Significance and Use
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.
3.3.2 A conical tow net 0.5 m in diameter or larger shall be used to reduce avoidance by organisms.
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.
1.1 This practice covers the procedure for obtaining qualitative samples of a zooplankton community by use of conical tow nets. Nets will collect most zooplankton, but some forms will avoid nets.
1.2 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.
1.4 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.