503 Journal of Oceanography, Vol. 59, pp. 503 to 513, 2003 Keywords: ⋅ California Current, ⋅ Gulf of California, ⋅ California sardine, ⋅ productivity, ⋅ interchange, ⋅ transition zones. * Corresponding author. E-mail: dlluch@ipn.mx Copyright © The Oceanographic Society of Japan. Baja California’s Biological Transition Zones: Refuges for the California Sardine DANIEL LLUCH-BELDA 1 *, DANIEL B. LLUCH-COTA 2 and SALVADOR E. LLUCH-COTA 2 1 Centro Interdisciplinario de Ciencias Marinas (CICIMAR-IPN), P.O. Box 592, 23000 La Paz, Baja California Sur, Mexico 2 Centro de Investigaciones Biológicas del Noroeste, S.C. (CIBNOR), P.O. Box 128, 23000 La Paz, Baja California Sur, Mexico (Received 31 May 2002; in revised form 7 January 2003; accepted 17 January 2003) The information on the transitional areas between the temperate and tropical do- mains at the southern extent of the California Current System is reviewed and de- scribed, particularly searching for the relative isolation or interchange between the western coast of the Baja California peninsula and the Gulf of California, as well as mechanisms that permit the existence of sizeable stocks of California sardine. Bio- logical Action Centers that have high productivity throughout the year, as opposed to the rest of the coastal area, are found in both the western coast of the peninsula at the Sebastián Vizcaíno—Punta Eugenia region and in the Ballenas Channel inside the gulf; these features support large biomasses of sardine throughout the full year and serve as long term refuges during adverse periods. The role of the Sebastián Vizcaino sardine stock as the primary group for expansion is examined. The CalCOFI (California Cooperative Fisheries In- vestigations) program has been a major investigative ef- fort since the late 1940s and its contribution has been in- valuable. Other institutions have participated in the study of the northern limits of the CCS and beyond into the north Pacific, resulting in a comprehensive knowledge of the area. Thus, what may be called the temperate- subarctic transition is well known. However, while during the 1950s to the early 1970s the CalCOFI grid covered most of the latitudinal extent of the CCS (mostly south to Magdalena Bay, plus some cruises within the Gulf of California; Fig. 2), since the late 1970s it has been restricted to the north of the inter- national boundary, and the southern part has lacked that continued research effort (Figs. 3 and 4). Partly due to this, the southern part of the CCS (the temperate-tropical transition) has been poorly defined; further, the Gulf of California has been perceived as an isolated body of water, mostly because it is surrounded by high topography, connected to the open ocean only at its southern end. The perception of its uniqueness has resulted in its designation as a Large Marine Ecosystem (Anon., 1991). Oceanographic descriptions of the CCS normally reach south to about 25° N (Hickey, 1979; Lynn and Simpson, 1987), the southern extent of most CalCOFI cruises. Equatorward from this latitude (southward from the north of Magdalena Bay), there are no equivalent data 1. Introduction The California Current System (CCS) is one of the better known marine areas of the world. It is a huge tran- sitional area, the eastern boundary of the North Pacific Gyre (Lynn and Simpson, 1987). Basically, it consists of a surface current (down to 300 m) transporting water from the subarctic divergence equatorward, together with quan- tities of eastern North Pacific Central water entering from the west along its path; Equatorial Pacific water penetrates through the southern limit of the system in the form of a deeper countercurrent. Seasonally variable wind-driven upwelling incorporates cool, nutrient-rich waters alongshore (Huyer, 1983). Inshore, a narrow countercur- rent often flows poleward during fall and winter (Lynn and Simpson, 1987, Fig. 1). Essentially, four major faunal assemblages result from the oceanographic conditions in the region: transi- tional, or the California Current System itself, limited to the north by the subarctic, to the west by the central and by the equatorial domains at the south. The California Current System fauna is a mixture of species from each of these domains plus some endemic, reflecting its tran- sitional nature (Moser et al ., 1987). It is a huge subarctic- tropical ecotone, in the sense defined by Odum (1959).