Figure 1 ) has an outside diameter of 

 305 mm, and Herke (1971) also illustrated one 

 with a 580-mm outside diameter that utilized part 

 of a 208-1 steel drum for the outer cylinder, a 19-1 

 bucket for the inner cylinder, and plywood for the 

 false floor. 



The subsampler in Figure 1 was constructed 

 primarily of Plexiglas about 6 mm thick. Plexiglas 

 joints were bonded with solvent (methylene 

 chloride and trichlorethylene). The major parts 

 and their functions are as follows (numbers refer 

 to the parts labeled in Figure 1): 



1. Base. 



2. Brass hinge for attaching base to edge of table 

 top. 



3. Outer cylinder bonded to base; in addition to 

 solvent, a suitable cement may be required to 

 ensure a watertight seal to the base. 



4. Gentral pillar of Plexiglas tube bonded to base 

 at exact center of circle formed by the outer 

 cylinder (3). 



5. Rubber stopper in (4) to prevent material from 

 falling inside the piller. 



6. Inner cylinder, which slides smoothly up and 

 down over (4). 



7. Locking pin for holding (6) in the raised posi- 

 tion. Rubber bands around (6) and over a peg 

 through the shaft of (7) hold the pin in place 

 (these are omitted from the diagram to avoid 

 cluttering). 



8. Vane bonded to (6); the outer edge almost 

 touches the outer cylinder. In the raised posi- 



^Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



490 



