hand, they may simply be contingents 

 of the main aggregation which have 

 chanced upon satisfactory feeding 

 grounds after spawning in the Mediter- 

 ranean, and stayed there rather than 

 returning to the Atlantic. At present, 

 there is no basis for determining which 

 explanation is correct. 



The main group of fish leaves the 

 Atlantic wintering area in late March 

 or April and migrates to the Ibero-Mo- 

 roccan Bay forming the "arrival" run 

 (see Section VICldi for details) which 

 moves along both shores of the Bay 

 toward the Strait of Gibraltar in May 

 and June. A fraction of these fish con- 

 tinue to migrate eastward and spawn in 

 the Mediterranean. The others presum- 

 ably remain in the Ibero-Moroccan Bay 

 and spawn there. Further study is nec- 

 essary to determine what percentage of 

 the "arrival" fish do remain and what 

 percentage spawns in the Mediterra- 

 nean. 



The major spawning grounds of 

 the large fish in the Mediterranean are 

 probably off western and northern Sic- 

 ily, western Sardinia and western Tuni- 

 sia, where the waters are somewhat 

 cooler and less saline than those farther 

 east, which are predominantly the 

 spawning areas of the smaller fish. 

 Spawning is believed to occur between 

 mid-June and mid-July in the Ibero- 

 Moroccan Bay and central Mediterra- 

 nean. The numbers offish which con- 

 centrate off Gibraltar and the ft^action 

 of these which enter the Mediterranean 

 depend considerably on the environ- 

 mental conditions in and over the east- 

 em Atlantic during the preceding weeks. 

 The number offish entering the Medi- 

 terranean depends greatly on the 

 strength of the inflowing Atlantic sur- 

 face current, in which they travel. 



After spawning, the fish in the 

 Mediterranean initiate the westward re- 

 turn run, which occurs through July 

 and August. They follow the deeper 

 Mediterranean current, skirting the east 

 and south coasts of Sicily, and go out 

 through the Strait of Gibraltar, contrib- 

 uting relatively small catches to the 

 traps on either side of the eastern en- 

 trance. There they evidently join, or 

 follow, the fish which have remained 

 in the Ibero-Moroccan Bay in their 

 westward migration along the southern 

 coasts of Spain and Portugal, forming 

 the "return" run. 



The positions where these repro- 

 ductive migrations come near the coasts 

 are clearly indicated by the traditional 

 locations of the traps. Those which 

 fished the arrival run in the Ibero-Mo- 

 roccan Bay extended along the Iberian 

 coast from Cabo de Santo Maria in 

 Portugal to the Strait of Gibraltar, and 

 along the Moroccan coast from near 

 Kenitra to the entrance of the Strait. 

 The traps nearest Gibraltar took the 

 largest fish. In the Mediterranean, the 

 traps off western and northem Sicily, 

 western Sardinia, and western Tunisia 

 have usually taken larger fish than those 

 off eastern Sicily, Calabria, eastern Tu- 

 nisia and Libya. The waters in the latter 

 areas are somewhat warmer and more 

 saline than those in the latter and the 38 

 o/oo isohaline, which runs from the 

 Gulf of Sirte to Sicily, has been pro- 

 posed as the eastward limit of the 

 spawning of large bluefin. Pelagic fish- 

 eries have recently indicated that large 

 bluefin are extensively distributed in 

 the westem and central Mediterranean 

 from late April into September, and 

 that spawn ing occurs extensively in off- 

 shore waters ol the south Tyrrhenian 

 Sea. The fish of the "local" groups in 

 the western and central Mediterranean, 

 presumably spawn with the more nu- 

 merous migrant aggregation, but tiiis is 

 conjectural. Larvae and/or small (less 

 than 10 cm) juveniles have been found 

 in many of these central and westem 

 areas. It seems very probable, how- 

 ever, that the large bluefin which feed 

 and winter in the Sea of Marmara spawn 

 in the Black Sea, where collections of 

 bluefin larvae have been reported. Since 

 large maturing fish are said to be rare in 

 the Mediterranean east of the central 

 Libyan coast and the eastern Ionian 

 Sea, it seems improbable that tiie very 

 large fish taken near Istanbul are mi- 

 grants from the Atlantic. Tlie Bosporus 

 would be an ideal location for tagging 

 and sonic tracking experiments. 



The "return" passage is completed 

 along the northem shores of the Ibero- 

 Moroccan Bay in July and August and 

 then the fish turn north toward the feed- 

 ing grounds. Some of them follow the 

 Iberian coast into the Bay of Bisca>, 

 where they usually remain for only a 

 month or six weeks. The majority, how- 

 ever, has usually proceeded to feeding 

 grounds off the coast of Norwaj' and in 

 the North Sea. They reached these 



northem areas by a circuitous course 

 passing west and north of the British 

 Isles, rather than following the more 

 direct route to these areas through the 

 English Channel. Until 1962, most of 

 the larger giant bluefin moved north- 

 ward along the Norwegian coast and 

 subsequently migrated southward into 

 the North Sea. Since then, however, 

 nearly all of them have moved south- 

 ward along the Norwegian coast with 

 the smaller members of this group, and 

 the species has been scarce or absent 

 off northem Norway and in the North 

 Sea. Tliese large bluefin usually enter 

 Norwegian waters in July, spend the 

 remainder of the warm season there, 

 moving southward along the coast and 

 feeding voraciously, and depart from 

 them in October. They again pass north 

 and west of the British Isles and reach 

 their wintering grounds east of north- 

 em Afi-ica and the Ibero-Moroccan Bay 

 in November or December, complet- 

 ing their migratory cycle. 



The south central Mediterranean 

 appears to be the prime reproductive 

 area for the bluefin of the Mediterra- 

 nean. Its importance to the reproduc- 

 tion of eastem Atlantic bluefin is more 

 difficult to assess. This is one of the 

 most important questions in regard to 

 the spawning, and the identification of 

 stocks that remains unanswered. 



The Ibero-Moroccan Bay has been 

 generally regarded, at least in this cen- 

 tur>', as the prime bluefin tuna spawn- 

 ing ground in the eastem Atlantic, but 

 not one bluefin larva, or small (less 

 than 10 cm long) juvenile, has been 

 collected in its waters. Research, in- 

 cluding plankton collections, has been 

 carried on there almost continually for 

 many decades by scientists of several 

 nations, and the waters of the Bay, as 

 well as adjacent parts of the Atlantic 

 and the Mediterranean, have twice been 

 surveyed vei^y thoroughly during the 

 spawning season. The failure to collect 

 an> early stages of bluefin there cannot 

 be attributed to lack of effort. It has 

 been suggested that the products of 

 spawning in the Ibero-Moroccan Bay 

 might have been passively transported 

 into the Mediterranean by the inflow- 

 ing surface current before the early 

 stages had attained full mobility. This 

 seems unlikely, since the surveys men- 

 tioned above covered all, or most of, 

 the Alboran Sea (the westemmost part 



138 



