Vol. 620: 139–154, 2019 MARINE ECOLOGY PROGRESS SERIES Published June 18 https://doi.org/10.3354/meps12962 Mar Ecol Prog Ser Trophic ecology of range-expanding round sardinella and resident sympatric species in the NW Mediterranean Marta Albo-Puigserver 1, *, Diego Borme 2, Marta Coll 1, Valentina Tirelli 2, Isabel Palomera 1, Joan Navarro 1 1Institut de Ciències del Mar - CSIC, Barcelona 08003, Spain 2Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS), Trieste 34151, Italy ABSTRACT: The recent northward expansion of the round sardinella Sardinella aurita in the Mediterranean Sea has been documented as a consequence of rising sea temperature. At the same time, declines in sardine and anchovy biomass have been observed in the NW Mediterra nean Sea, necessitating an assessment of whether the expansion of round sardinella may affect sardine and anchovy populations. Here, we combined stomach content and isotopic analyses to describe the trophic habits of round sardinella in the NW Mediterranean Sea and its trophic relationships with 2 sympatric small pelagic fish, European anchovy Engraulis encrasicolus and European pilchard Sardina pilchardus . Results revealed changes in the diet of round sardinella during the year. In summer, the most important prey were copepods ( Acartia spp.) and cladocerans ( Penilia avirostris ). During winter, the diet was composed mainly of copepods and tunicates (mainly appen- dicularians), but microplankton was also numerically important in adult diets. In contrast to previ- ous studies, during spring, round sardinella principally fed on salps (Thaliacea). To our knowledge, this is the first time that salps have been identified as an important prey for round sardinella. When compared to coexisting small pelagic fish, we found that round sardinella adults had a different trophic niche than anchovy and sardine. In contrast, round sardinella juveniles partially overlapped the trophic niche with the juveniles of the other 2 species. Therefore, the range expansion of round sardinella probably would not affect sardine and anchovy populations. Only in a situation of food limitation could juveniles of round sardinella compete with and affect both sympatric species. Our results provide new insights into the ecological role of this range-expanding species in the NW Medi terranean Sea, and highlight the importance of gelatinous zooplankton as prey. KEY WORDS: Gelatinous zooplankton · Sardine · Anchovy · Small pelagic fish · Trophic segregation · Stable isotopes · Stomach contents · Trophic pathways Resale or republication not permitted without written consent of the publisher 1. INTRODUCTION they do not currently exist) have been well docu- mented, the potential ecological and socioeconomic Climate change has biological effects on physio- consequences of these expansions are not well logical, phenotypical and distributional patterns of understood, due to the amount of ecological informa- marine species that can affect biological interactions tion required at different levels of organization (spe- (Hughes & Grand 2000). The expansion of certain cies, community and ecosystem; Madin et al. 2012). species outside of their native range may have impor- The basic knowledge of how these species use tant effects on ecosystems (Sorte et al. 2010, Last et trophic resources is crucial information to evaluate al. 2011). Although shifts or expansions in the geo- their impact on the new community (Sunday et al. graphic distribution of non-native or range-expand- 2015). Overall, 2 main trophic pathways have been ing species (i.e. species that enter a habitat in which suggested to explain resource acquisition by non- *Corresponding author: [email protected] © Inter-Research 2019 · www.int-res.com 140 Mar Ecol Prog Ser 620: 139–154, 2019 native or range-expanding species: (1) they may ex - related Clupeiformes, such as the European pilchard ploit novel niche opportunities that most native spe- Sardina pilchardus (hereafter referred to as sardine) cies are unable to use (opportunism hypothesis), or and the European anchovy Engraulis encrasicolus (2) they may exploit the resources used by native (hereafter referred to as anchovy), 2 well-established species, displacing the native species from their pre- small pelagic fish species in the northern Mediter- ferred niches (competition hypothesis) (Tilman 2004, ranean Sea. The 3 species have been described as San Sebastián et al. 2015). planktivorous fish that mainly prey on copepods, Traditionally, the round sardinella Sardinella aurita cladocerans and diatoms (Table 1). Previous trophic has been widely distributed in the southern part of studies conducted in the eastern Mediterranean Sea the Mediterranean Sea and subtropical waters of the revealed that round sardinella, sardine and anchovy Atlantic Ocean, due to its preference for spawning in exploit similar food resources throughout the year warm waters (Ben-Tuvia 1960, Sabatés et al. 2006, but with differences between seasons (e.g. for round 2009). The northward expansion of round sardinella sardinella, copepods were described as the main in the Mediterranean Sea has been documented over prey in fall, while for sardine, copepods were the the last decade as a consequence of the increase in main prey in winter; Karachle & Stergiou 2014). On sea surface temperatures associated with global the contrary, an isotopic niche analysis of these 3 clu- warming (Sabatés et al. 2006, Tsikliras 2008). Simi- peids in the NW Mediterranean Sea found different larly, in other areas of the Atlantic Ocean such as the trophic niches for adult stages (Albo-Puigserver et al. coast of Mauritania, Morocco and the Canary Archi- 2016). Although the trophic habits of sardine and pelago, higher abundances of this species have been anchovy in the western Mediterranean Sea have recorded in recent years due to high surface temper- been well studied (e.g. Costalago et al. 2012, Le atures (Zeeberg et al. 2008, Alheit et al. 2014). Bourg et al. 2015, Brosset et al. 2016, and see refer- As a consequence of the high feeding intensity and ences in Table 1), to our knowledge, only 1 study the wide plasticity and adaptability to environmental conducted in this area found high dietary overlap fluctuations and food availability of round sardinella between round sardinella and anchovy at larval (Cury & Fontana 1988, Tsikliras et al. 2005, Morote et stages (Morote et al. 2008). No information on the al. 2008, 2010), this species may have an impact via diet composition of juveniles and adults of round sar- trophic competition or direct predation on closely- dinella in the western Mediterranean is available. Table 1. Summary of the available published data on diet, including results of the present study (grey shading), based on stom- ach content analysis and the main prey groups reported for round sardinella Sardinella aurita , anchovy Engraulis encrasicolus and sardine Sardina pilchardus in the NW Mediterranean Sea. Seasons with available information are indicated with an ‘x’ (1: winter; 2: spring; 3: summer; 4: fall) Life Main prey groups Season References stage 1 2 3 4 Round sardinella Larva Copepod nauplii and postnauplii, cladocerans x Morote et al. (2008), present study Juvenile Copepods, cladocerans, appendicularians, salps x x x Adult Copepods, cladocerans, appendicularians, salps, x x x diatoms, tintinnids Anchovy Larva Copepod eggs, nauplii and postnauplii, x x Costalago et al. (2014), Le Bourg et al. cladocerans (2015), Morote et al. (2010), Plounevez & Juvenile Copepods, cladocerans, appendicularians x x x Champalbert (2000), Intxausti et al. (2017), Adult Copepods, cladocerans, decapods, x x Tudela & Palomera (1997), Brosset et al. appendicularians (2016) Sardine Larva Tintinnids, copepod nauplii and postnauplii x Costalago et al. (2014), Costalago & Juvenile Copepods, cladocerans, appendicularians, x x x x Palomera (2014), Le Bourg et al. (2015), mysids, diatoms Morote et al. (2010), Brosset et al. (2016) Adult Copepods, cladocerans, appendicularians, x x diatoms Albo-Puigserver et al.: Trophic ecology of round sardinella 141 This impairs our current capability to assess the eco- 2. MATERIALS AND METHODS logical consequences that the northward expansion of this species has on other pelagic species. 2.1. Study area and sampling procedure In addition to the northward expansion of round sardinella, in recent decades important declines in The study area was located on the continental biomass, landings and body condition of sardine shelf of the Ebro River Delta (NW Mediterranean and anchovy have been observed in the NW Sea; Fig. 1). This area is an important fishing Mediterra nean Sea (Van Beveren et al. 2014, Bros- ground and spawning area for small pelagic fishes set et al. 2017). The stocks of both species are and has been identified as a priority area of con- overfished in this area (Coll & Bellido 2019). In servation (Palomera et al. 2007, Coll et al. 2012, contrast, round sardinella landings have increased, 2015, Piante & Ody 2015). In the NW Mediterran- with large fluctuations due to its low commercial ean Sea, sea surface temperatures and primary value (Palo mera et al. 2007, Coll et al. 2019). At production follow annual cycles characterized by present, the potential factors controlling the popu- strong seasonality. Although the coastal zones of lation of small pelagic fish in the NW Mediterran- the area are considered oligo trophic, inputs from ean are still unc lear and there is an urgent need to the Ebro outflow and episodes of strong