The Open Sea: The World of Plankton. Alister Hardy

to 15 to 20 fathoms to get a good haul. To reach this depth you will require a good length of line—50 to 60 fathoms—and you will also require a much heavier weight, say a 20 lb lead, to take your net and all this line down. Care must be taken, of course, to know just how deep the sea is at the point where you are working so as not to run the risk of trawling the bottom with your net and either bursting it by filling it with mud or tearing it to ribbons by dragging it over a rough bottom. If you have not a chart you can consult, you should take a sounding with your lead and line before starting.

      It is often very interesting to take a series of samples from different depths at the same place as near together in time as possible to enable you to study the depth distribution of the various animals; if you repeat the series again at night you may be very surprised at the different results you will get. As you let the net run out on its line to a deeper level it will fish very little on its way down, for it is moving backwards with the water as it runs out and sinks; when you haul it up, however, at the end of a tow, it will of course fish all the way up. This difficulty is got over in modern oceanographic practice by having in front of the net a special closing mechanism which is operated by a brass messenger weight sent sliding down the cable; this releases the bridles when a trigger is struck and the net falls back to be closed and held by a throttling noose which passes round it behind the mouth. The net is thus hauled up to the surface closed like a sponge-bag with the strings drawn tight and you know that all the animals in it must have been caught at the actual level at which it was fishing. A simple example of this arrangement is shown opposite in Fig. 18. These devices, however, are perhaps rather elaborate to be practised by the amateur, especially as a smooth steel cable is required for the messenger and this means the use of a winch; they will not be further described but full information about them will be found in the descriptions of the equipment used on the Discovery Expeditions (Kemp and Hardy, 1929). To minimise the effect of catching whilst hauling up an open net, it is well to make rather longer hauls with it down; the time taken in coming up will then only be a small fraction of that during which the net was fishing at its proper level.

      If you are going to take a number of such hauls for study you will soon accumulate far more material than you can hope to keep alive successfully; in this case it will be best to keep only a small part of one or two samples fresh and preserve the rest for study dead. The living plankton will give you the greatest pleasure in studying the swimming movements and behaviour of the animals; the dead samples may nevertheless give you interesting information about the depth distribution of the same animals, which you could not otherwise obtain. The best general preservative for plankton, and the easiest to use, is formalin, i.e. a 40% solution of formaldehyde in water; this, which can easily be obtained at any chemist, can be added to the sample in quite small quantities to give a mixture (about 5% formaldehyde) strong enough to keep it indefinitely in good condition. Remember always to reserve separate jars for preserved samples—never mix them with those used for fresh; a good plan is to stick a red label on them for danger! The dead formalined samples can of course be concentrated into a smaller space; 1lb honey jars with screw-on tops are convenient for their storage. If you are going to keep the samples for any length of time it is well to use what is called neutral formalin, i.e. that to which just sufficient borax—from 5 to 10 grams per litre—has been added to neutralise its acidity; ordinary formalin nearly always contains formic acid which if not so neutralised will very soon dissolve away the calcareous shells and skeletons of many of the animals.

      FIG. 18

      Sketches of a simple release mechanism for closing the mouth of a tow-net before hauling it to the surface. A, the rig of the net when towed; B, enlarged view of release gear about to be struck by messenger weight; C, the towing bridles released and the net closed by throttling rope.

      What has just been said will have been sufficient to have corrected that very common misconception that the plankton exists almost entirely near the surface of the sea. Some people seem to have thought of it as existing as a kind of scum on the very surface itself; this is no doubt due to a misunderstanding of the expression often used that the plankton is the ‘floating life’ of the sea. The plants, as we have already seen, do in fact only flourish for a little way below the surface; but animal members may be found at all depths. Later on—in Chapter 12—we shall describe the plankton and nekton that is to be found at various levels in the ocean between the surface and the bottom, thousands of fathoms deep beyond the continental shelf. There is another erroneous impression about the plankton that is frequently held: the idea that it is more or less evenly distributed over quite wide stretches of the sea; it is often thought that if we used a tow-net in one place and another two or three miles away on the same day, the two samples would be almost exactly alike. This indeed may occur, but it is by no means always so.

      A great many surveys have been made in the past, often in relation to some fishery problem, attempting to give some idea of the varying quantities of the major plankton organisms over a particular area. I have already described how a research ship will proceed in such a survey to traverse the area, stopping or slowing down to take tow-net samples at regular intervals. If the area to be covered is a big one, the stations—as the different points of observation are termed—cannot be very close together or the survey would take much too long; they are frequently spaced twenty miles apart. It has usually been assumed that a sample at one point, will, within a reasonable range of error, give a fair representation of the plankton in the area for ten miles around it; thus it has been felt that a series of such stations twenty miles apart will give an adequate quantitative survey. Some plankton organisms are much more patchy in their distribution than others; for some kinds such a method may give quite an adequate picture, but for others it may be hopelessly misleading. Very early in my career as a marine naturalist I had an experience which I will recall because it so well illustrates this very point; it was an episode which had a marked effect on much of my later work. In 1921, soon after leaving the University, I was appointed as Assistant Naturalist on the staff of the Fisheries Laboratory of the Ministry of Agriculture and Fisheries at Lowestoft and was delighted to be allowed to study the plankton in relation to herring. In March of the following year, through the illness of a senior, I found myself, at the last moment of sailing, as naturalist in charge of a cruise on that grand old research trawler the George Bligh.