Effects of hauling speed on zooplankton catches

Abstract

Three plankton nets, similar in all respects but size, were hauled vertically through the water column (100 m) at speeds ranging from 0.4 to 2.4 m/sec. Several components of the catch were counted, and the counts compared to test the effect of hauling speed. With increased hauling speed, the number of juvenile and adult euphausiids decreased (beyond 0.6-0.8 m/sec) ; the number of small organisms increased; and the condition of the catch deteriorated (beyond 1.5 m/sec). It is suggested that the mesh selectivity of a net depends in part upon the number of contacts between organism and mesh in transit from mouth to cod-end. This could explain why fewer small organisms were taken at low hauling speeds than at high hauling speeds.

The spatial and temporal abundance patterns of chaetognaths in the western North Atlantic Ocean

“…Tranter (l967) and Tranter and Smith (l968) failed to include g in their equations, yet it is clear from Table 5 in Tranter and Smith (1968) …”

“…Given a mesh size, the width of the organism appears to be the most important factor determining its retention, although flexibility of both the organism and the mesh strands and variability in mesh size can result in loss of organisms somewhat larger than the mean mesh width (Saville, 1968;Omori et al, 1965;Heron, 1968 Sands (1980) reported about a 5% reduction in length for hamata, but it is not clear whether or not body width changes after preservation.…”

“…In practice the mouth area of the MOC 1 was Because there was no flowmeter within the mouth of the net, the filtration efficiency could not be measured directly. Tranter (l967) and Tranter and Smith (1968) provided several equations useful for the determination of the initial filtration efficiency of a net given several characteristics of the gauze and towing procedure. The filtration efficiency (F) is calculated by the empirical relation:…”

“…Gehringer (1962) did not include any data, so it was not possible to determine the extent of the differences or the conditions under which the samples were collected. Tranter et al (1968) reported highly significant day/night differences in total chaetognath abundance off the southeast coast of Australia. Examination of their data, however, suggests that patchiness is an equally likely explanation of the day-night differences observed (see Different sized net comparisons for a further discussion of their results).…”

“…Day/night abundances of chaetognaths were also examined from data presented by Anraku (l954, 1955), Gehringer (l962), andTranter et al (1968). Where the author has not performed a test of significance for greater night abundance, I have performed a t-test on the logiO (X+l) transformed day and night abundances.…”