Samples were collected over a 36 h period in October 1987 at two stations .... Figure 1) and gut passage time (3.74 h at 12.5°C; Stuart and Verheye, 1991), but.
Journal of Plankton Research Vol.14 no.2 pp.249-259, 1992
Diel feeding and vertical migration of Sagitta serratodentata Krohn tasmanica Thomson (Chaetognatha) in the southern Benguela M.J.Gibbons Marine Biology Research Institute, Zoology Department, University of Cape Town, Rondebosch 7700 and Sea Fisheries Research Institute, Private Bag X2, Roggebaai 8012, Cape Town, South Africa
Introduction Chaetognaths are generally the most numerous, if not the most conspicuous, exclusively carnivorous predators in the marine zooplankton. Unlike other members of the zooplankton from southern African waters, especially copepods (e.g. Verheye, 1989) and euphausiids (Pillar and Stuart, 1988; Pillar et al., 1989; Stuart and Pillar, 1990; Gibbons et al., 1991a,b), the chaetognaths are poorly studied and little understood (Shannon and Pillar, 1986). Community diversity is low in both the southern (Heydorn, 1959) and northern (Venter, 1969) regions of the Benguela (cf. Agulhas current; Stone, 1965) and assemblages are dominated by only two or three species. These are abundant, however, and together with euphausiids and amphipods can comprise the bulk of offshore zooplankton biomass (Verheye and Hutchings, 1988). Sagitta friderici is a neritic species and the commonest chaetognath reported from the area (Heydorn, 1959; Lazarus, 1974), where it can represent up to 90% (Venter, 1969) of all chaetognaths caught from inshore waters. Sagitta minima and S.serratodentata tasmanica are both epiplanktonic species that tend to dominate the offshore waters of the southern (Heydorn, 1959) and northern (Venter, 1969) Benguela respectively. Such distribution patterns are subject to seasonal changes and are not mutually exclusive, with both species being common components of zooplankton trawls in both areas. Stuart and Verheye (1991) have recently reported on the diel migration and feeding patterns of S.friderici. This study compliments the latter, and is a useful addition to the coincident work of Peterson et al. (1990) and Gibbons et al. (1991a,b) on copepods and euphausiids collected during the same anchor-station study in October 1987. Method Sample collection Samples were collected over a 36 h period in October 1987 at two stations on a 249
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Abstract. The migration and diel feeding of Sagitta serratodentata tasmanica were investigated at two stations in the southern Benguela. Pronounced diel migration and feeding was observed offshore where the food environment was markedly stratified. The inshore population, because of the shallow bottom, did not migrate out of their prey field and did not exhibit a pronounced diel feeding pattern. Copepods comprise almost the entire diet of both populations and, although Centropages brachiatus was heavily selected for, the data do not differentiate between size or species selection.
M.J.Gibbons
Sample processing In determining patterns of vertical migration, all S.s.tasmanica from all samples were counted and measured in 2 mm size intervals under a stereomicroscope.
Table I. Depth layers (m) from which chaetognaths were selected for gut content analysis: these correspond to WMDs of Figure 1 Date (Oct. 1987)
Time
Depth
Station
N
25 26 26 26 26 26 26 26
22.50 11.03 16.10 17.15 18.15 19.10 20.35 22.50
0-15 60-100 160-184 100-184 100-184 75-100 0-25 0-25
Offshore Offshore Offshore Offshore Offshore Offshore Offshore Offshore
100 19 100 100 100 100 100 190
27 28 28 28 28 28
22.80 10.48 12.10 16.25 19.26 22.15
0-10 60-66 40-58 50-70 20-40 0-20
Inshore Inshore Inshore Inshore Inshore Inshore
100 100 40 50 100 100
Copepods
D
ID
4 5 6 55 154
9.61 0.72 61.10 71.33 26.03 2.86 24.65 69.14
2.18 0.29 8.45 32.86 12.34 9.21 5.49 10.61
52 13 21 21 48 54
62.97 15.72 4.52 2.29 6.59 19.81
36.71 7.60 2.47 1.14 5.74 11.75
96 19
3
The number of S.s.tasmanica (N) examined and the total number of copepods recovered from them is indicated. The density of S.s.tasmanica recovered from the sampled depth is shown (£>), as is the mean density of chaetognaths recovered from that sampling time integrated over the whole water column (ID): both values per cubic metre. 250
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transect running seaward from the Olifants river mouth, South Africa (31°37' S 18°18' E; Gibbons et al., 1991a). The offshore station was located at a distance 57 nautical miles from the coast (depth 190 m) and the inshore station at 5 nautical miles (depth-§0 m). At each station a CTD/rosette cast to the bottom provided continuous profiles of temperature and salinity. Water samples were collected at six depths (according to the temperature profile) throughout the water column during ascent of the rosette. Chaetognath collections from each station were made at regular time intervals over 12 h (Table I), using a 200 n-m meshed 1 m2 multiple opening-closing Rectangular Midwater Trawl (RMT 1*6). The net was towed obliquely at 2 knots in five different depth strata, from just above the bottom to the surface (offshore: 200-100, 100-60, 60-40, 40-20 and 20-0 m; inshore: 80-60, 60-40, 40-20, 20-10 and 10-0 m). Flow was estimated by means of a digital flowmeter mounted centrally above the mouth of the net. Supplementary mesozooplankton was collected in the upper 80 m from six depths using a diaphragm pump (Peterson etal., 1990) and used in selection trials. Approximately 2 m3 of water was delivered from each depth through a 7.6 cm2 pipe and filtered on deck through a 200 |xm mesh. All zooplankton samples were preserved in buffered (CaCO3) saline-Formalin for processing later.
Feeding and migration of S.serratodenlata tasmanica
Body length was measured from the anterior tip of the head to the end of the tail, excluding the tail fin. Large collections were subsampled using a Folsom splitter and a minimum of 100 individuals counted. Maturity stages of S.s.tasmanica were classified according to Thomson (1947), and juveniles defined as animals
