129 



XI. Abyssal Southern Borderland Basins and Outer 

 Continental Slope, 1,800 to 4,122 Meters: 



This is not only the deepest of environments studied in the Gulf of 

 California and along the West Mexican coast, but it is also one of the most 

 fascinating especially with regard to the types of creatures living there. 

 The majority of deep-sea expeditions have concentrated on the more 

 accessible upper slope regions, or the true abyssal sea floor and deep 

 trenches. For instance, the Galathea Expedition (Bruun, 1959) con- 

 centrated on the deeper parts of the oceans, particularly the trenches and 

 the abyssal and hadal depths below 6,000 meters. The Russian expeditions, 

 using the Vityaz and Ob (Filatova, 1959; Zenkevitch, etai, 1959; 

 and Zenkevitch and Beljaev, 1955) also carried out most of their sam- 

 pling on the great abyssal plains or at hadal depths in the trenches. The 

 American Albatross concentrated more on slope than did any of the 

 other expeditions, since they were originally interested in marine resources 

 which were close enough to shore to be utilized by man. 



Even though the lower portions of the continental slope off most con- 

 tinents are characterized by so-called abyssal depths and temperatures 

 (2,000 to 4,000 meters, and 1" to 2.5° C), the richness and complexity of 

 the faunas are not duplicated anywhere else in the deep sea, and in many 

 cases not even on the continental shelf. An example of the richness of 

 fauna from the continental slope region off Central America is given by 

 Wolff (1961) in a partial list of one trawl hawl taken off Panama and 

 Costa Rica. This was the richest deep sea haul ever made, although similar 

 rich hauls were taken with smaller gear by the present author off Mexico. 

 Several of these hauls produced almost as many individuals and were of 

 nearly the same composition as Galathea Station 716. For instance, two 

 3-meter Agassiz beam trawl hawls were taken during this study in from 

 2,000 to 4,000 meters off Baja California in June, 1961, and each contained 

 between 100 and 300 kilograms (200 to 600 pounds) of one species of 

 Macrourid fish alone. Plates XI-XV illustrate the benthic life in the re- 

 gion within a few miles of the Beam Trawl stations of the present study. 

 On the average, otter trawls taken on the lower slope off Mexico produced 

 much larger catches per hour on the bottom, than trawls taken with the 

 same gear in depths of 50 to 100 meters. 



There are several reasons why the standing crop of benthic invertebrates 

 and probably vertebrates is so high on the lower continental slope and 

 borderland basins. One of these is that the slope is, morphologically, 

 closely associated with the continental land masses and thus still close 

 enough to shore to be affected by coastal surface productivity. Many of 



9 \'idensk. Medd. fra Dansk naturh. Foren. Bd. 126. 



