The fish and invertebrates harvested from the tropical Pacific Ocean and the coastal waters of the region, and the ecosystems that support these species, are influenced not only by surface climate; they also depend intimately on the oceanic environment. Large-and small-scale circulation patterns influence larval dispersal and the migration of species; water temperature, salinity, nutrient availability, dissolved oxygen concentration and pH affect biological activity; and oceanic currents, waves and sea level shape coastal habitats. Together, these properties of the tropical Pacific Oceanii have a profound effect on the productivity of oceanic and coastal fisheries, and coastal aquaculture.

Understanding how these features of the ocean vary is one of the primary keys to forecasting production from the fisheries sector. Resolving this puzzle is a major challenge, however, because the tropical Pacific Ocean varies across an enormous range of spatial and temporal scales. For example, within just a few days, cloud cover, local upwelling, surface mixing due to storms or oceanic eddies can substantially modify sea surface temperature, salinity, currents and nutrient supply at the scale of reefs and islands. On larger spatial scales, seasonal changes in nutrient upwelling have a significant effect on primary productivity (Chapter 4). Closer to the equator, key properties of the Pacific Ocean are dominated by the interannual variations of the El Niño-Southern Oscillation (ENSO)(Chapter 2). On longer time scales, variations in the strength of the mid-latitude westerly winds produce climate ‘regime shifts’, like those recorded in 1925 and 19431, and in 1976–19772. Regime shifts are characterised by abrupt ENSO-like changes that can last for several decades, commonly associated with the Interdecadal Pacific Oscillation (IPO) or the closely related Pacific Decadal Oscillation (PDO) 3–5. Profound changes over and above natural climate variability are also occurring, due to the build-up of anthropogenic greenhouse gas emissions in the atmosphere (Chapters 1 and 2).