Vecchione: Abundance of squid paralarvae during El Nino of 1987 



1029 



Pacific is generally limited by an oxygen minimum 

 layer beneath the thermocline (Saltzman and 

 Wishner, 1997). Thus, the distribution of these shal- 

 low-living paralarvae may have been compressed 

 toward the surface by the oxygen-minimum layer. 



Expendable bathythermograph measurements by 

 the Jordan during this period showed the mixed- 

 layer depth to be 20-40 m, except where the ther- 

 mocline was depressed within an anticyclonic eddy 

 located very close to the stations with maximum 

 squid abundance (Hansen and Maul, 1991). Conver- 

 gence of surface currents associated with the 

 downwelling that depresses the thermocline could 

 aggregate surface plankton such as these paralarvae 

 ( Bakun and Csirke, 1998 ). Although the area of maxi- 

 mum abundance was near the Costa Rica Dome, an 

 upwelling feature where the thermocline typically 

 shoals from 60 m to 30 m depth ( Balance at al., 1997 ), 

 thermal topography in this area was anomalously 

 flat during El Nino of 1987 (Fiedler et al, 1992). 



A few other records of cephalopod distribution and 

 abundance are known from this region. In an exten- 

 sive multicruise survey of the eastern tropical Pa- 

 cific in 1967-68 (Okutani, 1974), few.?, oualaniensis 

 or other ommastrephids were collected. Similarly, in 

 a cruise in this region from October 1969 through 

 February 1970, during which surface zooplankton 

 were sampled, no large numbers of paralarvae were 

 encountered, although the most abundant cephalo- 

 pod family sampled was the Ommastrephidae 

 (Ueynagi and Nonaka, 1993). 



Typical numbers of cephalopod paralai"vae taken 

 in zooplankton samples worldwide number to per- 

 haps 20 squids per sample, usually about 1-5 squids 

 (Vecchione, 1987). The highest abundance previously 

 reported anywhere of which 1 am aware was almost 

 1000 loliginid paralarvae in a 15-min surface plank- 

 ton tow in the western North Atlantic (Vecchione et 

 al., 1986). The maximum abundance found in the 

 present study was over an order of magnitude greater 

 than that. 



The samples reported in this study were collected 

 during the peak of a moderate El Nifio event 

 (McPhaden and Hayes, 1990). Surface temperatures 

 in the area averaged 3.5°C warmer than during the 

 same period of the following year (Fiedler et al., 

 1992). The temperature distribution of these samples 

 indicates that high abundances of rhynchoteuthions 

 would not be expected at temperatures <27.5°C. 

 Therefore, warm El Nino waters probably were the 

 primary reason for the high abundance of squid 

 paralarvae found here. These abundant surface- 

 dwelling paralarvae may have been concentrated to 

 extraordinary densities by convergent surface cur- 

 rents in the vicinity of an anticyclonic eddy. Seasonal 



occurrence is likely important, because surface tem- 

 peratures remained high until the end of these 

 cruises, whereas squid abundance decreased dra- 

 matically during the final 30 days. The sampling for- 

 tunately was conducted during the season when these 

 squid were hatching. 



Acknowledgments 



These samples and data were collected by biologists 

 from the Marine Mammal Program of the National 

 Marine Fisheries Service Southwest Fisheries Sci- 

 ence Center (SWFSC), primarily by R. Pitman and 

 J. Carretta. The samples were sorted in R. Charter's 

 laboratory at SWFSC, and he and G. Moser, also of 

 SWFSC, sent the cephalopods to me. I was assisted 

 in cephalopod sorting and measuring by K. Jackson. 

 The following people provided helpful comments on 

 a draft of this paper: R. Young, G. Moser, B. Collette, 

 and C. Roper. In addition, R. Young graciously pro- 

 vided unpublished data on vertical distribution of Ha- 

 waiian paralai-vae for an early draft of this paper. 



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