17 



Surveys are taken with small fine-meshed trawls in known or suspected 

 spawning areas. Such surveys have proven useful for the purpose of 

 forecasting subsequent stock size changes due to fluctuations in year- 

 class strength. However, prior to an estimate of the population size, 

 the following information is necessary: 



• Whether the sampled spawn is from only the chosen species or 

 from others as well ; 



• The extent of the spawning area and the period during which 

 spawning occurs; 



• The rate of development over the range of temperatures 

 encountered in the survey area during the spawning season. 



Advantage : Ease of collection. 



Disadvantage : The distribution of eggs and larvae may have little 

 relationship to the distribution of the adult stock during a large part 

 of the year; consequently, this information alone is insufficient to plan 

 a harvest strategy. It is also "jery hard to estimate future year-class 

 abundance if the surveys do not cover the entire spawning area. 



3. Examination of Stomachs 



It has been found that examination of the stomachs of predatory fish 

 (higher trophic levels) goes far toward determination of the availability 

 of lower trophic forms. Such studies provide information concerning the 

 food base upon which the larger forms subsist. 



Advantage : Capture of such fish and examination of their stomachs 

 is relatively cheap. 



Disadvantage : As in the two previous production indices, any method 

 that depends on existing sampling techniques is prone to bias. 



