42 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



populations farther south, and second, that they are made up of individuals of mixed 

 origin — that is, that the northern stocks are added to by the migrants from the south. 



The southernmost return of a striped bass tagged in Connecticut and Long Island 

 waters was from the northern tip of Pamlico Sound, N. C. It is probable that the 

 striped bass of the Southern Atlantic Bight — that part of the coast of United States 

 south of Cape Hatteras — are a completely separate population, that may possibly be 

 added to under rare circumstances by the stock from the Middle Atlantic Bight — 

 Cape Hatteras to Cape Cod — and it seems reasonable to expect that the striped bass 

 population of tbe Gulf of Mexico, which presumably extends as far west as Louisiana 

 is entirely isolated. 



The Middle Atlantic Bight is undoubtedly the center of abundance for the striped 

 bass over its entire range, and tagging experiments indicate that there is compara- 

 tively little encroachment by this stock on the populations to the north and south. 

 This is well in keeping with the conclusions of Parr (1933), who has shown that the 

 shallow-water fish population of the highly heterothermal Middle Atlantic Bight is 

 bounded on the north by a cold-water barrier in the Cape Cod-Nantucket Shoals 

 region in the summer, and on the south by a warm-water barrier at Cape Hatteras in 

 the winter. Parr (loc. cit.) has pointed out that " . . . in neither locality are such 

 barriers found to be a permanent feature during all seasons." But in the case of the 

 striped bass they exist at those times of year when they are most effective in keeping 

 the bulk of the population of the Middle Atlantic Bight from encroaching on the areas 

 to the north or south. Thus the cold-water barrier at Cape Cod in the summer marks 

 the end of the northern migration in normal years, and the warm-water barrier at Cape 

 Hatteras in the winter may play some part in delimiting the extent of the southern 

 migration, and so at least partially separate the populations north and south of this 

 boundary. 



The question as to how much temperature influences the migration of the striped 

 bass is one of particular interest. This is a highly eurythermal species, yet tempera- 

 ture variations well within the maximum and minimum limits appear to play some 

 part in determining the time of migration. It seems to be more than coincidence 

 that the times when the first striped bass of the year were taken — in April 1936, 1937, 

 and 1938 — and the times that the last ones of the year were caught — in November 1936 

 and 1937 — in the Niantic River, Conn., were always when the temperature of the 

 water was approximately the same, 6.0° to 7.5° C. (42.8° to 45.5° F.) (see fig. 30). 

 Moreover, the migration of striped bass on the outer coast of North Carolina in late 

 March and early April 1938 was observed to take place over a period when the water 

 temperatures averaged 7.0° to 8.0° C. (44.6° to 46.4° F.). 



The migrations north in the spring and the return to the south in the fall do not 

 include all striped bass, for this species is caught consistently through the summer in 

 southern waters and not uncommonly in northern waters in the winter. It is a rela- 

 tively small percentage of the stock that remains north in the winter months. How- 

 ever, those that do stay north are of two types — the individuals that form the resident 

 more or less isolated populations of the north Atlantic, and those that may have had 

 their origin farther south but spend an occasional winter in northern waters. The 

 latter may possibly bolster the northern spawning stocks, but are often composed of 

 individuals that are not spawning in that particular year, for this species is not neces- 

 sarily an annual spawner (see p. 16). Striped bass that do remain in the north 

 through the winter months apparently become dormant and inactive in many cases 

 and actually hibernate to much the same extent that lias been described for the black 

 bass (Micropterus dolomieu) in the northern part of its range by Hubbs and Bailey 

 (1938). Their easy capture through the ice by scoop nets and by gigging testifies to 

 their sluggish state in cold water, and the outward appearance of individuals taken in 

 the winter and extremely early spring often shows that they are in poor condition. 

 Striped bass certainly undergo partial hibernation as far south as New Jersey, the 

 extent of this southern limit undoubtedly being determined by the prevailing tempera- 

 tures. Dormant individuals are most commonly taken in northern waters during the 

 winter in shallow bays and in the brackish waters of estuaries. Thus it appears that 

 although temperatures from 6.5° to 8.0° C. play some part in causing the migrations of 

 this species, their effect is not universal. It may be that the first and last fish of the 



