SISP 2 Manual for the Midwater Ring Net sampling during IBTS Q1
The Midwater Ring Net is at present the standard gear for sampling fish larvae during the North Sea International Bottom Trawl Survey in the first quarter. The standard rigging and procedures for undertaking MIK sampling during IBTS Q1 is described.
The Midwater Ringnet is the present standard gear for the sampling of fish larvae during the North Sea International Bottom Trawl Survey in the first quarter. The sampling for fish larvae during the IBTS Q1 was initiated in 1977, and during the period 1977–1991 an Issacs Kidd Midwater Trawl was used as the standard gear. When changing to the present ringnet (which use a long two-legged bridle attachment as for the frame trawl described by R. D. Methot) the used abbreviation for the new gear – MIK- was erroneously interpreted as an abbreviation of the names Methot, Isaacs and Kidd. This is not the case, and the present gear has only a passing resemblance to the gears described by R. D. Methot or Isaacs and Kidd. Hence the names to be used for the gear are either “MIK” or “Midwater Ringnet”.
The construction of the MIK is strong and robust because of the often harsh conditions in the first quarter period in the North Sea. Hence compared to the smaller conical nets traditionally utilized for plankton sampling, the gear is made more robust by using a very strong (and heavy) ring frame, and a net strengthened with nylon or canvas reinforcing straps. The main target of the MIK-sampling is larval herring between 2 and 3 cm length, which at that size are of quite low densities. Therefore, the gear has a relatively large opening of 2 metre in diameter. Furthermore, because of the fast escape behaviour of the large herring larvae, sampling is carried out at night (with a black net) at a towing speed of 3 knots. The speed constraints the mesh size and a relatively coarse mesh of 1.6 mm is standard.
The standard rigging and procedures for undertaking MIK sampling during IBTS Q1 is described in the sections below. Minor differences in MIK design may occur among participants, however, these differences are all believed to be insignificant with respect to catchability of the gear. Nevertheless, the comparability among participants is verified for each survey. This is done by comparing the results of participants that sample the same ICES rectangles.
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