HISTORY OF THE AMERICAN MENHADEN. 65 



isothermals for July indicate the actual temperature of the sea from day 

 to day. In reality the marine isothermals are constantly varying, and, 

 in this respect are different from those printed upon a chart. A glance 

 at the tables in Appendix F, and the conclusions deduced from them in 

 regard to the menhaden (paragraph 85), will show that schools of fish 

 do not find it necessary to force their way through walls of sea tempera- 

 ture, but that their movements from south to north are exactly corre- 

 lated with the seasonal rise of temperature. As soon as the water at 

 a given point reaches the necessary temperature, which for the mackerel 

 on our own coast appears to be as much as 45°, the fish make their 

 appearance, and with the advance of the season they appear farther and 

 farther to the north. Mackerel do not appear on the coast of Maine un- 

 til the water is as warm as it was off Cape Hatteras at the time of their 

 first arrival. This is the case whether we suppose their general move- 

 ment to be parallel with or vertical to the coast line. 



I have entered the discussion of this question not with any idea of at- 

 tempting to prove that mackerel migrate south from the Gulf of St. 

 Lawrence, but to show that a comparatively rapid northward movement 

 in May and June does not necessitate a "sudden plunging from high to 

 low zones of temperature." 



Arguments against extended migrations of menhaden. 



89. There is no satisfactory evidence that the menhaden pursue ex- 

 tended migrations north and south. The same evidence which tends to 

 show that the shad, salmon, and alewife do not follow this course will 

 will apply, with modifications, to the menhaden. 



The menhaden schools at different points along the coast appear to 

 have individual peculiarities, corresponding to those of the shad in the 

 different rivers. A Maine menhaden may easily be distinguished from 

 a Long Island menhaden, a Chesapeake or a Florida one, by certain in- 

 describable characters, easy to perceive but difficult to define. The 

 presence of the crustacean parasite in the mouths of southern menhaden, 

 and its constant absence from those of the north is a very strong argu- 

 ment in favor of local limitation in the range of menhaden schools. 



That the same schools of menhaden return year after year to the 

 same feeding grounds is rendered very probable by the statements of 

 Mr. Miles in paragraph 72. 



The schools in the southern waters do not receive any apparent incre- 

 ment at the time of desertion of the north coast, nor are the southern 

 waters deserted at the time of abundance in the north. There is, how- 

 ever, a limited north and south migration. The Maine schools on their 

 departure in the fall appear to follow the southward trend of the coast 

 until they strike the hook of Cape Cod, where they are detained for some 

 days ; they then round the cape and are again detained by the hook of 

 Montauk Point. They first strike the shore at Point Judith and are 

 5 p 



