OXYCLINE CHARACTERISTICS AND SKIPJACK TUNA DISTRIBUTION 

 IN THE SOUTHEASTERN TROPICAL ATLANTIC 



Merton C. Ingham, 1 Steven K. Cook, 1 and Keith a. Hausknecht* 



ABSTRACT 



A shallow layer of low oxygen concentration, containing minimum values frequently less than 1.0 ml/1 

 and a strong oxycline, was measured on two cooperative cruises in the southeastern tropical Atlantic 

 Ocean and found to be consistent with previous portrayals and hypotheses based on fragmentary data. 

 The low-oxygen layer was in the form of a thick wedge off the southwestern coast of Africa, extending 

 from about lat. 18° to 3° S. The oxycline overlying the low-oxygen layer was generally coincident with a 

 pycnocline and was found at depths of 20-50 m in most of the area surveyed, as revealed by the 

 topography of the 3.5 ml/1 iso-oxygen surface. It is believed that a shallow oxycline has a strong 

 influence on the distribution and availability of skipjack tuna schools. The hypothesis was tested by 

 overlaying school sighting positions on the 3.5 ml/1 topography. The association between sightings and 

 oxycline depth was further defined by developing a linear "equation" relating the two variables as 

 follows: s = 23.15 - 0.59z, where s is the number of school sightings, z is the depth of the 3.5 ml/1 

 surface, and 23.15 and 0.59 are constants. A similar correlation was attempted with school sightings 

 and habitat layer thickness, but the results were less systematic and convincing than the oxycline 

 correlation. 



A shallow oxycline containing low values of dis- 

 solved oxygen concentration should serve as a 

 lower boundary of the environment habitable by 

 surface schooling tunas. In a study of the relation- 

 ship of thermocline depth to success of purse sein- 

 ing of tuna in the tropical Pacific, Green (1967) 

 stated that an oxycline approximately coincident 

 with the thermocline could play a major role in 

 restricting the fish to near surface waters. Work 

 on the oxygen requirements of captive skipjack 

 tuna in the Southwest Fisheries Center Honolulu 

 Laboratory 3 by R. M. Gooding and W. H. Neill 

 indicated a 4-h TL m (median tolerance limit) be- 

 tween 2.4 and 2.8 ml 2 /l, and in experiments with 

 gradually declining oxygen concentrations an 

 alarm threshold was found near 3.5 ml/1. If we 

 regard the 3.5 ml/1 iso-oxygen surface to be the 

 "floor" of habitable environment of surface school- 

 ing tunas in tropical waters, then the topography 

 of this surface becomes significant in describing 

 their environment. 



The shoaling of the oxycline, the floor of the 

 habitable environment, may serve not only to 



•Northeast Fisheries Center Atlantic Environmental Group, 

 National Marine Fisheries Service, NOAA, Narragansett, RI 

 02882. 



2 Graduate School of Oceanography, University of Rhode Is- 

 land. Narrangansett, RI 02882. 



3 Neill, W. H. Unpubl. exp. data, Southwest Fish. Cen. Hon- 

 alulu Lab., Natl Mar. Fish, Serv., NOAA, pers coramun., 1974 

 and 1976. 



Manuscript accepted February 1977. 

 FISHERY BULLETIN: VOL. 75, NO. 4, 1977. 



crowd the skipjack tuna schools to the surface, but 

 also to influence the lateral distribution of the fish 

 schools through other ecological factors associated 

 with the shoaling. The oxycline is imbedded in the 

 thermocline, which is brought up to or near the sea 

 surface under conditions of upwelling which sea- 

 sonally occur off the southwestern coast of Africa. 

 Such conditions, when well developed, will lead to 

 the development of fronts, which tend to concen- 

 trate forage, and higher rates of primary and sec- 

 ondary productivity to sustain larger forage popu- 

 lations; both processes tending to concentrate 

 predators such as tunas, as described by 

 Blackburn (1965). 



BACKGROUND INFORMATION ON 



OXYGEN MINIMA IN 

 THE SOUTHEASTERN ATLANTIC 



The oxygen minima in the Atlantic have been 

 studied since the early part of this century. These 

 studies have not, however, resulted in a definitive 

 explanation of the mechanisms of formation of 

 these low-oxygen layers. While many theories 

 have been proposed to explain the origin of these 

 layers, the mechanisms generally cited as being 

 most significant are either an extremely high 

 biochemical oxygen consumption or low rates of 

 oxygen replenishment by mixing processes. Some 

 recent papers have dealt with a synthesis of these 



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