3.1 The advantages of using conical tow nets are as follows:

3.1.1 They are relatively inexpensive and highly versatile in a variety of inland, estuarine, coastal, and marine waters.

3.1.2 They can be used from a small or large powered boat with a minimum of auxiliary equipment.

3.1.3 They can be used to collect qualitative samples and semiquantitative samples when fitted with a flowmeter and even better samples when fitted with a companion meter on the outside of the hoop to monitor filtering efficiency.

3.2 The disadvantages of conical tow nets are as follows:

3.2.1 When equipped with a flowmeter they require frequent maintenance including calibration and, in some types, lubrication.

3.2.2 They are effective only where drawn through a stream of water having considerable thickness. They are not suitable for collecting samples from a small or restricted region.

3.2.3 They are not suitable for collecting in very shallow water.

3.2.4 They are clogged by grass beds, coelenterates, and filamentous algae.

3.2.5 When used with a flowmeter, they collect only qualitative samples, or semiquantitative samples.

3.2.6 When sampling discrete depths using a horizontal tow, the sample can be contaminated from other depths during the deployment and retrieval of the samples if opening and closing devices are not used.

3.3 There are several special considerations that shall be observed when using conical tow nets. They are:

3.3.1 Conical tow net samplers are designed to be towed at speeds less than three knots; however, greater speeds have been used for the larger nets with a concomitant increase in capture.²

3.3.2 A conical tow net 0.5 m in diameter or larger shall be used to reduce avoidance by organisms.²

3.3.3 The nets shall be washed frequently and inspected for pin-size holes, tears, net deterioration, and other anomalies.

3.3.4 Nets should be allowed to dry while suspended full length in air and in subdued light prior to storage.

3.3.5 Lower catches per sample may result when collections are made during the day. These are particularly noted in the larger zooplanktons.

TABLE 1 Size of Common Zooplankton^A

^A Sage, L. E., "Zooplankton ," In: Methods for the Assessment and Prediction of Mineral Mining Impacts on Aquatic Communities: A Review and Analysis, Fish Wildlife Service/Office of Biological Services, Vol 78/No. 30, April 1978, pp 55-65.