Tim Baumgartner¹, Guillermo Auad², Arthur J. Miller²

¹ Centro de Investigación Científica y Educación
Superior, Km 107 Carr. Tijuana-Ensenada,
Ensenada, B.C., C.P. 22860, Mexico,
E-mail: tbaumgar@cicese.mx
² Scripps Institution of Oceanography, University of
California, San Diego, 9500 Gilman Drive, La Jolla,
California 92093, USA,
E-mail: guillo@ucsd.edu, miller@horizon.ucsd.edu

The Pacific sardine (Sardinops sagax caerulea) is one of several small coastal pelagic, schooling, planktivorous species that provide important trophic links within the ecosystem of the California Current. The regional ocean dynamics regulating sardine habitat have been subject to relatively abrupt reorganization in the basin-wide ocean-atmosphere circulation three times during the 20^th century (1925-26; 1943-44; 1976-77). These interdecadal changes appear to have occurred as shifts between two predominant physical regime states, each associated with a coherent pattern of basin-wide sea-surface temperature distribution similar (but not identical) to those distinguishing the interannual ENSO mode that oscillates between warm (El Nino) and cool (La Nina) phases.

This presentation explores the mechanisms by which overall productivity of the sardine population is mediated by the large-scale climate variability over the North Pacific. We approach this problem from two directions. The first is through a biological model to reconstruct natural variability of the population eliminating the effects of fishing to provide an estimate of density-independent variability in biomass production that would have occurred under the varying quality of the natural extrinsic habitat had there been no harvesting. The second approach is based on an ocean model of the habitat changes associated with the 1976-77 climate shift. The ocean model is forced with the atmospheric fields representing a canonical regime change.

This combined approach indicates significant association between density-independent growth rates of the population and access to favorable feeding and spawning habitat, controlled primarily by ocean temperature. Although greatly exacerbated by the effects of fishing, the collapse of the population in the first half of the 20^th century was underlain by progressively diminishing rates of population growth (recruitment failures after the mid 1940s) associated with a southward contraction of the spawning habitat to waters limited to the region with lower food production off Southern California and Baja California, south of Point Conception (34×N). Recovery of the population coincided with early season access to spawning habitat north of Point Conception in waters off central California with abundant and concentrated zooplankton. The ocean model shows that the overall size, quality and seasonality of the area of favorable habitat is linked to the large-scale climate regime through its influence on the distribution not only of near-surface temperature, but also on the underlying ocean dynamics producing more favorable large-scale conditions north of Point Conception after the 1976-77 regime shift. We conclude that periods of warm coastal regime allow spawning to begin earlier in the year in the more northerly region of habitat where food is more available due to stronger offshore and coastal upwelling, while stability in the transition region preferred by spawning sardines (between offshore oceanic and near coastal strip) is increased by anomalous large-scale onshore transport and deepening of the pycnocline