CONSERVATION OF THE HALIBUT — THOMPSON 375 



nitioii of overfishing as of two types: (1) The poor use of the avail- 

 able supply by an intensity that takes fish too early and (2) the 

 failure to leave enough spawning adults. 



The Commissi-on has seen in process the correction of the first 

 type of overfishing. The existing records of catch per unit and of 

 total catch might by themselves suffice to indicate whether the exist- 

 ing supply is being made better use of. But the matter is different 

 with the second type of overfishing. It may require restraint where 

 the first type of overfishing does not exist or where it has already 

 been corrected. The records of total catch cannot prove or measure 

 the increase in spawning adults, or the number of eggs and larvae 

 these produce, or the number of the latter that survive to become 

 young in the commercial catch. In fact, we know already from 

 other species that the relation between these successive stages is not a 

 simple one and that the increase may at some time reach its limit 

 in any one of them. A slight decrease in the present high intensity 

 of fishing should increase disproportionately the numbers of spawn- 

 ing adults, and hence of eggs. But the fishery may concentrate upon 

 and destroy these adults, or natural mortality may increase as the 

 average age is increased. It is, in fact, necessary to prove the fact 

 that more young are produced and to correlate their numbers with 

 changes in the production of spawn; or to show that these are now 

 sufficient and need no increase. 



Modern fishery science indicates that the high mortality due to 

 fishing is in part balanced by the more favorable feeding and survival 

 conditions which a sparse population encounters. These cause a 

 decrease in the rate of mortality due to natural factors and an in- 

 crease in the growth rate. Such changes constitute a " safety factor " 

 which may be exceeded when fishing becomes too extensive. It may 

 be the factor w^hich has enabled species to survive great changes in 

 natural conditions. Therefore, it is to be expected that a depleted 

 fishery in which this safety factor has been exceeded has mortality 

 and growth rates which are favorable to a greater production when 

 changes in fishing allow a longer life. The limit to favorable regu- 

 lation may in consequence be found in the reversion to normal of 

 these rates as the stock on the banks is increased. Hence the neces- 

 sity for constant observations of the results, however favorable they 

 may now be. 



These considerations have led to a program of measuring the hali- 

 but in the commercial catch to ascertain the changes in the numbers 

 of adults and of young. Vessels whose catches are known to origi- 

 nate from banks chosen as typical are met, and as many halibut in 

 their cargoes as possible are measured. The resultant data are 

 analyzed statistically to show any increase in adults due to reduced 



