Humboldt (Northern) - Ecology

The Northern Humboldt is the region where El Niño, and climate variability in general, is most notable (Chavez et al., 2008). The main impacts of El Niño events in the Peruvian coastal ecosystem are the inshore intrusion of warm and poor oceanic waters, deepening of the thermocline and as a consequence, the upwelling of nutrient-deficient waters (e.g. Clark, 1976; Valdivia, 1978; Arntz and Fahrbach, 1996; Bertrand et al., 2008). Depending on the particular characteristics of each El Niño event, the incoming oceanic waters may be subtropical surface or equatorial surface waters, and the geographical pattern of their intrusion is not regular. Species associated with cold and rich waters (e.g. anchovy) may experience dramatic mortality depending on the event duration, tend to become distributed deeper and/or closer to the coast (Valdivia, 1978; Bertrand et al., 2004) and may move southward following cold cores (Zuzunaga, 1985). However, some very coastal upwelling cells may remain active (for instance during El Niño 1997-98 but not during El Niño 1982-83) and provide refuge areas where high anchovy biomass can concentrate (Bertrand et al., 2004). The geographical distribution of species related to warmer waters (e.g. pelagic longnose anchovy Anchoa nasus or dolphinfish Coryphaena hippurus) extends southward while their availability increase.
In a broader context, long-term physical and biological fluctuations have also been identified. Until recently the environment, productivity, and fish abundance and dominance were supposed to have varied from cool “anchovy regimes” to warm “sardine regimes”, each one of about 25 years (Chavez et al., 2003). Paleoclimatic studies put into question part of this paradigm (Gutiérrez et al., 2009; Salvatteci, 2013). Indeed even if a warm regime occurred during the 1970s-1980s there is no systematic alternance between warm-cold regimes at a decadal scale. Anchovy is the dominant fish in the system and sardine occurs only at specific environmental periods. It seems that the OMZ plays an important role in both fishes dynamics (Bertrand et al., 2011; Salvatteci, 2013). During the recent decades, cool conditions are suggested to have occurred until 1975 and from the late 1990s to the present, characterized by negative temperature anomalies, increased plankton production and expanded distribution of anchovy and spawning by anchovy. A warm regime occurred from 1975 to mid 1990s and was associated with a deeper mixed layer, a flux of warm subtropical oceanic water towards the coast and a reduction in the range over which the cold coastal waters (CCW) extended and which anchovy inhabits (Swartzman et al., 2008). This led to reduction of large zooplankton (e.g. copepods or euphausiids), the main prey of anchovy (Espinoza and Bertrand, 2008), while smaller prey more favourable to sardine dominated (Espinoza et al., 2009; Ayón et al., 2011; Bertrand et al., 2011).