Applications to fishery oceanography 



What is the use of surface chlorophyll measurements in specifying and under- 

 standing the distribution of surface fish? I propose to answer this question by 

 giving the results of some of my own recent studies. 



During the years 1964-1966 I made a detailed study of the ecology of two 

 species of tropical tuna along the west coast of Baja California, where these 

 species occur from about June to December in each year. The results are in press 

 in the Fishery Bulletin of the U.S. Fish and Wildlife Service. The following 

 four figures show the main results. Each figure shows: the surface isotherm 

 at 20° C, below which temperature these tunas rarely occur; the area of maxi- 

 mum standing stock of surface chlorophyll a; the area of maximum standing stock 

 of the pelagic crab Pleuroncodes planipes , which in this area is the principal 

 consumer of phytoplankton and the principal species that the tuna eat; and the 

 areas in which tuna were caught by the fishermen. 



Figure 1 shows the situation off western Baja California at an early 

 stage in the tuna season, such as July, or August in a cold year. The seasonal 

 upwelling regime has begun to decay, whereby some waters have surface temperatures 

 at or over 20° C, and tropical tunas can enter the area. Tongues of upwelled 

 water protrude offshore from the coastal upwelling centers. They are rich in 

 chlorophyll and herbivorous crabs, but too cold for tropical tunas (< 20° C.) 

 except at the edges. On the other hand the warmer waters, where the temperatures 

 are suitable for the tunas, are relatively poor in chlorophyll and tuna food. 

 The tunas, therefore, are found at the boundary between the warm blue biologi- 

 cally poor water and the cool green biologically rich water, and not elsewhere. 



3-2 



