258 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



boundaries were established from data collected 

 on regular survey cruises of the California Coop- 

 erative Oceanic Fisheries Investigations (Farris, 

 1958). Spawning areas are approximately bound- 

 ed by the 26th parallel on the south, the 45th 

 parallel on the north, the coast of North America 

 on the east, and the 150th meridian on the west. 



SOURCES OF ERROR AND BIAS IN 

 SAMPLING EGGS 



These data and subsequent interpretations are 

 subject to errors inherent in the collection proce- 

 dures. The types of error investigated and 

 evaluated were (1) completeness of retention of 

 eggs by the nets; (2) completeness of sampling of 

 the vertical range; (3) sampling error owing to a 

 variable distribution of eggs in space and time; 

 and (4) incomplete sampling of the horizontal 

 range of jack mackerel spawning. 



RETENTION OF EGGS BY THE NETS 



The eggs are fully retained by the net once 

 they are in it, because the plankton nets have a 

 stretched mesh of 0.5-0.7 mm. (Ahlstrom, 1953), 

 and the spherical eggs range in diameter from 

 0.9-1.1 mm (Ahlstrom and Ball, 1954). It would 

 appear, therefore, that no eggs are lost through 

 the mesh of the sampling net. 



SAMPLING OF THE VERTICAL RANGE 



Investigation of the vertical distribution of 

 jack mackerel eggs (Ahlstrom, 1959: table 7) 

 with a horizontally towed closing net reveals that 

 most of the eggs occur in the upper 40 meters of 

 water. Jack mackerel eggs have rarely been 

 taken below 100 meters, and never below 140 

 meters. The bulk of the eggs have occurred 

 above the thcrmocline. Since plankton hauls are 

 routinely made from a depth of 140 meters, which 

 has always included the thermocline depth, it 

 seems likely that the vertical distribution of jack 

 mackerel eggs is completely sampled. 



VARIABLE DISTRIBUTION OF EGGS 



The distribution of jaclt mackerel eggs is vari- 

 able with respect to both time and space. An 

 illustrative example is given in figure 5. The 

 standard numbers of jack mackerel eggs for line 

 97 have been plotted by station for 4 months. 



5,000 



45 



STATIONS 



Figure 5. — Standard numbers of jack mackerel eggs fovmd 

 on line 97 during 4 months. 



The average number of eggs per station by month 



is given below. 



Year: April May 



1953 181 1,116 



1954 762 143 



The average station on hne 97 in May 1953 

 contained six times as many eggs as the average 

 station in the preceding month. Assuming that 

 this change was rectihnear in time, the estimate 

 would be altered considerably, depending on which 

 day of the month the sample was taken. 



Spatial variability is indicated by the data for 

 April 1954, in which a change of 1 order of magni- 

 tude occurs within 20 miles. There is at least 

 one such combination of adjacent stations having 

 as great a change in the distribution of jack 

 mackerel eggs for each cruise illustrated. The 

 grid of stations occupied is too coarse except for 

 fairly rough estimates of egg abundance. Al- 

 though more frequent sampling of more closely 



