Trophic niche overlap between round sardinella (Sardinella aurita) and sympatric pelagic fish species in the Western Mediterranean

Abstract

The northward expansion of round sardinella (Sardinella aurita) in the Mediterranean Sea, together with declines and fluctuations in biomass and landings of European sardine (Sardina pilchardus) and anchovy (Engraulis encrasicolus) observed in recent decades, may suggest potential inter-specific competition in the pelagic domain. The coexistence of sympatric zooplanktivorous fish species might therefore be exposed in part to trophic niche overlap and competition for food. Combining visual diet characterization under the microscope with DNA metabarcoding from stomach contents of fish collected in spring results show that predation on relatively large krill is equally important for sardinella than for the other two niche overlapping species. Furthermore, an important overlap is found in their isotopic niche, especially with anchovy, using nitrogen (δ¹⁵N) and carbon (δ¹³C) stable isotopes in muscle tissue. In fact, the three fish species are able to feed effectively in the whole prey size spectrum available during the sampled season, from the smallest diatoms and copepods to the larger prey (i.e., decapods and euphausiids), including fish larvae. Moreover, effective predation upon other large prey like siphonophores, which is observed only when multi-proxy analyses in stomach contents are applied, might also be relevant in the diet of sardinella. The overlapping diet composition in spring, together with the effective use of food resource by sardinella, can be of special interest in potential future scenarios with warmer water temperature leading to lower zooplankton and/or higher jellyfish availability, where sardinella may take advantage over other species due to its feeding plasticity.

Keywords: diet dissimilarity; multi‐proxy diet analysis; prey preference; small pelagic fish; trophic interactions; trophic niche width.

FIGURE 1

Sampling area in the Western Mediterranean Sea where sardinella (Sardinella aurita), anchovy (Engraulis encrasicolus), and sardine (Sardina pilchardus) were collected. All samples in the marked stations were used for stomach content characterization (under the microscope and with DNA metabarcoding) and for stable isotope analysis (see Table 1 for details). Figure generated with QGIS v. 3.2.1‐Bonn117 (https://qgis.org/en/site/)

FIGURE 2

Standard ellipses area (SEA_C) for sardinella (Sardinella aurita), anchovy (Engraulis encrasicolus), and sardine (Sardina pilchardus). Within the inlet, the isotopic niche width (SEA_B) is shown

FIGURE 3

Prey group composition from stomach content analysis, as percentage of (a) mean prey abundance and (b) of mean prey biomass (mg) per stomach, for sardinella (Sardinella aurita), anchovy (Engraulis encrasicolus), and sardine (Sardina pilchardus), based on the corrected diet characterization

FIGURE 4

Occurrence percentage of (a) phyla within “Others” prey group and (b) diatoms, determined with DNA metabarcoding and presented for sardinella (Sardinella aurita), anchovy (Engraulis encrasicolus), and sardine (Sardina pilchardus)

FIGURE 5

Relative frequency distribution of predator–prey size ratios (PPSR, left side) and predator–prey biomass ratios (PPBR, right side) in the three fish species: sardinella (Sardinella aurita), anchovy (Engraulis encrasicolus), and sardine (Sardina pilchardus), based on the corrected diet characterization. Cumulative PPSR and PPBR frequencies are indicated by continuous lines (note the secondary Y axis)

FIGURE 6

The partition of beta‐diversity transformed as (a) Sørensen dissimilarity index (β _SOR), and the partition in the (b) turnover (β _SIM) and (c) nestedness (β _NES) components, for sardinella (Sardinella aurita, red), anchovy (Engraulis encrasicolus, light green), and sardine (Sardina pilchardus, blue)

FIGURE 7

Mean abundance of parasites in stomach contents (left side of the plot, grey scale) and mean anthropogenic fiber ingestion (right side of the plot, blue–purple scale), classified by type and weighed by the total number of sampled fish