Euphausia mucronata: A keystone herbivore and prey of the Humboldt Current System

Euphausiids are important components of many ecosystems, especially in productive regions of temperate and high latitudes. The present paper makes the case that E. mucronata plays a keystone role in the food web of the Humboldt Current System (HCS) based on a synthesis of new and published data supporting its potential role as a primary grazer, as well as a principal prey for upper trophic level fish. E. mucronata is an endemic species, concentrated in the coastal upwelling belt of the HCS, with morpho-physiological adaptations to vertically migrate into the Oxygen Minimum Zone (OMZ). Within the 100-km coastal belt of the HCS it accounts for ca. 50% of the meso zooplankton wet weight in winter. In the mixed layer, it is a herbivore with high night ingestion rates (612.2 ng Chl eq ind−1 h−1 or 1013.9 μg C ind−1 d−1, in winter), and accounted for a 19.3% impact on primary production in winter, at an intermediate population abundance (3.8 ind m−3). At higher abundances (50 ind m−3) equivalent to swarms, impact on primary production could reach 254%.Additionally E. mucronata is a common prey of numerous upper trophic level predators. The diet of jack mackerel (Trachurus murphyi) off central Chile (34-39°S) indicates a striking dependence on E. mucronata prey (average of 75% of stomach content in weight). The fishing season off central Chile extended from austral fall (March–April) and continued at least until the end of austral winter (September). The average daily ration of jack mackerel was 17.4 g, which is equivalent to 2.3% of fish body weight per day. The total E. mucronata consumed in 1991 by the landed population of fish (3.7 million tons yr−1) amounted to 23.2 million tons yr−1. The total estimated population of jack mackerel that year (17.6 million tons) would have consumed ca. 110.2 million tons of E. mucronata. Based on stomach contents, consumption of E. mucronata by other nektonic predators off Chile and off Peru is also outstanding.Four different food web scenarios are offered for the northern and southern sectors of the HCS, according to regime shifts of small pelagics such as anchovy and jack mackerel, under the assumption that E. mucronata is a keystone link.