Maxwell et al.: Fishery dynamics of Loligo opalescens 



667 



clearly delimited by the 100-m contour. The landings 

 data, reported by fishing blocks, were much cruder in 

 geographic scale and failed to catch this subtlety. 



The ground-truthing work conducted by aerial sur- 

 veys indicated that detected light pixels are useful in 

 estimating the number of squid vessels in operation. 

 This result is consistent with examination of the fish- 

 ery for the squid Illex argentinus in the southwestern 

 Atlantic, where vessels use powerful lamps to attract 

 the squid to lures (Waluda et al., 2002). In the latter 

 fishery, analysis of images acquired by the DMSP-OLS 

 satellites revealed a good fit between the recorded num- 

 ber of vessels in operation on a given night and the 

 number of light pixels detected (Waluda et al., 2002). 



In the present study, the fishery data showed a 

 strong response to the 1997-98 El Nino event, which 

 was one of the strongest events on record (Wolter and 

 Timlin, 1998). Fishing effort and landings tended to 

 peak in the Oct-Dec and Jan-Mar quarters before 

 the 1997-98 El Nino. Both data series dramatically 

 dropped during the 1997-98 El Nino and showed recov- 

 ery afterwards. Squid abundance, measured as LPUE, 

 also showed a pronounced drop and rapid increase in 

 response to the El Nino. It is interesting to note that 

 another index of market squid abundance, the occur- 

 rence of squid beaks in the scat of sea lions, showed 

 similar responses to earlier El Nino events (Lowry and 

 Carretta, 1999). Squid beak occurrence dropped steeply 

 during the strong 1983-84 El Nino, and increased 

 afterwards. Beak occurrence also dipped and rose in 

 response to a milder El Nino in 1992-93. Significantly, 

 Lowry and Carretta (1999) examined southern Chan- 

 nel Islands: Santa Barbara, San Clemente, and San 

 Nicolas. Our present study reflects squid abundance 

 primarily around northern Channel Islands (e.g., Santa 

 Rosa, Santa Cruz, Anacapa). Taken together, these 

 studies may indicate that El Nino exerts a bight-wide 

 influence on squid abundance. 



We suggest that a strong El Nino event changes the 

 reproductive conditions for the market squid in the 



-2.0-1 1 1 1 1 1 1 1 h 



1992 1993 1994 1995 1996 1997 1998 1999 2000 

 Year 



Figure 4 



Multivariate ENSO index iMEI) for the tropical Pacific 

 (between 10 C N and 10°Si, by month. Data were obtained 

 from http://www.cdc.noaa.gov/~kew/MEI/table.html. 



Southern California Bight. With regard to spawning, 

 the spawning population becomes less abundant on the 

 traditional shallow-water spawning grounds. Research 

 on a congener, the South African chokka squid (Loligo 

 vulgaris reynaudii), points to possible environmental 

 influences on spawning for loliginid squid (Roberts and 

 Sauer, 1994). Off South Africa, a strong El Nino can 

 lead to reduced upwelling and increased turbidity. In 

 normal years, upwelling, presumably detected by the 

 squid as an influx of cold water, may trigger spawning 

 behavior (Roberts and Sauer. 1994). In El Nino years 

 off South Africa, reduced upwelling and increased tur- 

 bidity on the inshore spawning grounds are thought to 

 force the spawners into deeper water, beyond the reach 

 of the fishery (Roberts and Sauer, 1994). In a recent 

 study, catch for the chokka squid increased with strong 

 easterly winds, which caused upwelling, and decreased 

 with increased turbidity (Schon et al., 2002). In the 

 California Current System, upwelling decreases during 

 strong El Nino events (Schwing et al., 2000). Upwelling 



