Biological collecting
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Various organisms inhabiting the ocean are collected for the purpose of research on the physiology and ecology of marine organisms, surveys of fishery resources, and clarification of the material circulation system via biological pumps. As shown in Figure 1, marine organisms are classified into three categories according to their mode of life: surface organisms (Neuston) that live above and below the ocean surface, organisms drifting in the ocean (plankton), swimmers (Nekton) that can swim freely and move by themselves, and benthic organisms (Benthos) that live on the sea floor or in sediment, each of which uses specific collecting tools.
Fig. 1 Classification of marine organisms by mode of life
海面: sea surface 水中: underwater 海底: bottom of the sea
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Plankton are organisms that have little or no swimming ability and live by drifting in the water. Plankton can be broadly classified into plant species such as diatoms and dinoflagellates and animal species such as copepods and jellyfish. Organisms that live temporarily suspended in the water during their growth stages (life history) are also considered to be plankton such as fish fry, larvae of crabs, starfish, and sea anemones.
There are two ways to collect plankton: by using a nylon net called a plankton net or by collecting water.
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【Plankton net】
Plankton nets are made of nylon mesh (Fig. 2) sewn together into a cone, cylindrical cone, or square cone shape, with the opening (mesh mouth) fixed to a metal frame. By moving the net in the water, plankton is filtered out of the seawater.
The most typical and simple structure of a plankton net is shown in Fig. 4-a. Three bridle ropes and three manropes are connected to the top and bottom of the net mouth ring, respectively. The bridle rope is connected to the winch wire rope via a retractable fitting called “swivel”. A weight is suspended from the manropes to keep the net submerged in the sea. At the tail of the net, there is a cod-end that collects the sample, which is opened to remove the sample. By attaching a flow meter to the net mouth, the amount of seawater that has passed through the net mouth (filtration volume) can be estimated.
To facilitate comparisons between samples collected in different areas and at different times, Dr. MOTODA Shigeru (Professor Emeritus, Hokkaido University) devised the North Pacific Standard Net (NORPAC net), which has a net mouth ring with 45 cm diameter and a 180 cm filtration side length (length of the net). It was also decided that the vertical towing method should be used to haul the net from a depth of 150 m at a rate of 1 m per second. The "twin NORPAC net" (Fig. 4-b), which consists of two NORPAC nets connected together, allows plankton nets with different mesh sizes to be attached to each ring, so that samples with different properties can be obtained simultaneously in a single tow.
Link to observation method “Collection of planktonic organisms by plankton net”
Fig. 2 Enlarged view of nylon mesh
目開き（目合い）: mesh size of net
ナイロンモノフィラメント: nylon monofilament
Nylon mesh is available in various standards depending on the thickness of the yarn (nylon monofilament), its weaving method, and the size of the mesh size. Nylon mesh is also widely used for sifting products in flour mills and for filtering out impurities in industrial products.
Fig. 3 NORPAC net: North Pacific Standard net
a) Composition of “NORPAC net” b) “twin NORPAC net”
スイベル: swivel ブライドルロープ: bridle rope リング: net mouth ring ろ水計: flow meter
ろ過部（ナイロンメッシュ）: filtration section (nylon mesh)
コッドエンド: cod-end 力綱: manrope 錘: weight
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Nektons are the organisms that have the ability to swim and live freely in the water, including many fish, cephalopod squids, and marine mammals such as dolphins and whales. These nektons are collected by quantitative sampling tools developed for research and surveys as well as by commercial fishing gear and fishing methods.
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【Research and study collection tools】
For quantitative collection of small swimming organisms such as juveniles, a collection device similar to a plankton net is used. Nektons are distributed sparsely or patchily in the sea, so they are collected by filtering a large amount of seawater. For this reason, swimmers are collected using nets with large mesh openings and good water drainage (filtration efficiency). The mesh size is larger than that of plankton nets, and the nets are strong enough to be towed as fast as possible in order to collect organisms with high swimming ability. Typical examples are fry nets (Fig. 4) and MOHT (Fig. 5).
Fig.4 Fry net
吊りロープ: wing pendant リング: net mouth ring (diameter 130 cm) モジ網: minnow net (mesh size: 2 mm) ナイロンメッシュ: nylon mesh
曳網索: towing rope ブライドル: bridle
A surface horizontal tow net for collecting larvae and juveniles. The net is placed near the surface of the water on the broadside and towed horizontally with the vessel running. A coarse mesh minnow net is used at the front of the net to reduce reduction of the efficiency of water filtration due to clogging. When towing at night, the vessel's lights are turned off to prevent the light from attracting organisms.
Fig. 5 MOHT
浮子: float 網口フレーム: gape frame
潜行板: depressor トロール網: trawl net コッドエンド: cod-end
曳網速度: Trawling speed 潜行力: Diving force
A type of frame trawl. It is used for quantitative collection of small fish and large plankton such as krill. It is equipped with a special depressor that changes its diving force in response to changes in towing speed, and has excellent depth stability during towing.
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【Fishing gear and method】
Fishery methods, which are conventionally used, are sometimes chosen as the method to collect marine biological samples. Fisheries methods, which are commercially efficient and large-volume sampling of marine organisms, are an effective means of surveying large numbers of individual organisms or estimating the amount of living marine resources. There are various kinds of fishing gears and methods, not only gill nets, purse seines, set nets, and other net fishing gears, but also angling such as longlines and fishing rods, harpooning, and hooks used to pull fish off rocks.
Fig. 6 Examples of fishing gear and method
立て縄: flag line 延縄: longline 流し網: drift net オッタートロール: otter trawl
Link to ocean observation method "otter trawl"
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Organisms that live on the seafloor surface or in the sediment (mud and sand) are called benthos. They include organisms from diverse taxonomic groups such as seaweed, sea anemones, corals, shellfish, gobies, starfish, and flatfish.
Benthos can be collected by dragging a net or bucket across the seafloor or by using a mud sampler to collect the entire sediment that is their habitat. The efficiency of collecting organisms with a mud sampler depends on the hardness and characteristics of the bottom sediment (e.g., sediment or gravel), but compared to the method using a net, it is possible to obtain samples of organisms living below the seafloor surface, i.e., in the sediment.
Collecting large benthic fishes requires the use of large commercial fishing gear, such as otter trawls, which require extensive specialized fishing equipment and experience. The collection gear used for research is designed to be used by research vessels without fishing equipment.
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【Towing type collector】
Benthic organisms are collected by dragging a net or bucket over the seafloor. The collection gear is selected according to the community to be sampled, taking into consideration the towing speed and the size of the net opening. If the population density is high and the organisms can be collected by simply scraping the seafloor surface, they are collected using a dredge, as shown in Fig. 7. On the other hand, if the population density is low and a larger seafloor surface needs to be scraped, or if benthic fish with swimming ability need to be collected, a "sledge net", a kind of the beam trawls (Fig. 8) is effective.
Fig. 7 Dredge
By scraping the bottom of the seafloor, organisms that live on the seafloor surface and in the bottom sediment are collected.
a) organisms dredge b) cylinder dredge
Fig. 8 Sledge net
A type of beam trawl with sledge shape frames on both sides of the net opening. The net is towed by sliding the frames across the seafloor. Since the width of the net opening is constant, the area of the towed net can be easily calculated by adding up the towing distance. The population density can be estimated by dividing the number of individuals collected by the area of the towed net.
ビーム: beam 内網: inner net 外網: outer net ソリ状フレーム: sledge shape frame
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【Bottom sampler】
Organisms that live on or under the seafloor are collected with the seafloor sediment where they live. Unlike methods using nets, samples do not fall through the mesh, so even small organisms are collected. Since the entire habitat of the organisms is sampled, the possibility of live capture is high and damage to the biological sample is minimized. There are two types of mud samplers: those that grab the sediment with a bucket (grab type bottom samplers), and those that stick a pipe into the seafloor and pull it out while maintaining the stratigraphy of the sediment (column shaped corers).
Fig. 9 Smith-McIntyre Bottom-sampler
This is a type of grab type bottom sampler that collects sediments by grabbing them with a bucket. When the bucket touches the bottom, the trigger is activated, and the bucket is thrust into the seafloor by the spring force. When the bucket is pulled out, it closes and grabs the benthic organisms along with the sediment.
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Sea water strider, flow algae, and other organisms that live while floating on the surface of the water are called neuston. They are collected with a net called a neuston net, which is used to tow them across the surface of the water.
Fig. 10 Neuston net
The net is towed on the sea surface to filter seawater and collect neuston. Since floats are attached to both sides of the frame, the net can be towed while always catching the surface of the water without the net mouth being submerged in the water.
曳航ブーム: towing boom 曳網索: towline ブライドルロープ: bridle rope フレーム: frame
浮子: float 網: net コッドエンド: cod-end
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