Table 35. Patterns of spring and summer-early fall distribution ol bluelln tuna on the two sides of the Atlantic. 

 Season Fish Size Eastern Atlantic Western Atlantic 



Spring 



Small Morocco; Bay of Biscay 



Medium Ibero-Moroccan Bay, Medit.? 

 Large Ibero-Moroccan Bay, Medit.? 



Off edge of Cont. Shelf (N.E., U.S.) 

 Gulf Stream-Edge of Cont. Shelf 

 Bahamas-Gulf of Mexico 



Early Summer 



Mid-Summer- Early Fall 



Small Bay of Biscay; Morocco 



Medium Ibero-Moroccan Bay, Medit.? 



Large Ibero-Moroccan Bay, Medit? 



Norway 



Small Bay of Biscay; Morocco 



Medium Scandinavia, Bay of Biscay 



Large Scandinavia, North Sea 



Middle Atlantic Bight 



Middle Atlantic Bight; Gulf of Maine 



Cape Cod-Newfoundland 



Middle Atlantic Bight 



Middle Atlantic Bight; Gulf of Maine 



Cape Cod-Newfoundland 



Late Fall-Winter-Early Spring 



Small Morocco-Canary I. +? 



Medium Morocco-Canary I. -t? 

 Large N.E. Atl. -Canary 1. -Morocco 



N. edge GulfStream N of 35°N 

 Gulf of Mex. W. Indies, Atlantic from 

 35°N to lO'S (or farther) and from 

 the American coasts to about 40°W. 



winter and summer habitats. Major de- 

 partures from the ocean into relatively 

 land-locked spawning areas, and re- 

 turns to the ocean, take place on both 

 sides of the Atlantic. 



The patterns may be compared to 

 gyres which are greatly elongated in 

 the north-south direction and distorted 

 by the contours of the land masses and 

 by detours into and out of the European 

 and American Mediterranean, where 

 important spawning occurs. They are 

 also influenced by hydrological condi- 

 tions and coincide with favorable cur- 

 rents for much of their extent, particu- 

 larly in the western Atlantic. The south- 

 ward migrations are probably more dif- 

 fuse than the northward ones, and ex- 

 tend much farther into the middle of 

 the ocean, affording considerable op- 

 portunities for mixing. The movements 

 in the eastern Atlantic gyre are gener- 

 ally counterclockwise, while those in 

 the western Atlantic gyre are generally 

 clockwise. Thus, one pattern is roughly 

 a mirror image of the other. 



The medium apd smaller fish on 

 both sides of the Atlantic move within 

 these same patterns but to a much lesser 

 extent in the north-south direction. For 

 both groups migration appears to be 



much simpler in the western than in the 

 eastern Atlantic. The range of the fish 

 tends to increase somewhat with each 

 year of growth. 



D. IDENTIFICATION OF 

 STOCKS 



Much important information ob- 

 tained recently supports the tentative 

 identifications of Atlantic-Mediterra- 

 nean bluefin tuna stocks by Mather et 

 al. ( 1 974). These authors noted that an 

 infinite number of combinations of 

 stocks, and degrees of mixing between 

 them, might theoretically exist in the 

 Atlantic and connected seas, but con- 

 sidered the following basic combina- 

 tions only: 



1 . A unit stock in the Atlantic and con- 



nected seas. 



2. One stock in the Atlantic system and 



another in the Mediterranean sys- 

 tem. 



3. One stock in the eastern Atlantic and 



the Mediterranean, and another in 

 the western Atlantic. 



4. One stock in the Mediterranean, a 



second in the eastern Atlantic and a 

 third in the western Atlantic. 



They found no reason to believe 

 that one or more separate stocks ex- 

 isted in the South Atlantic. 



As they noted, varying degrees of 

 mixing, or none, might occur between 

 the components of dual- or multi-stock 

 combinations. Tag return data for blue- 

 fin tuna, however, indicated that mix- 

 ing tendencies may be strong. This was 

 especially true of the large fish. 



Mather et al. (1974) concentrated 

 most of their discussion on two ques- 

 tions which still appear to be of para- 

 mount importance: 



1 . The identification of the stock or 



stocks of bluefin tuna in the Medi- 

 terranean and the eastern Atlantic. 



2. The identification of the stock or 



stocks of bluefin tuna in the Atlan- 

 tic Ocean. 



They briefly reviewed the histori- 

 cal opinions and developments in re- 

 search on the relationships between the 

 Mediterranean and Atlantic bluefin 

 tuna. They then discussed Sara's ( 1 964, 

 1 973) hypothetical migratory model for 

 the migrations of bluefin tuna in the 

 Mediterranean, which has also been 

 described in the present work, and some 

 of the evidence supporting tliis theory. 



140 



