ments. Dead fish were removed daily, and shed 

 tags were recovered weekly from the tanks. The 

 experiments were terminated after 4 to 16 weeks 

 when Type-1 tagging mortality and tag shed- 

 ding had ceased. Type-2 tagging mortality and 

 tag shedding, which occurs throughout the life 

 of a tagged fish, were not investigated in these 

 experiments. 



The tag-recovery methods were also investi- 

 gated. Electronic detectors and several types 

 of magnets were installed at different locations 

 in menhaden reduction plants (Fig. 2). Pri- 

 mary magnets and electronic detectors recover 

 tags early enough in the processing to identify 

 the time of capture of the fish. Secondary mag- 

 nets recover tags too late in the handling of the 

 fish scrap to determine the time of capture. 



The type of magnet installed was dependent 

 on the conveyor system of each plant because 

 magnets could be placed only in locations which 

 did not impede the flow of scrap and which were 

 accessible for cleaning. Two- and four-pole 

 plate magnets were placed in primary and sec- 

 ondary locations in chutes with steep slopes, 

 and grate magnets, rotating at 12 rpm to pre- 

 vent clogging, were mounted in locations where 

 fish scrap dropped through the bars of the mag- 

 nets (Fig. 3). Stationary grates and hump 

 magnets were unsatisfactory because they 

 clogged. 



Tag-recovery test consisted of putting one 

 tag in each of 100 whole fish that were to be 

 processed with the catches. To compare the 

 recovery efficiency of adult tags with that of 

 juvenile tags, one of each was placed in each of 

 100 fish. The tag-recovery rate for each plant 

 was determined as the mean percentage of test 

 tags recovered on the magnets from several tests. 



EXPERIMENTAL TAGGING 



The object of the experiments was to develop 

 methods of internally tagging adult and juvenile 

 menhaden with ferromagnetic tags that could 

 be recovered on magnets. The prerequisites 

 were that the tagging methods would not cause 

 high mortality or tag-shedding rates and that 

 they would be simple enough to permit us to 

 tag large numbers of fish in a short period of 

 time. We were interested not only in developing 

 an efficient tagging method, but also in deter- 

 mining the amount of tag loss that we might 



SECONDARY MAGNETS 



MENHADEN VESSEL \.\ REDUCTION PLAJfT 



SCRAP SMED 



Figure 2. — Tag recovery locations in a typical menhaden 

 reduction plant. 



1\ 



Figure 3. — Rotating grate and two 2-pole magnets 

 installed in a transfer chute of a menhaden reduction 

 plant. 



expect in the field from both mortality and 

 tag shedding. 



Experiment 1 



Objective: To determine if the loss of tags 

 treated with antibiotics or disinfectants from 



