274 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



Table 32. — Relative north-south, inshore-offshore distribu- 

 tion of 6.75-mm. {30-day old) jack mackerel larvae, by 

 2-month intervals, 1954 



[Standard haul totals] 



' Eeglon not occupied. 



occur always to the south of the spawning area. 

 If one considers the special conditions of low 

 current velocity and high larval mortality, a 

 sampling problem becomes apparent. Consider: 

 A small subarea X on the periphery of the spawn- 

 ing area contains a thousand eggs. At the end of 

 a month, one larva has survived. Subarea Y 

 in the center of the spawning area contains a 

 million eggs. Under conditions of uniform sur- 

 vival a thousand larvae would survive at the end 

 of a month. The sampler has a much better 

 chance of obtaining larvae from subarea Y than 

 from subarea X. The failure in obtaining month- 

 old larvae from peripheral areas would lead to a 

 conclusion of better survival in the center of the 

 spawning area, though in fact, survival was 

 uniform over the entire area. 



Differential current flow, uniform surmval. — 

 Under conditions of this model, the California 

 Current would have an average transport to the 

 south, but some parts of the water would move 

 faster than others, and eddy currents would be 

 present. The larvae, although surviving at a 



uniform rate, would not appear to do so because 

 of the postulated concentrating mechanisms. A 

 high proportion of the larvae would be found in 

 the eddies. 



Diflerential current flow, diflerential suruival. — 

 Using this model, it is virtually impossible to 

 predict the distribution of the older larvae, as 

 that distribution depends on the special conditions 

 of the current and survival. 



Smce the average flow, as determined from 

 dynamic topography of the California Current, is 

 slow (about 0.2 knot) and since it contains water 

 flowing both slower and faster accompanied by 

 eddies, no definite conclusion may be reached 

 concerning differential survival, although survival 

 appears to be better in some regions (table 33). 



Table 33. — Annual regional summary of distribution of 

 jack mackerel eggs and month-old larvae, 1952-54 



[Survival at the end of 1 month Is given for each region] 



I 6.76-mm. size class; estimated to be 1 month old. 



AVOIDANCE OF NET 



When Ahlstrom (1954b) computed the mortality 

 of sardines, he found that a correction was neces- 

 sary to account for the larvae which dodged the 

 sampling net. He demonstrated dodging in a 

 relative way by examining the ratio of average 

 number of sardine larvae per night haul (when 

 presumably the larvae cannot see the net) to the 

 average number of larvae per day haul. When 

 he computed the night/day ratio for each size class 

 he found that the ratio increased with size. The 



