Table 6. — Weekly numbers and food-level means for each tank during period of exploitation; ipostrenioval numbers for removal 

 weeks were obtained hi/ subtractiny numbers removed (table 7): exploitation rates are indicated in parentheses. Exploitatian 

 was started week 29 for 1.0 diet, week 30 for 0.5 diet, and week 34 for 1.5 diet 



Week No. 



0.5 diet 



TankB 

 (0.25) 



Tank F 

 (0.33) 



Tank II 

 (0.50) 



Tank 

 mean 



1.0 diet 



Tank A 

 (0.25) 



Tank D 

 (0.50) 



Tank (i 

 (0.33) 



Tank 

 mean 



l.S diet 



Tank C 

 (0.60) 



Tank E 

 (0.25) 



Tank I 

 (0.33) 



Tank 

 mean 



No. 



29.. 



30.. 



31.. 



32.. 



33.. 



34.. 



35.. 



36.. 



37.. 



38.. 



39-- 



40.. 



41.. 



42.. 



43.. 



44.. 



45.. 



46 



47-. 



48.. 



49- 



50-. 



51.. 



52.. 



53.. 



54.. 



55-- 



5« - 



57- 



58.- 



59.. 



60-- 



61.. 



62.- 



63. 



64. 



65- 



67.. 

 68.. 

 69.. 

 70-. 



71.. 

 72.. 



127 



1.'7 



101 



97 



100 



90 



98 



82 



70 



73 



93 



82 



85 



92 



106 



114 



121) 



107 



110 



inii 



87 

 88 

 94 

 71 

 70 

 70 

 63 

 70 

 HO 

 74 

 75 



109 

 90 

 86 



115 

 97 



103 



108 

 97 



121 



118 



118 



107 



102 



m. 



118 



114 



80 



S3 



87 



65 



72 



69 



50 



62 



72 



62 



59 



97 



97 



99 



110 



91 



63 



122 



100 



131 



135 



104 



109 



125 



118 



117 



121 



116 



111 



105 



88 



104 



107 



81 



86 



88 



76 



76 



74 



64 



78 



74 



iVo. 



m. 



A7o. 



No. 



No. 



No. 



No. 



No. 



No. 



No. 



91 

 93 

 .W 

 67 

 54 

 34 

 36 

 46 

 27 

 34 

 45 

 83 

 72 

 60 

 70 

 71 

 66 

 fiO 

 51 

 53 

 38 

 85 

 84 

 63 

 69 



65 



42 



56 



6.1 



43 



47 



46 



25 



62 



61 



33 



33 



69 



49 



49 



47 



28 



29 



31 



112 

 111 



78 

 82 

 80 

 63 



49 

 66 

 70 

 76 

 72 

 83 

 91 

 96 

 101 

 86 

 76 

 94 

 75 

 101 

 104 

 79 

 83 

 87 

 74 

 81 

 89 

 7K 

 7H 

 S7 

 68 

 84 

 91 

 70 

 74 

 88 

 74 

 82 

 SO 

 70 

 71 

 69 



122 



104 



106 



119 



96 



99 



95 



73 



75 



90 



107 



92 



98 



82 



115 



94 



80 



82 



91 



69 



67 



71 



101 



88 



101 



109 



116 



150 



148 



141 



141 



139 



135 



150 



120 



120 



109 



8S 



93 



92 



68 



68 



79 



77 



158 



90 



104 



93 



52 



62 



71 



79 



77 



74 



71 



134 



131 



94 



113 



101 



104 



101 



108 



76 



88 



89 



53 



51 



50 



26 



39 



62 



49 



.IS 



44 



44 

 49 

 33 

 36 

 31 

 20 

 26 

 28 

 19 

 18 

 21 

 11 



148 



110 



110 



119 



91 



93 



123 



95 



106 



133 



134 



172 



131 



143 



167 



149 



100 



85 



91 



61 



75 



76 



52 



67 



81 



87 



101 



102 



95 



107 



98 



81 



122 



94 



78 

 60 

 65 

 50 

 47 

 44 

 58 



143 



101 



106 



110 



80 



85 



96 



82 



86 



99 



104 



133 



120 



106 



128 



115 



95 



89 



97 



69 



77 



79 



77 

 74 

 85 



101 

 97 



101 

 98 

 88 



100 

 98 

 77 

 77 

 76 

 62 

 60 

 62 

 46 

 44 

 48 

 49 



204 

 217 

 217 

 200 

 221 

 211 

 131 

 133 

 146 

 107 

 129 

 135 

 114 

 136 

 89 

 68 

 70 

 73 

 77 

 73 

 75 

 39 

 29 

 67 

 76 

 86 

 88 

 106 

 128 

 119 

 108 

 107 

 110 

 63 

 66 

 68 

 39 

 54 

 59 

 32 

 33 

 33 

 20 

 26 



126 



141 



143 



126 



142 



1.68 



109 



69 



74 



94 



109 



122 



112 



118 



97 



65 



83 



92 



46 



60 



112 



105 



98 



130 



145 



172 



159 



68 



81 



78 



87 



80 



95 



85 



129 



126 



106 



110 



112 



82 



81 



94 



66 



63 



133 

 141 



150 

 184 

 138 

 141 

 111 

 124 

 142 

 118 

 111 

 121 

 100 

 106 

 77 

 91 

 112 

 139 

 137 

 160 

 174 

 154 

 193 

 198 

 176 

 220 

 247 

 216 

 234 

 242 

 192 

 196 

 194 

 162 

 148 

 149 

 103 

 102 

 106 

 67 

 85 



66 



164 

 166 

 170 

 170 

 167 

 170 

 117 

 109 

 121 

 106 

 116 

 126 

 109 

 120 

 88 

 76 

 88 

 101 

 86 

 98 

 120 

 99 

 107 

 128 

 132 

 169 

 166 

 127 

 148 

 146 

 129 

 128 

 133 

 100 

 114 

 114 

 82 



60 

 66 

 73 

 51 



52 



YIELDS 



Removals during: the jjeriod of exploitation were 

 c'oni|)aral)le to the catches of commercial fisheries 

 and |)rovided information on stabilized yields. 

 Data were analyzed both to determine wlien rela- 

 tive stability beo;an and to measure the relation 

 of yield to amount of food consumed. 



COMPARATIVE YIELDS 



Yields as well as population sizes were more 

 stable in weights than in numbers; therefore, 

 j-ields were studied in terms of weight. The course 

 of yield for each jjopulation during the e.xploita- 

 tion period (table 9 and fig. 7) included an initial 

 period of decline as the populations adjusted 

 themselves to removals. This decrease was fol- 

 lowed by a period of relative stability beginning 

 about week 49. 



Even within the relatively stable period, yields 

 showed considerable irregularity. This phenome- 

 non resulted from variations in the response of 



the populations to exploitation and from de\ia- 



tions of tlie jiercentages removed from the exact 



nominal exploitation rates. The latter de\iations 



occurred becau.se the removal rates were appUed 



on the basis of numbers of fish rather than weights. 



Some of this random variability is averaged out in 



means of yields for three 14- or 15-week subperiods 



covering the entire period of exploitation (table 9 



and fig. 8). These means again reflect the initial 



period of decline, followed by more stable yields. 



The final jjeriod, including weeks 59 to 72, was 



one of fairly stable yield (fig. 7). Mean yields for 



this ])eriod (fig. 8) ranked the same according to 



exploitation rate for the 1.0 and 1.5 diets. Yields 



at the 0.5 diet were nearly identical. The period 



inclucfing weeks 59 to 72 fairly well fulfilled the 



|)lanned objective of "a reasonably stable yield" 



(section, "Plan of the Experiment"), and data 



from it were used in the study of relation between 



food level and exploitation rates. Results from 



this period had the additional advantage of being 



434 



U.S. FISH AND WILDLIFE SERVICE 



