NOTES 



A LARGE, OPENING-CLOSING MIDWATER 



TRAVC L FOR SAMPLING OCEANIC NEKTON, 



AND COMPARISON OF CATCHES WITH 



AN ISAACS-KIDD MIDWATER TRAWL 



Avoidance of nets by agile micronekton and nek- 

 ton is one of the major problems with small mid- 

 water trawls routinely used for oceanic sampling 

 by oceanographers and marine biologists (e.g., 

 Harrisson 1967; Pearcy 1975; Roper 1977). A major 

 advantage of small trawls is that they can be 

 equipped with opening-closing devices so that 

 samples can be ascribed to discrete depths. This 

 capability is especially important for sampling 

 deep water where densities of animals are low. An 

 opening-closing rectangular midwater trawl with 

 a mouth area of about 25 m^ is the largest 

 opening-closing net described (Baker et al. 1973). 

 Another important advantage of small nets is that 

 they can be used from most oceanographic vessels. 



Large, commercial-size midwater trawls used to 

 sample oceanic micronekton (Berry and Perkins 

 1966; Harrisson 1967; Taylor 1968; Clarke 1973, 

 1974; Krefft 1974; Roper 1977) are usually not 

 evaded as successfully by nektonic animals. Big 

 nets which filter large volumes of water have the 

 added advantage of catching enough animals to 

 characterize species and size composition from 

 sparsely populated waters. However, these large 

 nets lack opening-closing capability, and most 

 oceanographic research vessels are unable to 

 handle large nets, otter doors, and bridles ( Pearcy 

 1975). 



This paper describes a 50 m"^ pelagic trawl es- 

 pecially designed for use with an opening-closing 

 cod end, attempts to evaluate its performance, and 

 compares its catches of mesopelagic fishes and 

 cephalopods with those from a 5.4 m^ Isaacs-Kidd 

 midwater trawl. 



Midwater Trawl Description and Operation 



The midwater or pelagic trawl was designed by 

 G. Loverich, Nor'Eastern Trawl Systems, Inc.,^ 

 Bainbridge Island, Wash., for sampling meso- 



' Reference to trade names or commercial firms does not imply 

 endorsement by the National Marine Fisheries Service, NOAA. 



FISHERY BULLETIN: VOL. 78, NO, 2, 1980. 



pelagic fishes and cephalopods in conjunction with 

 a 1 m^ five-net opening-closing device which is 

 attached to the cod end of the trawl (Figure 1). The 

 body of the trawl is lined throughout with 19 mm 

 (% in) stretch mesh. The net is 42 m long and was 

 constructed with a gradual taper from mouth to 

 cod end in order to provide a large netting area for 

 filtration in order to reduce the water velocity 

 through the meshes, stagnation and hang-up of 

 animals on the netting, and extrusion of animals 

 through the netting. The wings of the trawl are 

 made of large mesh ( 292 mm). It was assumed that 

 micronekton (fishes, squids, and shrimps up to 200 

 mm in length) escape or pass through this large 

 mesh rather than lead into the trawl body, giving 

 an effective diameter of the net for micronekton 

 equivalent to the small mesh body of the trawl. 

 Unfortunately data do not exist to evaluate herd- 

 ing or leading of oceanic micronekton by the wings 

 of trawls. 



The trawl has six seams with four identical 

 panels for the top and bottom (wing, body, and 

 intermediate) two identical side (wing, body, and 

 intermediate) panels, and four cod end panels 

 ( Figure 2). The meshes were hung at 29. 3"^^ in both 

 directions to allow formation of square openings 

 and laced to make six seamlines. Two riblines are 

 located along the middle of the side panels and 

 extend to the opening-closing device. 



A 1 m2 Multiple Plankton Sampler (MPS) with 

 five separate nets, each 4.6 m long (see Pearcy et 

 al. 1977 for details), was used as an opening- 

 closing cod end device on the trawl. The levers for 

 release of the five nets of the sampler are actuated 

 by a modular timer which employed a crystal oscil- 

 lator and a binary series of counters for selection of 

 release times (Evans 1975). The timer is mounted 

 on the MPS, started as the trawl is launched, and 

 is set to give the trawl time to stabilize at a 

 selected towing depth before net 1 is released 

 (usually 30-60 min). Thus the first net fishes 

 obliquely from the surface to the fishing depth of 

 net 2. The four remaining nets (nets 2-5) all fish 

 the same amount of time on a given tow (usually 

 40 min), either at the same depth (horizontal 

 series) or at different depths (vertically stratified 

 series). 



The footrope and headrope are each 28.6 m of 16 



529 



