3 3 o Memoir Sears Foundation for Marine Research 



Parasites. The parasitic acanthocephalan Echinorhynchus acus and the nematode 

 Heterakis foveolata have been listed (JSj: 742). A specimen with a large colonial 

 hydroid, Obelia commensuralis, attached to its back has been reported for Beaufort, 

 N.C. (50: 1-6). 



Relationships. This species and pseudoharengus are closely related, but they differ 

 prominently from chrysochloris and mediocris in the more numerous gill rakers. In general, 

 aestivalis has a more slender body and a smaller eye than pseudoharengus. However, 

 extensive overlapping occurs in a series of measurements of specimens from various 

 localities, even though specimens of equal size are measured. But in any one area, as 

 in Chesapeake Bay, the separation of the two species by a combination of the two 

 characters mentioned generally is not difficult. 



Northward, as in New England, the species run somewhat more slender, and often 

 some northern graybacks are quite as slender as some of the Bluebacks from the southern 

 part of the range. Although the proportionate size of the eye is a somewhat more reliable 

 character than depth of body, some specimens are difficult to distinguish by this char- 

 acter also. Fresh examples generally are distinguishable by the color, the Bluebacks 

 being bluish above, the graybacks grayish green. However, this difference in color 

 disappears in preserved specimens. The Bluebacks usually have more gill rakers and 

 vertebrae than the graybacks, but again intergradation occurs in each instance; and the 

 use of the number of gill rakers is made more complex because of their increase in 

 number with the age of the fish. The range and intergradation of all the proportions 

 and enumerations mentioned are shown in the Description. 



The peritoneum (lining of abdominal cavity) is said to be black in the Bluebacks 

 and pale or pale gray in the graybacks. Because this character was supposed to be diag- 

 nostic, it has been used in keys many times, but in the numerous preserved specimens 

 at hand, it was found unreliable; Herbert E. Warfel, who has worked with Pomolobus 

 in Connecticut, has informed me (verbally) that this character is unreliable in fresh 

 material also. It cannot be trusted, because the peritoneum in the Bluebacks varies all 

 the way from black to pale or pearly gray. 



It is evident from the foregoing discussion that aestivalis and pseudoharengus often 

 can be distinguished only by a combination of characters, and that, even then, some 

 troublesome examples occur, with small specimens more difficult to identify than 

 large ones. 



Abundance and Commercial Importance. The relative abundance of the Bluebacks 

 and graybacks is not definitely known. Although many fishermen recognize the two 

 species, they are not separated in the market, and the catches are listed merely as 

 "alewives" in the government's statistical reports. It has been stated that the two species 

 are about equally abundant in Chesapeake Bay and that each contributes an equal 

 share to the commercial catch {^g: 91); also, the Bluebacks are more abundant than 

 the graybacks in the southern states and less numerous northward {y2, 1896: 426). 

 For the Gulf of Maine, Bigelow and Welsh found it difficult to arrive at a fair idea 

 of the relative abundance of the two {16 : 112); although it is generally agreed that the 



