FISHERY BULLETIN: VOL. 86, NO. 1 



(McEachran et al. 1976). This condition, however, 

 is not necessarily true when the sought-after re- 

 sources are not in short supply (Zaret and Rand 

 1971). This may be the case of M. thurstoni and 

 M . japanica feeding together on A^. simplex when 

 the abundance of euphausiids is at its peak. Com- 

 petition should occur, however, in late summer, 

 when prey numbers decline. It would be interest- 

 ing to determine whether the slight morphologi- 

 cal and behavioral differences between poten- 

 tially competing species pairs (M. thurstoni/ 

 M. munkiana in winter, M. thurstoni IM . japan- 

 ica in spring and summer) influence or reflect 

 partitioning of their habitat when food resources 

 become limiting, as was described for both fresh- 

 water (Werner and Hall 1977) and marine 

 teleosts (Hixon 1980; Larson 1980). 



This overview of the ecology and natural his- 

 tory of mobulids in the Gulf of California is based 

 on field investigations made chiefly in 1983, a 

 year in which the El Nino perturbation was par- 

 ticularly severe (Cane 1983). Although in terms 

 of fishermen's experience the year 1983 was not 

 unduly different, as far as mobulid relative abun- 

 dance and seasonality are concerned, the abnor- 

 mally high water temperatures resulting from El 

 Nino may have affected the devil rays studied in 

 subtle ways; therefore this investigation should 

 be repeated in a normal year. 



According to the fishermen, the abundance of 

 sharks (mostly carcharhinids and sphyrnids) on 

 which their activity is based is declining. This 

 decline will probably result in an increase of mob- 

 ulid fishing effort. It is of concern that 12% of the 

 specimens of M. thurstoni caught were immature 

 (DW <1,500 mm). 



ACKNOWLEDGMENTS 



I owe deep gratitude to the many persons who 

 assisted me in this investigation: Richard H. 

 Rosenblatt, Theodore H. Bullock, Paul K. Dayton, 

 William E. Evans, and Walter H. Munk, mem- 

 bers of my doctoral committee; Edward Brinton, 

 Robert Cowen, Abraham Fleminger, Nicholas 

 Holland, Margaret Knight, Spencer Luke, 

 William Newman, Mark Grygier, Jeff 

 Schweitzer, George Shor, George Snyder, and 

 Fred White of the Scripps Institution of Oceanog- 

 raphy; Thomas Bowman (United States National 

 Museum, Washington, D.C.); Daniel Brooks (Uni- 

 versity of British Columbia, Vancouver); Greg 

 Deets (Long Beach State University); Dennis 

 Bedford and Robert Lea (California Department 



of Fish and Game); Fay Wolfson (Hubbs Marine 

 Research Institute); Alexis Fossi (Institut Na- 

 tional des Techniques de la Mer, Cherbourg, 

 France); Felipe Galvan Magaiia (Centro Inter- 

 diciplinario de Ciencias Marinas, La Paz, Mex- 

 ico); Lalo Cuevas, Marcelo Geraldo, Juan Lucero, 

 and their colleagues of the Cooperativa Pesquera 

 de Punta Arena de la Ventana; Steven Kramer 

 (National Marine Fisheries Service, Honolulu, 

 Hawaii); Carl A. Jantsch and Steven D. Kamol- 

 nich (Sea World, Inc., San Diego). The Hubbs 

 Marine Research Institute (San Diego) loaned a 

 sailing vessel, the "Fling", for the field study; 

 Rodney Black helped in the outfitting of the ves- 

 sel and in sailing it to the Gulf of California. This 

 investigation was supported in part by a grant 

 from the Foundation for Ocean Research (San 

 Diego). 



LITERATURE CITED 



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1980. Euphausiids in the Gulf of California - the 1957 



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