Worldwide large-scale fluctuations of sardine and anchovy populations

Decade-scale regimes of sardine #Sardinops sagax and anchovy #Engraulis spp. have been observed in the productive coastal waters of the North-Western, North-Eastern and South-Eastern Pacific and the South-Eastern Atlantic. In each of these systems, the two genera fluctuate out of phase with each other. The subdominant genus may initiate a recovery while the other species is still abundant, so population growth is not necessarily a response to a vacant niche. Rather, it appears to be triggered by formation of one or a few powerful year-classes. At high population levels, quality of sardine and their eggs decreased in Japan, leading to decreased production and survival of eggs, poor year-classes and stock collapse. Excessive fishing of strong year-classes early in the recovery stage may prevent a species from assuming dominance, so influencing the natural succession of species. This may greatly alter the structure and functioning of an ecosystem. For example, a mesopelagic forage fish may replace an epipelagic one, with severe repercussions for predators that can only feed in the upper ocean, e.g. some seabirds. Biological factors also may influence the succession of forage fish. For example, off California, peaks in abundance of predatory species such as bonita #Sarda, and chub mackerel #Scomber separate those of the planktivorous sardine and anchovy. In the Pacific Ocean, sardine distribution has changed greatly. Compared with its range when scarce, a population at a high level of biomass can extend 1 000-1 800 miles farther along the coast and 400-2 200 miles farther out to sea. In different periods of high biomass, sardine did not always have the same distribution. In the 1930s and 1940s, near simultaneous fluctuations of sardine in the North-Western and North-Eastern Pacific were in phase. In the 1980s and 1990s, sardine in the North-Western and South-Eastern Pacific were fluctuating in phase... (D'après résumé d'auteur