302
D. H. UUSHING
catches more material than the Indian Ocean net, but it was not stated
whether the additional material was of nauplii or of algae ; however, a
correction factor for converting catches of the Juday net t o the 1 m
was derived. From such nets, a proportion of the zooplankton population escaped through the meshes and a further proportion evaded
capture, and it is likely that the displacement volume included an unspecified volume of algae. There is no information on the mesh selection
of these nets in terms of displacement volume of animals of different
sizes. It is hoped that the loss of zooplankton through the meshes is
balanced by gain of algae and nauplii by clogging ; so the low volumes
might be underestimated and the high ones might be overestimated.
Loss by escape has not been measured for nets of this type, although it
has been shown for some larger animals (Fleminger and Clutter, 1965;
McGowan and Fraundorf, 1966), so it is possible that euphausids are
improperly sampled. However, as sampling was carried out both by
night and day, it is possible that the underestimate is not too great. At
the present stage of zooplankton sampling, the nets used probably
represent the best compromise for estimates of zooplankton displacement volume over extensive areas. The best way would doubtless be to
count the animals and estimate their volumes from an array of nets,
each sampling a band of sizes properly. But the labour for treating the
very large number of samples would never become available.
Many expeditions have sailed over the Pacific Ocean in the last two
decades. The zooplankton hauls are well summarized by Reid (1962b) ;
those in the very rich area in the eastern tropical Pacific have been
analysed by Blackburn (1966). Most hauls were made from about 200 m
to the surface (but some were hauled from 100 m and some from 300 m),
Observations in the Peru Current have recently been confimed with
large quantities of information on Hensen net hauls from 50 m t o the
surface. During the International Indian Ocean Expedition, the Indian
Ocean net was hauled through a water column of 200 m. In the Atlantic
Ocean, zooplankton operations in a standard way are lacking. Frontier
(1963) hauled a Hensen net (0.33 ma; 0.41 mm mesh) in the Guinea
Current off Abidjan; with the same net, Mensah (1969) has described
in some detail the seasonal cycle in the continental shelf off Ghana, i.e.
in the Guinea upwelling. There are a few observations made off Cap
Blanc in the Canary Current (Khromov, 1962; Pavlov, 1968) with a
Juday net. Between Dakar and Freetown, Khromov (19658) has recorded some observations during the early upwelling with a Juday net.
Elsewhere in the Canary Current and in the Benguela Current, the best
observations were those made on the " Meteor " expedition. On that
expedition, numbers of " metazoa " were counted from 4 1. of water
D. H. UUSHING
catches more material than the Indian Ocean net, but it was not stated
whether the additional material was of nauplii or of algae ; however, a
correction factor for converting catches of the Juday net t o the 1 m
was derived. From such nets, a proportion of the zooplankton population escaped through the meshes and a further proportion evaded
capture, and it is likely that the displacement volume included an unspecified volume of algae. There is no information on the mesh selection
of these nets in terms of displacement volume of animals of different
sizes. It is hoped that the loss of zooplankton through the meshes is
balanced by gain of algae and nauplii by clogging ; so the low volumes
might be underestimated and the high ones might be overestimated.
Loss by escape has not been measured for nets of this type, although it
has been shown for some larger animals (Fleminger and Clutter, 1965;
McGowan and Fraundorf, 1966), so it is possible that euphausids are
improperly sampled. However, as sampling was carried out both by
night and day, it is possible that the underestimate is not too great. At
the present stage of zooplankton sampling, the nets used probably
represent the best compromise for estimates of zooplankton displacement volume over extensive areas. The best way would doubtless be to
count the animals and estimate their volumes from an array of nets,
each sampling a band of sizes properly. But the labour for treating the
very large number of samples would never become available.
Many expeditions have sailed over the Pacific Ocean in the last two
decades. The zooplankton hauls are well summarized by Reid (1962b) ;
those in the very rich area in the eastern tropical Pacific have been
analysed by Blackburn (1966). Most hauls were made from about 200 m
to the surface (but some were hauled from 100 m and some from 300 m),
Observations in the Peru Current have recently been confimed with
large quantities of information on Hensen net hauls from 50 m t o the
surface. During the International Indian Ocean Expedition, the Indian
Ocean net was hauled through a water column of 200 m. In the Atlantic
Ocean, zooplankton operations in a standard way are lacking. Frontier
(1963) hauled a Hensen net (0.33 ma; 0.41 mm mesh) in the Guinea
Current off Abidjan; with the same net, Mensah (1969) has described
in some detail the seasonal cycle in the continental shelf off Ghana, i.e.
in the Guinea upwelling. There are a few observations made off Cap
Blanc in the Canary Current (Khromov, 1962; Pavlov, 1968) with a
Juday net. Between Dakar and Freetown, Khromov (19658) has recorded some observations during the early upwelling with a Juday net.
Elsewhere in the Canary Current and in the Benguela Current, the best
observations were those made on the " Meteor " expedition. On that
expedition, numbers of " metazoa " were counted from 4 1. of water