BIOLOGY OF THE ATLANTIC MACKEREL 



169 



took very few eggs that were not otherwise accounted for (by the contamination 

 correction based on the average catch of the upper net and on the time taken to pass 

 through the upper stratum). It is safe to conclude therefore, that the pycnocline 

 forms a barrier to the downward extension of mackerel eggs. Further, the pycnocline 

 is sufficiently well indicated by the thermocline in this region so that the latter may be 

 used an an indicator of the lower limit of mackerel eggs. 



SURFACF 



5 



N'JMO-R OF THOUSANDS OF MACKEREL EOOS PER HAUL 

 5 10 !5 in 30 



TEMPERATURE, DEGP.FES C. 

 « 8 1? 12 



BOTTOM 



335 3-10 3Z3 3?-0 



SALINITY, PARTS PER THOUSAND 



31.5 



t.C'255 



I.C250 '.0245 

 Of.NSITY IN SITU 



Figure 4.— Vertical distribution of mackerel eggs in relation to temperatures, sa ii n it y , and density of water. Observations were 

 adjusted to the basis of standard speed of towing, except tbose indicated as questionable. 



The serial tows of May 17, 1929, also illustrated significant differences in the 

 vertical distribution of eggs in different stages of embryonic development. When the 

 eggs were separated into three stages of development occupying approximately equal 

 periods of tune, it was found that those of the early stage (A) were mostly near the 

 surface, those of the late stage (C) mostly between the 5- and 10-meter levels, and 

 those of the intermediate stage (B) intermediate between A and C in their vertical 

 distribution (table 3). Too few eggs were taken at greater depths to indicate reliably 

 the proportionate numbers at each of the three different stages of development. 



Table 3. — Vertical distribution of various stages of mackerel eggs according to noon series, station 20498, 



May 17, 1929 



[Stage A is fmin fertilization to complete epiboly; stage B is from complete eplboly tn embryo extending three quarters around tho 



circumference of the egg; stage C is from this point to hatching] 



i Adjustments the same as in table 2. 



The differential vertical distribution of the several egg stages could result either 

 from a decrease in specific gravity of the water after the eggs were spawned or an 

 increase in the specific gravity of the eggs as embryonic development proceeded. 

 Moore (1899, p. 14) concluded that the eggs increased in specific gravity during 

 development when he noted that mackerel eggs which he was incubating in the 

 laboratory sank during the third day. But he gives neither the specific gravity of 



