DESCRIPTION OF THE SYSTEM 



The electronic detector-recovery system is in- 

 stalled between the dock and processing plant 

 (Fig. 1). Fish being unloaded are transferred 

 from boat to plant by a dragline or screw con- 

 veyor. In the conveyor of a small menhaden 

 reduction plant in Beaufort, N.C., we have in- 

 stalled a trap door which when open allows fish 

 to slide down a transfer chute onto a conveyor 

 belt. The belt is made of 8 mm rubber, 41 cm 

 wide and 8.2 m long, and runs at a rate of 

 2.1 m/sec through a magnetic shield which 

 houses a detector head. The shield, 107 X 56 

 X 41 cm, is made of iron-nickel alloy which re- 

 duces extraneous magnetic fields that interfere 

 with the operation of the detector. The Jef- 

 ferts detector senses minute currents created 

 by the magnetic fields of stainless steel tags, 

 14 X 3 X 0.5 mm, in fish. The electronic unit 

 has a sensitivity adjustment that can be set for 

 different sizes of magnetic tags (Fig. IE). When 

 a tag is detected, an audio and electrical impulse 

 is produced. The impulse triggers an inter- 

 phase unit which activates a pneumatic gate on 

 the side of the belt conveyor system. Time delay 

 adjustments for closing and opening the gate 

 are built into the interphase unit (Fig. IF) . The 

 gate is hinged to close across the belt at an angle, 

 forcing the marked fish off the belt into a tub 

 (Fig. 2) . If the gate is not activated, unmarked 

 fish are carried by the belt into a screw con- 

 veyor and back to the plant's conveyor system. 

 The detector-recovery system does not interfere 

 with the plant's operation. Since some un- 

 marked fish are collected with the marked ones, 

 it is necessary to recycle the collected fish indi- 

 vidually to isolate the tagged fish. The propor- 

 tion of unmarked fish that must be recycled has 

 been considerably reduced in our two-phase elec- 

 tronic detector-recovery system built at a larger 

 menhaden reduction plant. This system consists 

 of two sequential detector-recovery operations 

 in which the second phase thins out most of 

 the untagged fish (Fig. 3). Since one-half of 

 this system is used only when a tagged fish is 

 detected, recoveries can be recycled through this 

 half during the actual recovery operation. The 

 double system costs less than $14,000 including 

 conveyors, compressor, construction, and elec- 

 tronics; this is about twice the cost of the single 

 system (Appendix) . However, it searches near- 



ly twice the number of fish per hour and is nearly 

 twice as efficient in recovering tagged fish. Thus, 

 a cost analysis favors the double system. 



OPERATIONAL PROBLEMS 



Several problems wei'e encountered when we 

 began using the electronic detector-recovery 

 system. False detections caused by moving 

 metal equipment and vibrations were eliminated 

 by shielding and isolating the detector head. 

 Small tagged fish sometimes wedge under the 

 closing gate and unless they are quickly removed, 

 they are lost to the screw conveyor. This loss 

 was reduced in our later detector-recovery sys- 

 tem by changing the type of belt lacing from a 

 thick diagonal splice to a thin horizontal splice 

 and by using a one-piece metallic gate assembly 

 insert. These changes permitted a close level 

 sweep of the gate over the conveyor belt. An- 

 other problem, overloading belts or screw con- 

 veyors, and thus stopping the units, was re- 

 solved by using higher amperage fuses. 



Tests indicate that' the efficiency of recovering 

 tagged fish of the single detector-recovery in- 

 stallation is 36 "^r and that of the double instal- 

 lation is 60 ''/f. The single installation can con- 

 vey all of the catch at a small processing plant 

 or 13 tons per hour. The double installation can 

 convey a fourth of the catch at a conventional 

 menhaden plant or about 22 tons per hour. Most 

 of the losses of tagged fish in our system occur 

 after large quantities of menhaden enter our 

 conveyor: detection occurs and the gate closes 

 for an instant, but as the fish are about to be 

 deflected, a pileup occurs. This often holds back 

 the tagged fish or causes it to flip over the gate. 

 In both instances it is not recovered. Although 

 up to 64 "^r of the tagged fish entering our de- 

 tector installations are missed, most of the tags 

 are eventually recovered on magnets in the 

 plants. 



The electronic detector-recovery system was 

 to play an additional role on the Gulf coast to 

 that on the Atlantic coast because methods of 

 fishing are not the same. Atlantic menhaden 

 fishermen normally return to port daily after 

 fishing over a small geographical area, and their 

 catch is usually processed immediately. Thus, 

 thousands of tags recovered by magnets are pro- 

 viding reliable data for estimating population 

 parameters in the Atlantic. Gulf of Mexico 



