FLUCTUATIONS IN ABUNDANCE OF MARINE FISHES 185 



"age distribution". From these age distributions it is possible to estimate the 

 mortality rates by comparing the abundance of the successive ages of fish. These 

 mortality rates in tmii (when properly treated) can be used in conjunction with 

 catch data to estimate the size of the stock from which the aged samples of fish 

 were taken. In some instances it is possible to apply a similar method to size 

 measurements of the fish where data on the growth rate are available. 



Tagging or Marking. By means of identifying individual fish with some type 

 of recognizable mark or attached tag it is possible to estimate the size of the 

 total population. There is a very large number of types of tags in use, their 

 variations depending upon the species of fish to which they are attached and 

 the means by which the fish are handled after they are caught. The common 

 strap cattle tag is used quite extensively on marine fish, being attached to the 

 operculum or tail. Another type in common use (particularly on salmon where 

 long life is imimportant) is the pin, consisting of two plastic disks which are placed 

 on either side of the fish below the dorsal fin. A wire of noncorrosive metal is 

 pushed through holes in these disks and also through the fleshy part of the back 

 of the fish. The disks are usually printed with an identifying number, information 

 regarding return of the tag to the marking agency, and the amount of reward 

 which will be paid, if any. 



For fish, such as herring, menhaden, and sardines, which are to be handled 

 in reduction plants, metal belly tags are usually used. These are small pieces of 

 metal about % inch long and % inch wide on which numbers are stamped. The 

 tags are inserted into the body cavity of the fish through an incision which 

 normally heals very rapidly. Recovery is made either by means of magnets placed 

 under the dry meal lines of the reduction plants or by electronic detectors placed 

 in the conveyors leading from the unloading hopper to the plant. These detec- 

 tors cause a few fish to be diverted from the conveyor each time a tagged fish 

 passes through. This method, although more expensive and difficult to operate, is 

 advantageous in that it permits identification of the tag with a specific load of 

 fish rather than with a full day or more of operation as is the case with the 

 magnets. 



Marking by mutilation of the fins is a widely used process, particularly with 

 young downstream migrating salmon. In this method one or more fins are clipped 

 off the young fish and instructions are given the cannery butchers as to the par- 

 ticular types of missing fin combinations to be expected on the adult fish and the 

 means for retiirning the information to the marking agency and receiving the 

 reward. The marking agency usually requires that the scars of the excised fins 

 be returned to it in order that the reward may be claimed. 



When data are at hand on the number of fish tagged or marked and the num- 

 ber recovered by the fishery, it is possible to estimate by simple proportion their 

 total population. Thus, for instance, if 1,000 fish were marked and 500 recovered, 

 it would be assumed that 50 per cent were taken and that the stock was, there- 

 fore, double the amount of the catch. In making such an estimation it is supposed 

 that the tagged or marked fish are uniformly distributed throughout the popula- 

 tion; however, this assumption must be carefully checked since it does not always 

 hold true. Certain mathematical relationships also often distmrb the simple direct 

 proportion indicated above. In such cases adjustments, based on theoretical formu- 

 lae, have to be made. 



