Clarifying a trophic black box: stable isotope analysis reveals unexpected dietary variation in the Peruvian anchovy Engraulis ringens

Pizarro J.; Docmac F.; Harrod C.

Abstract

Background. Small fishes play fundamental roles in pelagic ecosystems, channelling energy and nutrients from primary producers to higher trophic levels. They support globally important fisheries in eastern boundary current ecosystems like the Humboldt Current System (HCS) of the SE Pacific (Chile and Peru), where fish catches are the highest in the world (per unit area). This production is associated with coastal upwelling where fisheries target small pelagic fishes including the Peruvian anchovy (Engraulis ringens). The elevated biomass attained by small pelagics is thought to reflect their low trophic position in short/simple food chains. Despite their global importance, large gaps exist in our understanding of the basic ecology of these resources. For instance, there is an ongoing debate regarding the relative importance of phytoplankton versus animal prey in anchovy diet, and ecosystem models typically assign them a trophic position (TP) of similar to 2, assuming they largely consume phytoplankton. Recent work based on both relative energetic content and stable isotope analysis (SIA) suggests that this value is too low, with delta N-15 values indicating that anchovy TP is ca. 3.5 in the Peruvian HCS. Methods. We characterised the trophic ecology of adult anchovies (n = 30), their putative prey and carnivorous jack mackerel (n D 20) captured from N Chile. SIA (delta C-13 and delta N-15) was used to estimate the relative contribution of different putative prey resources. delta(15N) was used to estimate population level trophic position. Results. Anchovies showed little variability in delta C-13 (18.7 to 16.1 h) but varied greatly in delta N-15 (13.8 to 22.8 h)-individuals formed two groups with low or high delta N-15 values. When considered as a single group, mixing models indicated that anchovy diet was largely composed of zooplankton (median contribution: 95% credibility limits), with major contributions of crustacean larvae (0.61: 0.37-0.77) and anchovy (preflexion) larvae (0.15: 0.02-0.34), and the assimilation of phytoplankton was negligible (0.05: 0-0.22). The modal (95% credibility limits) estimate of TP for the pooled anchovy sample was 3.23 (2.93-3.58), overlapping with recent SIA-based estimates from Peru. When the two delta N-15 groups were analysed separately, our results indicate that the lower delta N-15 group largely assimilated materials from crustacean larvae (0.73: 0.42-0.88), with a TP of 2.91 (2.62-3.23). Mixing models suggested high delta N-15 anchovies were cannibalistic, consuming anchovy preflexion larvae (0.55: 0.11-0.74). A carnivorous trophic niche was supported by high TP (3.79: 3.48-4.16), mirroring that of carnivorous juvenile jack mackerel (Trachurus murphyi; 3.80: 3.51-4.14). Our results support recent conclusions regarding high TP values of anchovy from Peru and reveal new insights into their trophic behaviour. These results also highlight the existence of cryptic trophic complexity and ecosystem function in pelagic food webs, classically considered as simple.

Más información

Título según WOS: Clarifying a trophic black box: stable isotope analysis reveals unexpected dietary variation in the Peruvian anchovy Engraulis ringens
Título según SCOPUS: Clarifying a trophic black box: Stable isotope analysis reveals unexpected dietary variation in the Peruvian anchovy Engraulis ringens
Título de la Revista: PEERJ
Volumen: 7
Editorial: PEERJ INC
Fecha de publicación: 2019
Idioma: English
DOI:

10.7717/peerj.6968

Notas: ISI, SCOPUS