Zoological Studies

Vol. 46 No. 3, 2007

Different Spatiotemporal Distribution of Argentine Short-Finned Squid (Illex argentinus) in the Southwest Atlantic during High-Abundance Year and Its Relationship to Sea Water Temperature Changes

Chih-Shin Chen1, Wen-Bin Haung2, and Tai-Sheng Chiu3,*

1Institute of Fishery Biology, College of Life Science, National Taiwan University, Taipei 106, Taiwan
2Graduate Institute of Biological Resources and Technology, National Hualien University of Education, Hualien 970, Taiwan
3Institute of Zoology, College of Life Science, National Taiwan University, Taipei 106, Taiwan

Chih-Shin Chen, Wen-Bin Haung, and Tai-Sheng Chiu (2007) The catch of Argentine squid (Illex argentinus) in the Southwest Atlantic began in the early 1980s, reached a historical high in 1999, and dropped thereafter.  By using retrospective catch information of Taiwanese jiggers to represent years of low (1996) and median (1998) catches in contrast to the historical high, we outlined an ordinary pattern of abundance for the winter cohort during the dominant fishing phase from Mar. to May, and applied step-by-step generalized linear models to look into possible causes for the high catch.  The variations in catch per unit of effort (CPUE) corresponding to the 5 variable effects of year, month, latitude, position on the continental shelf, and body size were analyzed, and the findings were mapped spatiotemporally.  In the 1st step, we confirmed that the high abundance of 1999 was significant at p < 0.05 as compared to ordinary years (1996 and 1998).  In the subsequent intra-annual comparisons, effects of month, latitude, and body size affected the CPUE in ordinary years, while only latitude and body size were significant to the CPUE and monthly differences were irrelevant in the high-abundance year (1999).  The spatiotemporal patterns in 1999 were unique; characterized by a significantly high catch rate which was widespread over the fishing ground, relatively small body sizes, a concentrated geographic distribution prone to southern latitudes, and little signs of a northerly (pre-spawning) migration. The cause of these characteristics could be explained by deviations in subsurface water temperatures at fishing sites.  During the austral autumn of 1999, the thermal retention of waters on the Patagonian Shelf experienced a rapid decrease.  Specifically, the temperature began to drop in Apr., becoming lower than in ordinary years (supported by > 90% bootstrap possibility) in middle latitudes.  The lower water temperature in Apr. might have retarded the growth of the squid, consequently causing the population to remain on the nursery ground, and ultimately delaying the timing of the northerly migration of the squid for spawning.  The stagnation of a high concentration of squid in the middle and southern latitudes of the Patagonia Shelf may have resulted in extensive fishing practices that further reduced the size of the potential spawning population.

Key words: Illex argentinus, Distribution, Migration, Temperature, Patagonian Shelf.

*Correspondence: Tel: 886-2-33662448.  Fax: 886-2-23634014. E-mail:tschiu@ntu.edu.tw