328 Memoir Sears Foundation for Marine Research 



portionately about as deep as large examples, though the diameter of the eye continues 

 to exceed the length of snout until they are nearly full grown. The gill rakers (see 

 Description) increase in number with age, quite rapidly in the young up to about 100 mm 

 SL. It has been indicated also that the teeth are more prominent in the young than in 

 the adult, and that they disappear entirely from the premaxillary with age. 



In waters tributary to Chesapeake Bay, in the Potomac River in particular, where 

 most of the young fish were collected and measured, the rate of growth of the young 

 during their first summer and subsequently is shown in a table by Hildebrand and 

 Schroeder {^g: 88): 26 taken in June, 30-37 mm; 31 in July, 30-59 mm; 93 in 

 August, 35-64 mm; loi in September, 40-69 mm; 232 in October, 40-74 mm; 

 and 1 8 I in November, 50-74 mm. Collections of young of the same year-class from 

 Chesapeake Bay consisted of: 276 taken in January, 65-119 mm long; 280 in Feb- 

 ruary, 65-119 mm; 773 in March, 65-1 15 mm; and 8 in May, 95-120 mm in 

 length. These data suggest that the young grow to a length of about 50-75 mm 

 (2 or 3 in.) while living in fresh water and to a length of 65-120 mm (2.6-4.8 in.) 

 when a year old. 



Some older fish were taken in Chesapeake Bay, often in the same otter trawl hauls 

 with the younger ones, but they are not separable into year-classes by the lengths 

 given in the table referred to above. It is highly probable that, with few exceptions, 

 these older and larger fish are approaching an age of two years. The collections con- 

 sisted of: 8 taken in January, 160-194 mm in length; 15 in February, 150-209 mm; 

 and 8 in March, 140-200 mm. If the six largest examples, which may be about 

 three years old, are excluded, the range for the fish that apparently are two years 

 old is 140-184 mm. If a length of around 200 mm is attained at three years of age, 

 it is reasonable to assume that many individuals will attain a length of 250 mm (10 in.) 

 when four years old. As adult gravid fish often are 250 mm and even less, it is highly 

 probable that many individuals become sexually mature and perform their first migra- 

 tion to the spawning grounds at an age of four years. 



Spawning, Migrations, and Habitat. Bluebacks are anadromous, but apparently 

 they do not ascend rivers as far as their near relative, P. pseudoharengus. In the vicinity 

 of Woods Hole, spawning takes place in fresh to slightly brackish ponds having an 

 outlet to the sea; also, Bluebacks enter at least the mouths of the larger rivers in 

 New England. 



This species usually arrives in the coastal waters a month or so later than pseudo- 

 harengus — for example, in the lower parts of Chesapeake Bay during the first half ot 

 April and in the upper sections of the Bay about two weeks later. Hildebrand and 

 Schroeder {^g)-, in speaking of the migration in Chesapeake Bay, stated: "The peak 

 of the catch of bluebacks is usually taken between April i and 20 in the lower bay. 

 The numbers decrease throughout May, until after June i only stragglers are caught." 

 The time of arrival varies from year to year by as much as a couple of weeks, depending 

 upon the temperature of the water, which apparently must reach about 70° F before 

 Bluebacks will enter. Northward, of course, the runs take place correspondingly later. 



