ENSO and the anchovy


If one wishes to gain a “heads-up” as to imminent developments in ENSO and possible beginnings of an el Nino or La Nina event, I would advise turning to the Peruvian anchovy as an important but often overlooked oracle to the oceanography of the anchovy’s home ocean, the Pacific.

The Peruvian anchovy or “anchoveta” is an important fish to the global economy and to the diet of most of us. It is the world’s single largest fishery by landed tonnage, and is a principal component of fishmeal which is a major agricultural feed for farmed fish and animals. One can even order them direct as a pizza ingredient (the “Napoli” or “Siciliana”pizza for instance).


The anchovy is a filter feeder like a mini-whale, swimming with its mouth agape. This is the key to its huge success in exploiting the massive plankton productivity of the equatorial eastern Pacific off the coast of Peru. It is the archetypal pelagic (free-swimming) filter-feeder.



Figure 1. The anchovy (E. ringens) is a pelagic filter-feeder – there are lots of them off the coast of Peru.


Anchovy – the ENSO fish

The Latin classification of the Peruvian anchoveta is Engraulis ringens (Jenyns, 1842). Like almost no other species on earth (certainly not your average Homo) the anchovy has an astonishing and profound instinctive knowledge of the ebb and flow of the El Niño Southern Oscillation – the ENSO. One could indeed reasonably call the anchovy the “ENSO fish”. The migrations, dispersals and gatherings, and year to year biomass peaks and crashes of the anchovy fishery in the eastern Pacific off Peru are tuned with exquisite sensitivity to ENSO itself. In particular it is the Peruvian upwelling, one side of the Bjerknes feedback (the other being the trade winds) which both couple intermittently to provide the bursts of positive feedback that drive El Niño and La Nina episodes. These two systems are characterised by weakening and strengthening respectively of this upwelling.

The first to know of any developments in the dark deep ocean currents way off the Peruvian coast, signifying portentous shifts in the upwelling stemming from the Humboldt current from Antarctica, is Engraulis the anchovy. Long before any clanking fish-imitating Argo floats, before any TAU or TRITON moored bathyscaphes, or satellite imagery, still longer before any armchair climate punditry, the anchovies respond in real time to upwelling changes with variations in the first-feeding survival and size of their juvenile year classes and their spatial distributions. Thus it was inevitably the Peruvian fishermen, heirs of the ocean abundance provided by E. ringens, who were discoverers of what they called El Niño (“the boy” in Spanish), the periodic anomalous warming of the eastern Pacific surface waters. This event is accompanied by a crash in the anchoveta numbers and catches, and typically occurs in December-January, the time of the celebration by the Christian Church of Christmas, the incarnation of the Christ-child.

El Niño is bad for the Peruvian fishermen since it is bad for the anchovy. Why then, one must speculate, did the Peruvians name this cursed event after the divine infant of their religion? Was there a streak of resentment or protest against their Catholic faith and its priests and offices? Or, perhaps, were so many fisherman heard to shout in frustration “Oh Cristo! Su cálida de nuevo!” (Oh Christ! – it’s warm again!) – like skeptical climate bloggers after every uptick in the global temperature anomaly – that the event became named after the blasphemously invoked member of the Holy Trinity.

The baleful influence of El Niño on the anchoveta is related to the transport of nutrients from deep to surface waters which accompanies upwelling, and which fuels the phytoplankton bloom which in turn provides the primary production which nourishes the vast shoals of anchovy which teem off Peru’s coastline and in fishermen’s nets. This is basic first-year marine biology. Primary production, the photosynthetic algal base of the marine food chain, is nutrient limited and thus one talks of blooms, and also of “bloom-and-bust”, as for example with spring sunshine, in temperate waters, the phytoplankton first grow rapidly but then run out of nutrients and die back abruptly (Paul Ehrlich should have been a marine biologist).

This is why coastal regions are the most biologically productive seas where ocean floor topography causes the upwelling essential to bring nutrient rich cooler bottom water up to fertilise the depleted upper layers. The vast expanses of the oceans by contrast have nutrient limited surface water – therefore the strikingly visible transition from green to blue colour of the sea as you move from fertile coastal water to the barren ocean deeps. The world’s most productive seas are in places such as the south west coast of Africa and, biggest of all, the Peruvian west coast of south America, where cold deep water originating in Antarctica and the Humboldt current wells up to sustain the world’s largest fishery, that of Engraulis ringens the anchovy.




radiolarians blue

Figure 2. Microscope images of phytoplankton single-celled algae, foundation of the marine food pyramid. They are beautifully sculpted microscopic creatures with exotic names like diatoms (upper) and foraminiferans (middle) and radiolarians (lower) as well as dinoflagellates and coccolithophores.


In the presence of this upwelling, the east equatorial Pacific is cooler than surface waters further west, setting up a temperature and pressure gradient that drives the prevailing pattern of trade winds, the east to west (“easterly”) winds that for millennia have carried intrepid human seafarers from the Americas to populate the Pacific islands. As well as being impelled by this sea surface temperature difference, the trade winds further amplify the eastern Pacific upwelling by dragging the surface water westward, and this reinforcing positive feedback – the Bjerknes feedback – lies at the heart of the El Niño southern oscillation (ENSO).

However, this positive feedback can cut both ways. From time to time, initiated by no-one knows quite what (although hypotheses abound and swarm like the anchovies themselves) Kelvin waves of warm water surge eastward, interrupting the Peruvian upwelling. This warms the east Pacific surface water, reducing the temperature gradient on which the trade winds depend and thus choking them off, resulting in the dreaded “doldrums” – no wind and reduced upwelling off Peru. This slackening part of the Bjerknes feedback is the El Niño event. And in turn, the reduced upwelling is bad news for the anchovy as he has to stay deeper to access life-giving nutrients, and the Peruvian fishermen once more cry “O Cristo – El Niño!”.


El Niño now? What does the anchovy have to say?

So what significance does all this have to the current conditions in the Pacific? Endlessly the ENSO-watchers wait expectantly for El Niño to arrive (I call this “waiting for el Ninot”, theatrically adapted from the play “Waiting for Godot” by Samuel Beckett).

But El Niño, this year in 2020 as so often before, has stubbornly resisted all entreaties to manifest itself like in the good old days of 1998 and even 2010. Now, instead, the Nina 3.4 is falling toward La Niña territory.

Rather than trust conflicting forecasts, it’s better to consult the anchovy, the ENSO fish, about whether El Niño or La Niña are approaching. The latest on the Peruvian anchovy fishery can be found in the following article from the website “Undercurrent News” which gives up to the minute news on fisheries and fish markets around the world. Is there strong recruitment of juvenile anchovies? This points to strong nutrient-rich upwelling off Peru and thus the likelihood of either neutral or La Nina conditions with a cool eastern equatorial ocean surface. While if this recruitment is weak, this would indicate interrupted upwelling and the posibility of El Niño conditions.



Figure 3. Seine-net industral fishing for anchovy off Peru.


Therefore, continued strength of the anchovy fishery indicates that upwelling is still strong and yet to be interrupted by Kelvin waves and a nascent el Niño. It is possible that the inception of a proper, full-on el Niño requires not just meteorological conditions or other surface factors, but a simultaneous development in the ocean currents and mixing, linked to the thermohaline ocean circulation, to entrain the Bjerknes feedback to trigger a pause in the upwelling.

If we are interested in the ENSO status of the Pacific and what might lie ahead, where better to turn than the wise and all-knowing Engraulis ringens? It’s well worth checking out what’s happening with the Peruvian anchovy fishery before making pronouncements on the current or future status of ENSO. So remember nature’s bell-weather of ENSO, the ENSO-fish, next time you bite down and the salty anchovy “Napoli” or “Siciliana” pizza along with freshly baked pizza with cheese and tomato.


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