NATIONAL FISH AND SEA FOOD PARADE--OCTOBER 6-12, 1958 The United States Department of the Interior Bureau of Commercial Fisheries and the Fishing Industry are working together to encourage the greater use of all fishery products during the "Fish 'n' Sea Food Parade"--October 6-12, 1958. Reports indicate that a great deal of interest has been generated in this fall promotion. Mailed free to members of the fishery and allied industries. Address correspondence and requests to the: Chief, Branch of Market News, Bureau of Commercial Fisheries, U. S. Department of the Interior, Washington 25, D. C. Publication of material from sources outside the Bureau is not an endorsement. The Bureau is not responsible for the accuracy of facts, views, or opinions contained in material from outside sources. Although the contents of the publication have not been copyrighted and may be reprinted freely, reference to the source is appreciated. The printing of this publication has been approved by the Director of the Bureau of the Budget, May 21, 1957. CONTENTS COVER: A cod end full of red snapper and some grouper alongside the U. S. 5/31/60 Page A Practical Depth Telemeter for Midwater Trawls, by Richard L. McNeely Color and Quality of Canned Gulf of Mexico Yellowfin Tuna as Related to Weight of Fish, by Arnold W. 1 11 1957: The Year of Warm Water and Southern Fish off California Coast 15 September 1958 Washington 25, D.C. Vol. 20, No.9 A PRACTICAL DEPTH TELEMETER FOR MIDWATER TRAWLS By Richard L. McNeely* SUMMARY A direct-reading electrical depth telemeter for midwater trawls has been developed and used successfully in the northeastern Pacific. The system utilizes an electrical trawl cable to transmit continuous depth information from a pressuresensing unit on the gear to a pilothouse meter which shows trawl depth in feet and fathoms. Slip rings and brushes on the trawl winch complete the elec- Fig. 1 Pilothouse depth meter for the electrical telemeter, calibrated to show depth of the trawl in feet and fathoms. INTRODUCTION A depth telemetering system, utilizing a low-voltage electrified trawl cable for determining the depth of midwater trawls, was installed and used successfully aboard the U. S. Bureau of Commercial Fisheries exploratory fishing vessel John N. Cobb in the northeastern Pacific during the spring and summer of 1957. Accurate knowledge of the depth of the net is essential to successful midwater trawling. No matter how efficient the gear might be, unless it is placed at the proper depth indicated by fish signs on the echo-sounder or by other means, the school of fish will be missed entirely or only a small catch will be made. This problem has been apparent during the several years of intermittent midwater trawling research by the Bureau's Branch of Exploratory Fishing and Gear Research. A variety of methods have been used in various parts of the world to determine the depth of midwater trawls, but there has always been the need for a more practical instrument which is accurate, simple to use, and economically within reason for commercial *Electronic Scientist, North Pacific Fisheries Exploration and Gear Research, Division of Industrial Research and Services, U. S. Bureau of Commercial Fisheries, Seattle, Wash. fishermen. The electrical depth telemeter, which was designed, constructed, and installed at Seattle by Bureau personnel, appears to meet this need. Although midwater trawling by commercial fishing vessels thus far has been limited primarily to herring in northern Europe and British Columbia, there is evidence that at times other species of fish may be available to midwater gear, thus opening up vast new fishing areas of the ocean. Echo-sounders and sonar-type instruments have shown that schools of fish may be found at any depth between the surface and the bottom. A method to permit accurate positioning of the net is necessary because some schools of fish occupy a relatively thin vertical layer of water and can be missed easily if the net is a few fathoms too high or too low. During a single tow separate schools of fish may be found at different depth levels, necessitating raising or lowering the net at intervals (Richardson 1957). Also, when attempting to catch fish very near hard or uneven bottom, the position of the gear must be accurately known in order to avoid contact with the bottom which could snag the trawl doors and depressors or damage the net itself. Fig. 2 - Pressure-sensing unit attached to the end of the electrical trawl cable just in front of one of the trawl doors. EARLY METHODS OF TRAWL-DEPTH DETERMINATION Many methods and devices have been used in attempts to accurately determine the depth of midwater trawls. None have been entirely satisfactory due to inaccuracy, cost, depth limitation, operating difficul ties, or fragility. Calculation based on length and angle of towing warp to determine the gear depth is one of the oldest methods. Also, a second vessel has been used to sound the net as it is being towed at various depths. Bathythermographs have been attached to the trawl to record the depth range and to check other methods of calculation. Tables and graphs have then been prepared to show the probable trawl depth for each wire angle-length combination (Barraclough and Johnson 1956). Shortcomings in these methods arise from the effects of currents, wind, and tide on the wire angle at any given throttle setting and length of towing warp, and from the unknown changes in the underwater wire angle with varying sea conditions. Fig. 3 Bronze pressure vessel with cap removed to show pressure potentiometer, feed-throughs, and "O" ring seal. Several accurate depth telemetering systems have been used with a fair degree of success. An electronic-acoustic telemeter was built and tested by the Woods Hole (Mass.) Oceanographic Institute (Dow |