A Field Investigation of the Relationship Between Length of Tow, Size of Net and Sampling Error

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Abstract
A series of eight oblique net tows to 100 m was taken at night in oceanic waters off Equador to study the relationship between net size, tow length, and sampling error. Two replicate tows of each combination of two net sizes (1·0 m diameter, 0·25 m diameter) and two lengths of tow (2000 m, 500 m) were taken in randomized order. For the eighteen zooplankton species which were counted (13 copepods, 5 euphausiids), precision of the replicates increased with increasing volume of water filtered. However, for the seven most abundant species, the increase in precision was not directly related to volume filtered; increasing tow length resulted in a larger reduction of sampling error than did a corresponding (in terms of volume of water filtered) increase in net diameter. In addition, greater similarity of relative proportions of all species were obtained for replicates taken with 2000 m tows than with 500 m tows regardless of net size. The data support the relationship between net size, tow length, and sampling error derived from model experimentation.

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