Variability and capture efficiency of bongo and Tucker trawl samplers in the collection of ichthyoplankton and other macrozooplankton P. Pepin and T.H. Shears Abstract: We examined the sampling variability and capture efficiency of bongo nets and a modified Tucker trawl used in the sampling of ichthyoplankton and other macrozooplankton by taking seven replicate samples at each of two stations on two separate occasions. Sample variance was highly significantly related to sample mean for all major taxonomic categories (i.e., fish eggs, fish larvae, crustaceans, and medusae–chaetognaths). Sampling variability of the bongo nets was significantly greater than that of the Tucker trawl for both fish eggs and larvae. Sampling variability of macrozooplankton was significantly greater than that of ichthyoplankton. For larval fish of 5 mm or less, bongo nets generally yielded higher estimates of abundance than the Tucker trawl and the reverse was true for lengths above 10 mm, but there was a significant influence of species-specific morphological characters. The large sample volume of the Tucker trawl relative to the bongo nets resulted in significantly higher estimates of species diversity for fish eggs and larvae but not for crustaceans or medusae. Although bongo and Tucker samplers are efficient at catching a wide range of sizes of larval fish, the latter’s lower variability may make it a more effective sampler. Résumé : Nous avons étudié la variabilité d’échantillonnage et l’efficacité de capture des filets bongo et d’un échantillonneur Tucker modifié utilisés pour échantillonner l’ichtyoplancton et d’autres organismes macrozooplanctoniques en prélevant sept échantillons répétés dans deux stations différentes à deux occasions différentes. La variance de l’échantillon était corrélée de manière hautement significative à la moyenne de l’échantillon pour toutes les grandes catégories taxinomiques (c.-à-d. oeufs de poisson, larves de poisson, crustacés et médusaires–chétognates). La variabilité d’échantillonnage des filets bongo était significativement plus grande que celle de l’échantillonneur Tucker pour les oeufs et les larves de poisson. La variabilité d’échantillonnage du macrozooplancton était substantiellement plus grande que celle de l’ichtyoplancton. Pour les poissons larvaires de 5 mm ou moins, les filets bongo donnaient en général des valeurs estimées de l’abondance plus élevées que l’échantillonneur Tucker et l’inverse se produisait pour les longueurs supérieures à 10 mm, mais il y avait là un effet significatif des caractéristiques morphologiques spécifiques des espèces. Le grand volume d’échantillonnage de l’échantillonneur Tucker par rapport à celui du filet bongo entraîne des valeurs estimées significativement plus élevées de la diversité des espèces dans le cas des oeufs et des larves de poisson, mais non dans celui des crustacés ou des médusaires. Bien que les échantillonneurs bongo et Tucker soient efficaces pour capturer une grande variété de tailles de poissons larvaires, la variabilité plus faible de l’échantillonneur Tucker pourrait en faire un échantillonneur plus efficace. [Traduit par la Rédaction] Introduction Proper understanding of the processes that influence popula- tion dynamics requires that sampling programs and devices provide both accurate measurements of changes in abundance and levels of precision that permit the detection of variations in key parameters (e.g., growth or mortality rates). Survey de- sign must provide adequate spatial and temporal resolution and sampling devices must be chosen to minimize sampling bias for the organisms under study. Ichthyoplankton represent a difficult group of organisms to sample. This is partly due to their high and variable degree of patchiness (Smith 1973; Hewitt 1981; Matsuura and Hewitt 1995) as well as their changing vulnerability to sampling gear as a result of extrusion through the mesh by small individuals and avoidance by large individuals (Brander and Thompson 1989; Somerton and Ko- bayashi 1989; Johnson and Morse 1994; Shima and Bailey 1994). The problem is made somewhat more difficult in stud- ies of prey–predator interactions because of the need to sample both larval food resources (Suthers and Frank 1989) and preda- tors (Frank and Leggett 1982, 1985). Although several studies have dealt with variations in catchability of ichthyoplankton to different gear types (Herra and Grimm 1984; Brander and Thompson 1989; Johnson and Morse 1994; Shima and Bailey 1994), few have considered the properties of various gears in terms of sampling variability (McGowan and Fraundorf 1966; Cyr et al. 1992). The com- parison of catches of fish eggs and larvae has to consider that these organisms are a relatively scarce component of most planktonic communities (Smith and Lasker 1978) and that the relative variability in catches is likely to be high because of the power function for the variance-to-mean relationship for most aquatic organisms (Elliott 1977; Downing et al. 1987; Vézina 1988; Pace et al. 1991; Cyr et al. 1992). In addition, Smith (1973) argued that ichthyoplankton were highly aggre- gated, which may lead to greater sampling variability than Can. J. Fish. Aquat. Sci. 54: 765–773 (1997) Received July 24, 1996. Accepted September 27, 1996. J13571 P. Pepin 1 and T.H. Shears. Department of Fisheries and Oceans, Science Branch, P.O. Box 5667, St. John’s, NF A1C 5X1, Canada. 1 Author to whom all correspondence should be addressed. e-mail: pepin@athena.nwafc.nf.ca 765 © 1997 NRC Canada