There are three types of plankton net seine methods: vertical tow, oblique tow, and horizontal tow (Figure 3).
Vertical tow is the simplest seine method, in which the net is lowered into the sea from a stationary vessel and then pulled up. The plankton net is connected to the end of a wire rope that is wound into a winch, and the net is pulled by uncoiling the rope for the same length as the desired depth and then rewinding the rope again. Because the amount of seawater that can be filtered in a single haul is limited, this method is suitable for collecting organisms with relatively high population densities. Vertical tows are also used to collect samples for zooplankton community breeding experiments, which require "live" samples, since the towing time (distance) is relatively short and damage to samples during the towing process is minimized.
In the "oblique tow" and "horizontal tow," a large volume of seawater is concentrated by towing the net while the vessel is moving forward, making them suitable for collecting organisms with low population densities or scattered (patchy) aggregations. In the "oblique tow" method, the net is sunk to the desired depth and then immediately hauled up, so that samples are collected evenly from all layers from the depth at which the net is sunk to the water's surface. Normally, the net reaches the target depth in a short time by speeding up the unrolling speed of the wire rope to minimize the number of samples that enter the net during the sinking process, while the wire rope is slowly reeled up to set a longer towing distance when the net is pulled up. The trajectory of the net is therefore asymmetrical and V-shaped. Horizontal tow is a method of filtering a large amount of seawater at a specific depth by submerging the net at the desired depth and then keeping the net at that depth while the vessel continues to move forward. This method is used when the habitat depth of the target organisms is known, or when the biomass and composition of the community inhabiting a particular depth layer is being investigated.
