Large-scale long-term variability of small pelagic fish in the California current system

A number of recent studies have suggested that ocean-climate interactions can drastically change the productivity of small pelagic fish populations in different current systems, with seemingly parallel variations in the biomass of phytoplankton, zooplankton, and other fish. Sardinops and Engraulis populations, occurring in most eastern and western boundary currents, seem to be sensitive to climate-driven interdecadal regime shifts in the systems, and have been associated with long-term changes in distribution. Particularly, when sardine population size (Sardinops sp.) within a current system is high, it dominates the entire neritic zone. Alternatively, when the population size is low its distribution is greatly reduced or even totally absent from some areas. This spatial process is known as expansion-contraction. Live-bait boats fishing for tuna from the California coastline in the north to Cabo Corrientes in the south, including the Gulf of California, use as bait clupeoid fish caught alongshore. The records include catch of California sardine (Sardinops caeruleus) and northern anchovy (Engraulis mordax). For this wide region, the CPUE pattern for live-bait species, used as a proxy of abundance, is analyzed during a 49-year period (1931-1979) on large- and mesoscale basis, and related to environmental variability (SST anomalies were the proxy). Our large-scale long-term analysis in the California Current System suggests that when the sardine population size increased during warming conditions, the bulk of its biomass and the center of distribution were in the north. Before the 1960s, when the population size was decreasing during transition and cooling conditions, the center of distribution and bulk of abundance shifted from north to south and a part of it entered the Gulf of California during the cold 1960s. This population movement gave rise to a new fishery inside the Gulf of California. When the sardine population size decreased during the end of cool conditions, the bulk of its biomass and the center of distribution were in the south. The spatial process described here is different from that of homogeneous spread resulting from simple expansion and contraction. Sardine population changes are related to environmental variability, while a spatial pattern of anchovy abundance alongshore is seen assuming that the sardine is deleterious to the anchovy. Northern anchovies colonized and increased in abundance where the sardine population level was low or absent. Thus, concerning the long term and large scale, neither sardine nor anchovy populations conformed to the simple expansion-contraction model of range changes with population increase-decrease. The same information analyzed only for a restricted geographical area shows limitations and would lead to different conclusions.