Diet of sardine (Sardinops sagax) in the northern Humboldt Current system and comparison with the diets of clupeoids in this and other eastern boundary upwelling systems

Abstract

Sardines are one of the main small pelagic fish resources in eastern boundary upwelling systems (EBUS) where they play an important ecological role both as a predator of plankton and as prey of top predators. Sardine trophodynamics have been relatively well studied in three of the EBUS (the Benguela, California and Canary upwelling systems) but not in the Humboldt Current system. In this paper we describe the diet of sardine Sardinops sagax in the northern Humboldt Current system (NHCS) off Peru, using an analytical method which assesses relative dietary importance in terms of estimated prey carbon content. We assessed sardine diet by examining a total of 555 stomachs collected during six surveys conducted off Peru during the period 1996–1998, and compare our results with the diet of anchoveta Engraulis ringens off Peru and with the diets of sardines from the southern Benguela (also S. sagax) and the northern Canary (Sardina pilchardus) upwelling systems. The diet of sardine off Peru is based primarily on zooplankton, similar to that observed for anchoveta but with several important differences. Firstly, sardine feed on smaller zooplankton than do anchoveta, with sardine diet consisting of smaller copepods and fewer euphausiids than anchoveta diet. Secondly, whilst phytoplankton represents <2% of sardine dietary carbon, this fraction is dominated by dinoflagellates, whereas diatoms are the dominant phytoplankton consumed by anchoveta. Hence, trophic competition between sardine and anchovy in the northern Humboldt Current system is minimized by their partitioning of the zooplankton food resource based on prey size, as has been reported in other systems. Whereas sardine in the NHCS feed on smaller zooplankton than do anchovy in that system, sardine in the NHCS forage on larger prey and obtain a substantial portion of their dietary carbon from euphausiids compared to sardine from the northern Canary and southern Benguela Current systems.

Introduction

Sardines (the genera Sardinops and Sardina) and anchovy (the genus Engraulis) are the most important commercially-exploited small pelagic fish species worldwide, and are particularly abundant in the eastern boundary current systems (EBUS; including the Benguela, California, Canary and Humboldt Current systems). These fish feed on plankton and are the dominant forage of a variety of fish, marine mammal and seabird predators, and hence are also ecologically important.

A recent synthesis on the trophic dynamics of small pelagic fish from a variety of upwelling and other systems (van der Lingen et al., 2009) has shown that sardine and anchovy derive the bulk of their dietary carbon from zooplankton, a finding that contradicts many previous descriptions of exclusive or near-total phytophagy by these species. For example, Peruvian anchoveta (Engraulis ringens) was considered to feed directly on primary producers, which led to the belief that the large populations of anchoveta were supported by an unusually short and efficient food chain (Ryther, 1969, Walsh, 1981). This assumption was recently refuted by Espinoza and Bertrand (2008), who estimated the carbon content of prey items, as opposed to using the numerical method of stomach content analysis which is considered to be subjective (James, 1987), and showed that anchoveta forage mainly on zooplankton. Such findings improve our knowledge of the functioning of the northern Humboldt Current system (NHCS) off Peru, and illustrate the need for accurate and appropriate methods. Dietary information derived using carbon content is available for sardine and anchovy from the main EBUS (see van der Lingen et al., 2009) but not for sardine (Sardinops sagax) in the Humboldt Current system. To date, sardine in the Humboldt Current system was generally considered as feeding mainly on phytoplankton (Rojas de Mendiola, 1966, Sanchez de Benites et al., 1985, Alamo and Bouchon, 1987). This differs from recent studies on this and similar species’ diet in other ecosystems, that demonstrated that sardine derive the majority of their dietary carbon from zooplankton (van der Lingen, 2002, Garrido et al., 2008, van der Lingen et al., 2009). A single study on sardine diet in the NHCS (Konchina, 1991) mentioned that sardine consumed mainly small copepods and tunicates, but this study has seldom been considered.

In this paper we describe the diet of sardine in the NHCS based on a method which assesses relative dietary importance according to estimated prey carbon content (James, 1987, van der Lingen, 2002). We compare our results on sardine diet and prey-size spectrum with those of the anchoveta in the NHCS. We also compare the diet of sardine from the NHCS with data on the diet of sardines in other systems, including Sardinops sagax from the southern Benguela (SBCS) off South Africa, Sardina pilchardus from the northern Canary system off Portugal (NCCS), and with published work describing clupeid diets in the California Current system (CalCS). We discuss how new insights into the functioning of these systems may be extracted from such a comparative approach. In particular, we re-examine the hypothesis (Ryther, 1969) that upwelling regions are so efficient in terms of fish production because of the direct link between phytoplankton and filter-feeding fishes.