Revisiting Peruvian anchovy (Engraulis ringens) trophodynamics provides a new vision of the Humboldt Current system

Abstract

The Peruvian anchovy or anchoveta (Engraulis ringens) forages on plankton and is a main prey for marine mammals, seabirds, fish, and fishers, and is therefore a key element of the food web in the Humboldt Current system (HCS). Here, we present results from the analysis of 21,203 anchoveta stomach contents sampled during 23 acoustic surveys over the period 1996–2003. Prey items were identified to the genus level, and the relative dietary importance of different prey was assessed by determination of their carbon content. Variability in stomach fullness was examined relative to the diel cycle, the distance from the coast, sea surface temperature, and latitude, using generalized additive models (GAMs). Whereas phytoplankton largely dominated anchoveta diets in terms of numerical abundance and comprised >99% of ingested prey items, the carbon content of prey items indicated that zooplankton was by far the most important dietary component, with euphausiids contributing 67.5% of dietary carbon followed by copepods (26.3%). Stomach fullness data showed that anchoveta feed mainly during daytime between 07h00 and 18h00, although night-time feeding also made a substantial contribution to total food consumption. Stomach fullness also varied with latitude, distance from the coast, and temperature, but with substantial variability indicating a high degree of plasticity in anchoveta feeding behaviour. The results suggest an ecological role for anchoveta that challenges current understanding of its position in the foodweb, the functioning of the HCS, and trophic models of the HCS.

Introduction

The Peruvian anchovy or anchoveta Engraulis ringens, is ecologically and economically the most important pelagic fish species in the Humboldt Current system (HCS). Anchoveta is the major prey of the principal top predators including marine mammals, seabirds, fish and fishers, and more than 250 million tons of anchoveta have been harvested by the purse seine fishery since the 1950s. Anchoveta forage on plankton and is a key element of the marine food web in the HCS and have been the subject of many studies (e.g. the books edited by Pauly and Tsukayama, 1987, Pauly et al., 1989a).

The first trophodynamic studies on anchoveta in Peru concluded that anchoveta subsisted mainly on phytoplankton (Rojas, 1953, Rojas de Mendiola, 1969), and the ability of clupeoids like anchoveta to feed at low trophic levels (directly on primary producers) was suggested as the reason such large populations, biomasses and fisheries could be sustained in upwelling systems (Ryther, 1969). Later studies suggested that in addition to filter-feeding on phytoplankton, anchoveta could also particulate feed on zooplankton (Rojas de Mendiola, 1989, Alamo, 1989), and zooplankton was sometimes considered equally important as phytoplankton in anchoveta diets (Alamo, 1989, Pauly et al., 1989b; Jahncke et al., 2004). With the exception of Konchina (1991), who suggested that anchoveta preferentially consume zooplankton, all other recent work in the HCS has concluded that anchoveta depends mainly on phytoplankton (Alamo et al., 1996a, Alamo et al., 1996b, Alamo et al., 1997a, Alamo et al., 1997b, Alamo and Espinoza, 1998, Espinoza et al., 1998a, Espinoza et al., 1998b, Espinoza et al., 1999, Espinoza et al., 2000). However, these studies were based on counts of anchoveta prey, a method considered to be inadequate for estimating dietary importance (James, 1987, Konchina and Pavlov, 1995). In contrast, methods based on prey weight (e.g. gravimetric) or on nutritional value (e.g. carbon content, caloric or energetic value) may be more ecologically relevant (Hyslop, 1980, Koslow, 1981, James, 1987, Konchina and Pavlov, 1995, van der Lingen et al., 2006, van der Lingen et al., in press). In other upwelling systems, these latter methods indicate that zooplankton, rather than phytoplankton, support clupeoid populations (e.g. Koslow, 1981, James, 1987, James and Chiappa-Carrara, 1990, Chiappa-Carrara and Gallardo-Cabello, 1993, van der Lingen et al., 2006).

Konchina (1991) results highlighting the significance of zooplankton in the diet of anchoveta were based on gravimetric analysis of prey importance, but his study was based on a very small sample size (n = 65 fish). Here we revisit Peruvian anchoveta diet and feeding behaviour in Peru using a database which contains information on the stomach contents of 21,203 anchoveta sampled along the Peruvian coast (1996–2003). In particular we assess the relative importance of different prey types to anchoveta using a method which estimates the carbon content of prey items. We also describe variations in anchoveta stomach fullness in relation to the diel cycle, latitude, distance to the coast, and sea surface temperature, using generalized additive models. Our results confirm Konchina (1991) finding that Peruvian anchoveta subsist primarily on zooplankton, and suggest an ecological role for anchoveta that challenges current understanding of the functioning of the HCS.