765
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 population 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 design 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 Received July 24, 1996. Accepted September 27, 1996. J13571 P. Pepin1 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:
[email protected]
Can. J. Fish. Aquat. Sci. 54: 765–773 (1997)
through the mesh by small individuals and avoidance by large individuals (Brander and Thompson 1989; Somerton and Kobayashi 1989; Johnson and Morse 1994; Shima and Bailey 1994). The problem is made somewhat more difficult in studies of prey–predator interactions because of the need to sample both larval food resources (Suthers and Frank 1989) and predators (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 comparison 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 aggregated, which may lead to greater sampling variability than © 1997 NRC Canada
766
other plankters (Cyr et al. 1992). Thus, the choice of gear type–size and tow duration, and consequently the volume sampled, are likely to influence the level of regional integration, or averaging, of the abundance of fish eggs or larvae. As a result, the precision of surveys may be affected by the choice of sampling gear owing to differences in the effective representation of the population. In this paper, we examine both the sampling variability and capture efficiency of bongo nets and a modified Tucker trawl used to sample ichthyoplankton and other macrozooplankton. Bongo nets are standard gear used in many ichthyoplankton surveys (Smith and Richardson 1977; Johnson and Morse 1994; Shima and Bailey 1994). In this study, we used a modified Tucker trawl (Pepin 1993; Laprise and Pepin 1995; Pepin et al. 1995) to examine reduced sampling variability and increased capture efficiency for large ichthyoplankton. The objectives of this study were (i) to determine if the sampling variability in estimating abundance of ichthyoplankton and macrozooplankton is affected by gear types, (ii) to establish if the range of ichthyoplankton sizes effectively captured differs between gear types, and (iii) to establish if the two gears sample the entire macroplankton community equally well.
Materials and methods The study was conducted in Conception Bay, Canada (47°45′N, 53°00′W), a physically dynamic system (deYoung and Sanderson 1995) with a subarctic plankton community (Davis 1982; Laprise and Pepin 1995). Conception Bay is part of the Labrador – northeast Newfoundland shelf and is influenced by the inshore arm of the Labrador current as well as wind forcing on time scales of 5–15 days (deYoung and Sanderson 1995; Pepin et al. 1995). The experimental design consisted of taking seven replicate samples at each of two station locations, one site at the mouth (16 and 26 July 1994) and the other at the head (23 July and 3 August 1994) of the bay, using both bongo nets and a Tucker trawl. The experiment was repeated twice at each location. Different locations were chosen to sample plankton communities characteristic of shelf and coastal water masses. We alternated each gear type on replicate tows, with the choice for the first tow being decided by a coin toss. All sampling was conducted during daylight hours (07:00–19:00) to avoid potential bias associated with diurnal variations in net avoidance. Bongo nets were 60 cm in diameter (0.28-m2 mouth) and 4.5 m in length and were equipped with 333-µm mesh nitex. The Tucker trawl was 2 m on either side (4-m2 mouth) and 8 m in length, and it was constructed of three sections fitted with 1000-, 570-, and 333-µm mesh nitex consecutively from the mouth, which sampled 100, 23.6, and 5.9% of the mouth’s area, respectively. Trawl design was used as an approach to reduce relative sampling variability, by sampling a larger volume of water, and increasing capture efficiency for larger organisms while maintaining sample volume at a level that can be processed in the laboratory in a timely manner. Furthermore, the range of mesh sizes was chosen to reflect various gear types used in a wide range of ichthyoplankton studies. Each sample consisted of a single oblique tow of approximately 15 min at a towing speed of 2 kn (1 m/s). All sampling was conducted within a 1-km radius from the designated station location. Each net was lowered to 40 m at a rate of 0.25 m/s and retrieved at 0.064 m/s. Maximum tow depth was chosen to include the mixed layer in which >95% of the larval fish reside (Frank and Leggett 1982; deYoung et al. 1994). Volume filtered was estimated with a General Oceanics flowmeter positioned at the mouth of each net. On deck, nets were washed and samples preserved in 2% buffered formaldehyde. Fish eggs and larvae were sorted and identified to species or the lowest taxonomic level possible from each sample (Tucker trawl and
Can. J. Fish. Aquat. Sci. Vol. 54, 1997 one side of the bongo net chosen at random) by the Atlantic Reference Centre (Huntsman Marine Science Centre, St. Andrews, N.B., Canada) as were crab larvae (zoea and megalopa), euphausiids, mysids, medusae, and chaetognaths. Subsampling of an individual taxon was performed using a beaker technique (van Guelpen et al. 1982) for samples in which numbers of that species exceeded 200 individuals per stage (i.e., eggs or larvae) in the case of ichthyoplankton or 100 individuals per species, genus, family, or order in the case of macroplankton. The length frequency distribution for each species of fish larvae in each sample was estimated by measuring up to 200 larvae. Standard length was measured to the nearest millimetre using a dissecting microscope and a gridded background. Abundance (number per thousand cubic metres) of each taxon, stage, and length category (for fish larvae) was calculated for each sample and corrected for subsampling or for larvae that could not be measured because of damage _(
