Field Technique

Preliminary investigations in the summer of 1949–50 had shown that snapper eggs occurred in irregular patches and that these patches seldom persisted for more than one day. The short duration is not surprising since the egg stage from fertilisation to hatching usually lasts less than 48 hours. For this reason it was desirable to employ a sampling technique which covered the greatest possible area in the time available. From vertical closing net samples it was determined that the majority of eggs were to be found at or near the surface, so that surface sampling should give at least an approximate index of the relative abundance. Three successive models of high speed nets were designed and employed during the course of the work As each new type was brought into use it was calibrated against the previous type by towing the two nets together over the same course and making a volumetric comparison of catches It was found that differences in catching power between nets either of the same or different models were negligible.

Model I, which was employed in 1950–51, was based on a design by Mr. A. S. Fuller (then of the Zoology Department, Auckland University College). This consisted of a sleeve of bolting cloth 180 cms long, attached in front to a cylindrical brass mouthpiece 6 cms in diameter by 6 cms long, and behind to a removable brass cup 5 cms in diameter by 7 cms long. On opposite sides of the mouth two brass lugs served for attachment of a steel wire bridle The brass cup was fitted with a wire gauze draining panel so that, except when catches were particularly large, the volume of the sample could be reduced sufficiently to be stored in a four by one inch glass specimen tube Two grades of bolting cloth were used in different instruments of Model I: 16 and 30 meshes per cm. The coarser mesh was sufficient to retain all snapper eggs while releasing smaller organisms and hence facilitated counting, but the finer mesh was found to tear much less readily.

In 1951–52 Model II was introduced This had the same mouth and collecting cup, but was reduced in length to 90 cms, and the bolting cloth was replaced by brass gauze, 16 meshes per cm. Mouthpiece and cup adapter were joined by two struts of 0.65 cm diameter brass rod. This model was found more durable, though its length made it somewhat unwieldy, and there was still a tendency for the net to tear about the middle of its length after prolonged use Model III, which is illustrated in Fig. 2, was brought into use in 1953 and has not as yet been employed in any intensive survey. However, it has proved more durable and convenient to use, and samples just as efficiently as Model II, so that it would undoubtedly be preferred for any further surveys of similar nature. The length of the filtering gauze has been reduced to 45 cms, while greater rigidity and protection is gained by the use of four struts.

Fig. 2—High-speed surface plankton sampler, Model III Total length 60 cms (approximately)

The high-speed sampler was towed over the stern of the ship from the port quarter. When a second net was used this was towed from the starboard quartes, so that the two would be about 3 metres apart The one metre wire bridle was attached by a heavy galvanised swivel to a 40-metre towing warp of 2 ½ cm circumference Phormium rope, which was found to be relatively free from kinking and coiled well on the deck. During plankton sampling the ship travalled at full speed (approximately 8 knots.) The course was plotted by three-point sextant fixes, but