MACKEREL INVESTIGATIONS IN 1897. 5 



occasionally a spawning or even immature ovary was met with. It is 

 worthy of remark that, even in the same run, fish in very different con- 

 ditions of maturity were found. Most of them were nearly or quite 

 through spawning- : a smaller number might be immature, and for some 

 unknown reason would probably fail to ripen their eggs this season, 

 while a still smaller number would be spawning fish. In nearly all 

 cases the number of males was greater than the number of females. 

 The fishermen were almost unanimous in stating that a run of spawn- 

 ing fish much more productive than any which we encountered had 

 passed into Oasco Bay about two weeks previous to our arrival. 



If an ovary of a mackerel be examined just previous to the spawning 

 season, it will be found to contain ova of three sizes or generations. 

 Constituting the bulk of its lamellae are large, opaque ova measuring 

 from 0.35 mm. to 0.55 mm. (say about 0.02 inch) in diameter. These 

 have large, ill-defined nuclei, well-developed egg- membranes and egg- 

 follicles, protoplasm filled with yolk-spherules and a number of minute 

 oil droplets, which latter are scattered in the smaller, but more or less 

 closely aggregated in the larger of these ova. The opaque ova just 

 described are destined to produce the mature eggs of the approaching 

 sjjawning season. Packed between them are smaller ova of two sizes. 

 The larger ones measure about 0.12 mm. in diameter, are about as 

 numerous as the large, opaque ova, and are destined to produce a crop 

 of eggs a year hence. The smallest measure about 0.04 mm. in diame- 

 ter, are very numerous, aggregated in clusters, have large and distinct 

 nuclei, no follicles, and retain the character of simple cells. From 

 these future crops of ova will arise. 



As the spawning season approaches, part of the opaque ova increase 

 rapidly in size. The yolk-corpuscles, to which the opacity was due, 

 pass into solution, and as a consequence the yolk becomes more and 

 more transparent and of a pale amber color. The oil-droplets coalesce 

 into two or three larger drops and finally into a single sphere, which is 

 free to move about in the now fluid yolk. The nucleus has, meantime, 

 become invisible. Such ova are ready for extrusion and the final pro- 

 cesses of maturation. Owing to these changes the ovary as a whole 

 has increased greatly in bulk, and becomes spotted all over, both 

 externally and on the internal laminse, with translucent spots, due to 

 aggregations of the clear eggs. Unlike some fishes (those generally of 

 sluggish habit or bottom-livers) the mackerel matures only a portion 

 of the generation of eggs at one time. Thus is obviated any undue 

 enlargement of the ovary, which would cause the body walls to pro- 

 trude and doubtless obstruct the movements of the fish, a result which 

 might be particularly serious to an active pelagic species like the mack- 

 erel. The number of ova produced at one time is seldom more than 

 50,000 and is frequently much less, but the aggregate number matured 

 in one season by a female of average size is several hundred thousands. 



All of the evidence which could be gathered from examinations of 

 the ovaries of numerous fishes captured at different hours of the day 

 seems to indicate that the common mackerel, like the Spanish mackerel 



