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A modification of the Isaacs-Kidd midwater trawl for sampling at different depth intervals
Deep-Sea Research, 1964, Vol. 11, pp. 263 to 264. PergarronPressLtd. Printed in Great Britain.
A modification of the Isaacs-Kidd midwater trawl for sampling at different depth intervals WILLIAM G. PEARCY and LYLE HUBBARD Department of Oceanography, Oregon State University, Corvallis, Oregon
(Received 13 January 1964) A SCALED-DOWN model of the Multiple Plankton Sampler (MPS) (BE, 1962) was adapted as a cod-end collecting unit for a 6-ft (1.8 m) Isaacs-Kidd midwater trawl. The frame of the MPS is 35 cm on a side with holes along the leading edge for attachment of the trawl net (Fig. 1). In this sampler a pressure-actuated mechanism releases three rotating bars at preset depths, thus opening and closing the nets and sampling within three successive depth layers. The manufacturer* calibrated the pressurerelease system finding an error of about 2 ~ of the maximum depth or 20 m, and preset it to operate at depths of 150, 500 and 1000 m. We found by lowering the MPS vertically (wire angle < 5 °) to successive depths at sea that nets closed between 150-160 m, 480-520 m, and 940-960 m. Openingclosing operations take place during the descent of the trawl. Later models permit sampling during the descent or ascent. The weight of the MPS (13 kg or 29 lb) was originally thought to pose a flushing problem by sinking the cod-end below the level of the mouth of the trawl. However, where it could he observed alongside the R.V. Acona, the MPS travelled at the same level. Operational success of the MPS-IKMT unit has been good. Forty-two separate tows to 10~0 m, which should provide 126 samples, yielded 117 samples from discrete depths. All failures (7%) occurred because the deep net (500-1000 m) did not close. Water inside the cylinder of the pressurerelease mechanism may have caused the failures. An additional ' O ' ring was installed around the piston, and ridges in the cylinder that also may have prevented a full stroke of the oiston were ground down. With these minor structural changes, all nets have closed. The most serious problem encountered during sea trials with the modified trawl was insufficient flushing of the contents of the trawl past the MPS into the collecting nets. After tows to 1000 m depth, the net, which ascends with the cod-end open, still had animals adhering to the trawl netting. They were concentrated toward the end of the trawl near the attachment of the MPS, indicating that animals were not immediately passing into the cod-end. Under these conditions animals accumulating in the trawl at one depth layer may be a source of contamination for n~'ts open at lower depths. Flushing action was improved by : (1) using a knotless nylon liner instead of the one of knotted netting; (2) tailoring the trawl net with a gradual taper to eliminate the looseness produced when the ½ m cod-end diameter was constricted over the MPS; (3) adding a plastic fabric'[ collar to the end of the trawl to reduce accumulation of animals by furnishing a better means of attaching the MPS to the trawl net (Fig. 1). Bolts on four plates, extending the length of the inside, are inserted through holes in the plastic and the MPS frame and secured with wing nuts; and (4) adding a 2-m section of knotless nylon netting the same size as the liner between the MPS frame and the plankton nets to increase the flow of water through the MPS. With these changes the number of animals enmeshed in the trawl net was markedly ro:luced. However, some animals, such as fishes with depressible teeth, are still occasionally entangled in the trawl netting. Since these animals, whose depth capture is unknown, are a source of contamination for succeeding tows, they must be completely removed from the net after each tow. Providing sufficient flow of water through opening-closing devices to flush large nets will undoubtedly be a problem common to all cod-end units (see also FOXTON,1963). Some contamination is unavoidable since time is required for organisms to pass into the cod-end of the net, but it can be *G. M. Mfg. Co., 12 East 12th Street, New York, New York. "tHerculite ]No. 80, Herculite Protective Fabrics Corp., 661 4th Street, Newark, New Jer~.$t. 263
264
Instrumental Notes
minimized when the volume o f flow through the opening-closing device is increased or when the time fished at each layer is prolonged, i.e., when the ratio, flushing time/fishing time, is low. Besides the capability of sampling three depths per tow, another advantage of the MPS as a cod-end unit is its relatively large cross-sectional area ( > 1000 cm 2) permitting high flow rates into the collecting nels.
Acknowled,~mlents The authors are indebted to ALLAN B~ for his helpful correspondence and to the Captain and crew o f the R.V. Acona for their cooperation at sea. This study was supported by the Atomic Energy Commission, Contract AT (45-1) 1726. Shiptime was provided by grants from the Office of Naval Research and the National Science Foundation. REFERENCES
B~, A. W. H. (1962) Quantitative multiple opening and closing plankton samplers. Deep-Sea Res. 9, 144--151. FOXTON, P. (1963) An automatic opening-closing device for large plankton nets and midwater trawls. J. Mar. biol. Ass., U.K. 43, 295-308.
Fig. I. Multiple Plankton Sampler as cod-end opening and closing unit. Note the four plates used to attach the MPS to the plastic collar of the trawl. Two plates have deflectors to divert water away from the nets.
A modification of the Isaacs-Kidd midwater trawl for sampling at different depth intervals WILLIAM G. PEARCY and LYLE HUBBARD Department of Oceanography, Oregon State University, Corvallis, Oregon
(Received 13 January 1964) A SCALED-DOWN model of the Multiple Plankton Sampler (MPS) (BE, 1962) was adapted as a cod-end collecting unit for a 6-ft (1.8 m) Isaacs-Kidd midwater trawl. The frame of the MPS is 35 cm on a side with holes along the leading edge for attachment of the trawl net (Fig. 1). In this sampler a pressure-actuated mechanism releases three rotating bars at preset depths, thus opening and closing the nets and sampling within three successive depth layers. The manufacturer* calibrated the pressurerelease system finding an error of about 2 ~ of the maximum depth or 20 m, and preset it to operate at depths of 150, 500 and 1000 m. We found by lowering the MPS vertically (wire angle < 5 °) to successive depths at sea that nets closed between 150-160 m, 480-520 m, and 940-960 m. Openingclosing operations take place during the descent of the trawl. Later models permit sampling during the descent or ascent. The weight of the MPS (13 kg or 29 lb) was originally thought to pose a flushing problem by sinking the cod-end below the level of the mouth of the trawl. However, where it could he observed alongside the R.V. Acona, the MPS travelled at the same level. Operational success of the MPS-IKMT unit has been good. Forty-two separate tows to 10~0 m, which should provide 126 samples, yielded 117 samples from discrete depths. All failures (7%) occurred because the deep net (500-1000 m) did not close. Water inside the cylinder of the pressurerelease mechanism may have caused the failures. An additional ' O ' ring was installed around the piston, and ridges in the cylinder that also may have prevented a full stroke of the oiston were ground down. With these minor structural changes, all nets have closed. The most serious problem encountered during sea trials with the modified trawl was insufficient flushing of the contents of the trawl past the MPS into the collecting nets. After tows to 1000 m depth, the net, which ascends with the cod-end open, still had animals adhering to the trawl netting. They were concentrated toward the end of the trawl near the attachment of the MPS, indicating that animals were not immediately passing into the cod-end. Under these conditions animals accumulating in the trawl at one depth layer may be a source of contamination for n~'ts open at lower depths. Flushing action was improved by : (1) using a knotless nylon liner instead of the one of knotted netting; (2) tailoring the trawl net with a gradual taper to eliminate the looseness produced when the ½ m cod-end diameter was constricted over the MPS; (3) adding a plastic fabric'[ collar to the end of the trawl to reduce accumulation of animals by furnishing a better means of attaching the MPS to the trawl net (Fig. 1). Bolts on four plates, extending the length of the inside, are inserted through holes in the plastic and the MPS frame and secured with wing nuts; and (4) adding a 2-m section of knotless nylon netting the same size as the liner between the MPS frame and the plankton nets to increase the flow of water through the MPS. With these changes the number of animals enmeshed in the trawl net was markedly ro:luced. However, some animals, such as fishes with depressible teeth, are still occasionally entangled in the trawl netting. Since these animals, whose depth capture is unknown, are a source of contamination for succeeding tows, they must be completely removed from the net after each tow. Providing sufficient flow of water through opening-closing devices to flush large nets will undoubtedly be a problem common to all cod-end units (see also FOXTON,1963). Some contamination is unavoidable since time is required for organisms to pass into the cod-end of the net, but it can be *G. M. Mfg. Co., 12 East 12th Street, New York, New York. "tHerculite ]No. 80, Herculite Protective Fabrics Corp., 661 4th Street, Newark, New Jer~.$t. 263
264
Instrumental Notes
minimized when the volume o f flow through the opening-closing device is increased or when the time fished at each layer is prolonged, i.e., when the ratio, flushing time/fishing time, is low. Besides the capability of sampling three depths per tow, another advantage of the MPS as a cod-end unit is its relatively large cross-sectional area ( > 1000 cm 2) permitting high flow rates into the collecting nels.
Acknowled,~mlents The authors are indebted to ALLAN B~ for his helpful correspondence and to the Captain and crew o f the R.V. Acona for their cooperation at sea. This study was supported by the Atomic Energy Commission, Contract AT (45-1) 1726. Shiptime was provided by grants from the Office of Naval Research and the National Science Foundation. REFERENCES
B~, A. W. H. (1962) Quantitative multiple opening and closing plankton samplers. Deep-Sea Res. 9, 144--151. FOXTON, P. (1963) An automatic opening-closing device for large plankton nets and midwater trawls. J. Mar. biol. Ass., U.K. 43, 295-308.
Fig. I. Multiple Plankton Sampler as cod-end opening and closing unit. Note the four plates used to attach the MPS to the plastic collar of the trawl. Two plates have deflectors to divert water away from the nets.