TAGGING RESEARCH 



Tagging has been extensively practiced by the California State 

 Fisheries Laboratory, and the Fisheries Research Board of Canadaj also 

 to a lesser extent, by the Oregon Fish Commission and the Department 

 of Fisheries of Washington,, Having been an interested spectator rather 

 than a direct participant, my discussion may siiffer from lack of famil= 

 iarity with this technique, which is simple and direct in principle 

 but difficult and complex in its applicationo 



SOURCE DATA. 



Tag fisho - The tagging operation itself needs little discussion 

 but various aspects of the process require attention in subsequent adjust- 

 ments of the datao It may be remarked here, however, that the tags are 

 of metal and inserted into the body of the fish following in general 

 the method first developed by Rounsefell and Dahlgren (1933) « They are 

 recovered by electro-magnets installed in the fish-meal line of the reduc= 

 tion plants o The fish to he tagged are obtained, for the most part, from 

 commercial fishermen and are subject to the same selections that influence 

 the raw materials used in the vital statistics methodo Since the numbers 

 of "samples'* tagged are fewer, the danger of non-random selectivity is 

 greater© 



Tag mortality o - It is known that tagged sardines suffer high mor- 

 tal! tjrTr'^~"the~operati on itself or from the attendant handlingo The 

 only sources of data on the magnitude and variability of tagging mortality 

 ares (l) A limited nianber of experiments by the California State Fisheries 

 Laboratory in which tagged fish were held in live cars for direct cbser- 

 vation, and (2) a correlation establishing the relation between length 

 of time of confinement before tagging and the subsequent returns o 



The live-car experiments proved that mortalitj'- is high and variable, 

 that small sardines siiffer greater mortality than large ones, and that 

 the size of the tag affects the rate of mortalityo It is, of course, 

 difficult to perform a sufficient number of such experiments to establish 

 a reliable mean mortalityo Moreover^ there is uncertaintj' as to whether 

 the difference in treatment received when the fish are released in the 

 live-car rather than directly into the sea (as in the regular tagging 

 operations) has caused the live-car mortality to differ from that in the 

 sea releases. This uncertainty is especially acute with respect to libera- 

 tions in the Pacific Northwest, where tags have been applied with a dif= 

 ferent instrxjment (tagging-gum) from the scalpel and forceps used in 

 California and where the fish were tagged directly from the fishermen's 

 seine instead of being brailed into a holding net for tagging; and, also, 

 where the roughness of the sea generally imposed more difficult condi-= 

 tions for the handling than in Californiao 



Recover tags. - Since sardines are handled in bulk and mostly by 

 machine, few tags would come to light without a specific recovery system^ 

 That now in use consists of magnets installed in the meal-lines at can- 

 neries and reduction plants o Difficulties arise in achieving installation 



17 



