DISTRIBUTION OF EGGS AND LARVAE OF JACK MACKEREL 



273 



SAMPLING OF THE HORIZONTAL RANGE 



It was shown that incomplete samphng of the 

 entire range of jack mackerel spawning introduced 

 an error of at least 21 percent of the estimated total 

 egg population. This error appears to be reduced 

 for the estimates of larvae. A very small propor- 

 tion of the 30-day and older larvae is taken sea- 

 ward of stations 90 (tables 30, 31, and 32). These 

 tables were constructed using the standard numbers 

 of jack mackerel larvae (6.75 mm size-class) for all 

 stations sampled during the year. The data 

 were grouped by 2-month intervals about selected 

 sampling lines and further grouped to reflect the 

 offshore-inshore distribution of the larvae as was 

 done with the eggs (see p. 253). The data seem to 

 indicate that almost all the 30-day-old larvae are 

 to be found in an area bounded by line 80 on the 

 north, line 110 on the south, stations 90 on the 

 west, and the coast on the east. Some larvae are 

 found both to the north and south of the main 

 area. This area is far more restricted than that 

 exhibited by the distribution of the eggs (see 

 above) . 



Table 30. — Relative north-south, inshore-offshore distribu- 

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

 2-month intervals, 1962 



[Standard haul totals] 



Table 31. — Relative north-south, inshore-offshore distribu- 

 tion of 6.76-mm. (30-day old) jack mackerel larvae, by 

 2-month intervals, 1353 



[Standard haul totals] 



* Region not occupied. 



1 Region not occupied. 



There are several possible explanations for the 

 relatively restricted distribution of jack mackerel 

 larvae. Four interactions of ocean current and 

 fish survival are discussed as possible explanations 

 of the observed distribution. 



Uniform current flow, diflerential survival. — In 

 this model, it is assumed that the southerly flowing 

 California Current has a uniform velocity, i.e., 

 it has no eddies or countercurrents, and the 

 larvae have a differential survival. The eggs 

 spawiied in some part of the Current will survive 

 at a much higher rate over those deposited in 

 other parts. Under the conditions of this model, 

 the older larvae (survivors) would occur in regions 

 south of the one in which they were spawned, 

 never north of the spawning region. If the 

 current, though of uniform velocity, is very slow, 

 the southward displacement will be slight and 

 therefore very hard to measure. 



Uniform, current flotr, uniform survival. — If the 

 California Current flowing south contained larvae 

 which survived equally well in any part of the 

 current, one would expect the older larvae to 



