Emanuela Fanelli

Resource partitioning among early colonizing Siganus luridus and native herbivorous fish in the Mediterranean: an integrated study based on gut-content analysis and stable isotope signatures

The present study attempts to give timely information on the resource partitioning between the lessepsian migrant Siganus luridus and two ecological native analogues, Sarpa salpa and Sparisoma cretense. Sampling was carried out in concomitance with the first record of Siganus luridus in Linosa and fish were caught simultaneously, allowing direct comparison of diets. Gut-contents analyses provided a snapshot of the feeding habits of the three species: Siganus luridus fed on 34 taxa of benthic algae (mostly represented by Dictyota dichotoma) and a total of 27 taxa was identified in the stomach of Sarpa salpa, with the predominance of Sargassum vulgare. For the first time, a detailed picture of Sparisoma cretense diet was given (N=22 taxa of identified algae) albeit, due to the high percentage of digested food, this species was excluded from gut-content comparisons. A certain resource partitioning between Siganus luridus and Sarpa salpa was resolved on the basis of trophic indices and multivariate analyses, these latter also highlighting a more dispersed diet for Siganus luridus with respect to Sarpa salpa. Isotopic signatures were important towards defining the trophic level of the three species and particularly of Sparisoma cretense, whose gut-contents analysis was only partially informative. The values of δ 15 N confirmed a strictly vegetal diet for Siganus luridus and Sparisoma cretense while Sarpa salpa was significantly more enriched. According to δ 13 C, observed values matched the predicted ones for Siganus luridus and Sarpa salpa while both species presented less enriched δ 15 N values than expected.

Sewage pollution impact on Mediterranean rocky-reef fish assemblages

The effects of sewage outfalls on subtidal fish assemblages were studied along the NW coasts of Malta (Sicily channel, Mediterranean Sea) by means of underwater visual census. The presence of two spatially distinct outfalls discharging untreated wastewaters allowed to use a balanced symmetrical after control/impact (ACI) design that consisted of two putatively impacted locations and two controls, with four sites nested in each location. Surveys were performed in 2006 at two random dates. The study highlighted significant changes at both assemblage and individual species levels. Fish assemblages structures were different between controls and sewages, where total abundance of fish were higher. The responses of individual species to sewage pollution were mostly related to an anomalous increase of two small opportunistic species i.e. Gobius bucchichii and Parablennius rouxi and to a decrease of species of the genus Symphodus, particularly S. roissali and S. ocellatus. Moreover in correspondence of the outfalls, significant changes of the fish size distribution were detected for several species. These results support the use of fish assemblages as biological indicators for marine coastal waters and demonstrated the possibility to obtain sharp signals of environmental impact from some individual fish species.

Assemblage structure and trophic ecology of deep-sea demersal cephalopods in the Balearic basin (NW Mediterranean)

Despite the important ecological role of cephalopods in energy and material flow in marine ecosystems, they are poorly understood, particularly those inhabiting deep-sea bottoms below 800 m. To define cephalopod species assemblages from the upper to the lower slope, we conducted 13 oceanographic and trawl surveys in the Balearic basin (NW Mediterranean) in 1985–92 and 2007–10 at depths between 450 and 2200 m. Multivariate analysis suggests the existence of three assemblages on the upper (450–600 m), middle (650–1500 m) and lower (1600–2200 m) slopes. Although seasonal changes in species abundance and composition were evident, no significant variations in the assemblage structure were observed between the two time periods. Two main trophic pathways involving deep-sea cephalopods were apparent: species mainly linked to benthic resources and species mostly connected to the pelagic food web through active swimming and frequent vertical migrations. Accordingly, two environmental variables best explained the observed patterns: net primary production and river discharge. Phytoplankton availability controls the increase of zooplankton that influences the distribution and abundance of pelagic cephalopods. River discharge affects, in some instances with delays, transfer of organic matter to the benthic trophic pathway and thus to bottom-dwelling cephalopods.

Spatial variations in feeding habits and trophic levels of two small pelagic fish species in the central Mediterranean Sea.

Trophic ecology of adults of European sardine (Sardina pilchardus) and anchovy (Engraulis encrasicolus) was examined and compared among various regions of central Mediterranean Sea. Carbon and nitrogen stable isotope analyses (δ(13)C and δ(15)N) were adopted as a tool to determine changes in feeding behaviour of adults of sardines and anchovies. In the study period (summer) a clear geographical pattern was recognized in the isotopic composition of both species, with an increasing trend northward. The highest variations in isotopic signal were linked to the geographical positions of the samples and, especially, between pairs of areas: South Sicily/South Campania and Gulf of Gaeta/South Elba. Higher isotope values were found in the anchovies and sardines caught in northern Tyrrhenian Sea, while lower values were mostly estimated in the southern region. Higher carbon and nitrogen isotopes may reflect a more coastal behaviour of both species, being (13)C-enriched source from benthic primary producers in addition to phytoplankton. Variations in the nitrogen isotope ratio may reflect not only differences in the trophic level of prey species, but also variations in the baseline level of food webs. Our results support the hypothesis that feeding behaviour of both species is directly or indirectly influenced by local factors, or by resource partitioning based on zooplankton size. Findings can supply knowledge needed for improving fish stock management and promoting plans able to take into account also local ecosystem analysis.

Trophodynamic effects of trawling on the feeding ecology of pandora, Pagellus erythrinus, off the northern Sicily coast (Mediterranean Sea)

Because trawling disturbs benthic organisms, it could affect the diet of benthic-feeding fish with implications for food-web dynamics. The present study assessed the effects of commercial trawling on the trophodynamics and diet of pandora, Pagellus erythrinus, by comparing its stomach contents and stable-isotope (δ15N and δ13C) composition in two trawled and two untrawled gulfs in northern Sicily (central Mediterranean). Fish were collected on muddy bottoms at 50–100-m depth. Higher abundance and biomass and a slightly larger mean body length were found in the untrawled gulfs. The feeding habits were similar although more selective in the untrawled gulfs. The diet was mainly composed of decapod crustaceans (especially the brachyuran crab Goneplax rhomboides) and of polychaetes. The trophic level of pandora, estimated by its δ15N values, was higher in the untrawled gulfs. No clear trend between trawled and untrawled gulfs was found for the source of carbon in the diet (δ13C). The diet of a benthic feeder such as pandora may be used as an indirect indicator of trawling disturbance, as long as stomach contents and stable-isotope analysis are used jointly to assess the diet and trophodynamics of a species.

Trophic ecology of Lampanyctus crocodilus on north-west Mediterranean Sea slopes in relation to reproductive cycle and environmental variables

This study examined the population structure, reproductive cycle and feeding pattern of the lanternfish Lampanyctus crocodilus in the Balearic Basin (north-west Mediterranean Sea) from a depth of 450 to 1800 m and at a seasonal scale. Juveniles were mainly located at shallower depths, but also at deepest stations in autumn, while adults mostly inhabited intermediate depths with their centre of population density (CPD) located at 800-1000 m of depth. The migration of adults to deeper depths was detected in late summer to autumn, probably linked to the occurrence of nepheloid layers at c. 1200 m, which in turn enhances the biomass of the zooplankton prey. The diet was mainly based on euphausiids and mysids, with marked seasonal variations both on the upper (450-800 m) and lower (1000-1800 m), where suprabenthic gammariids and pelagic decapods were also dominant. Stomach fullness increased from winter to autumn on the US, while it had a maximum in spring on the LS, in parallel with high consumption of gelatinous zooplankton, which is probably more available after the phytoplankton bloom in late winter. Reproduction occurred in winter, confirmed by the higher percentage of mature females and high gonadosomatic indices (I(G)) at both depth ranges. Hepatosomatic indices (I(H)) showed an inverse trend to I(G) on the US, except in autumn, and was almost parallel on the LS, probably attributable to the migration of adults, which determined different temporal schemes in energy use and storage for reproduction on the US v. LS. Consistent with the different patterns observed at the two depth ranges, environmental drivers of fullness (i.e. feeding intensity) and I(G) (as a proxy of reproductive cycle) differed on the US and LS. The biomass of mysids and euphausiids was the greatest explanatory variables of fullness on the US and LS, pointing to the increasing feeding intensity when a resource was more available. I(H) also explained fullness, suggesting that greater feeding intensity in pre-reproductive periods enabled energy storage in the liver. I(G) was linked directly (i.e. mysids) or indirectly (i.e. surface primary production recorded 2 months before sampling) to food availability, implying a rapid response to vertical food inputs by deep-sea predators. Also, I(G) in L. crocodilus was related to population density, which suggests aggregations for reproduction. Estimates of L. crocodilus trophic levels, and of other accompanying mesopelagic fishes, indicated that the species feed through a continuum spanning the third trophic level, confirming the key role of mesopelagic fishes in transferring organic carbon between trophic levels. Trophic niche segregation among mesopelagic species was pronounced and non-overlapping groups could be distinguished because of the different vertical distribution and migratory behaviour. The study highlights the important role of the benthic boundary layer in sustaining benthopelagic communities in the deep Mediterranean Sea and the need to study the biology of a species throughout its whole depth range and not just at exploited depths (i.e. fishing grounds).

Food partitioning and diet temporal variation in two coexisting sparids, Pagellus erythrinus and Pagellus acarne

Resource partitioning in two congeneric sparids, pandora Pagellus erythrinus and axillary seabream Pagellus acarne, was investigated using stomach content analysis integrated with data on stable isotopes (δ(15) N and δ(13) C). The study was carried out on coastal muddy bottoms in the Gulf of Castellammare (southern Tyrrhenian Sea, western Mediterranean Sea) in seasons (autumn, November 2004; winter, March 2005; spring, early June 2005), at depths between 50 and 100 m. Stomach content analysis suggested low trophic niche overlap between the two species. Pagellus erythrinus mainly preyed on strictly benthic organisms (polychaetes, brachyuran crabs and benthic crustaceans). Although it consumed benthic prey, P. acarne preferred suprabenthic prey such as peracarid crustaceans from the benthic boundary layer a few metres above the bottom. The two species showed different isotopic values, with P. erythrinus exhibiting higher δ(15) N and more enriched δ(13) C than P. acarne, in accordance with its marked benthic behaviour and high predation on carnivore polychaetes. Significant temporal variability in both diet and isotopic values caused trophic differences between the two species. The autumn and winter diet differed from the spring diet and the trophic levels of both species increased from autumn and winter to spring, in accordance with variations in food availability and changes in prey δ(15) N and δ(13) C. These temporal variations may be linked to an increase in energy requirements for reproduction, together with the differing availability of preferred prey throughout the year. Significantly, lower δ(13) C was recorded in fishes collected in winter (March), suggesting the influence of river inputs as a source of particulate organic matter in this zone after the flooding season. In conclusion, these sympatric congeneric fish species displayed clear food partitioning throughout the temporal scale analysed.

Depicting the novel Eastern Mediterranean food web: a stable isotopes study following Lessepsian fish invasion

The introduction of Red Sea species through the Suez Canal has caused dramatic alteration of Eastern Mediterranean marine communities, however changes at the level of trophic interactions remain poorly understood. Capitalizing on this spectacular bioinvasion, we used stable isotope analysis to depict the novel fish food web occurring in the Eastern Mediterranean, off the Lebanese coast. Multivariate analyses were performed on six trophic groups contrasting the δ15N–δ13C ratios of native (MED) versus non-indigenous (Lessepsian-LES) species. Community-wide aspects of trophic structure were explored by means of recently developed quantitative metrics, such as the average distance to centroid and the standard ellipse area. Although the trophic positions of Lessepsian and native species were comparable, native fishes generally showed a greater trophic diversity than Lessepsians, the latter being mainly located in the central region of the available trophic space. We argue that the original trophic niche of native species has widened out due to the shift they have made towards less preferred food items, whilst highly competitive invaders are exploiting more energetic resources. In the absence of historical data, our observations agree with the idea of a rapid change of the Eastern Mediterranean food web, driven by niche displacements through trophic competition.

Trophic relationships between anchovy ( Engraulis encrasicolus ) and zooplankton in the Strait of Sicily (Central Mediterranean sea): a stable isotope approach

The study of the relationships among environmental factors, primary production, zooplankton community, and feeding behavior of fish species represents a key aspect to obtain a deeper knowledge of biological processes acting at ecosystem level. In this context, oceanographic data and biological samples were collected in two different surveys, carried out during late June 2011 and early July 2015 in the Strait of Sicily (Central Mediterranean Sea). Differences in abundance, biomass, and assemblage structure of zooplankton and anchovy (Engraulis encrasicolus Linnaeus, 1758) population were observed between the two surveys. Still, zooplankton biomass was significantly correlated to longitude, chlorophyll a recorded during the surveys (as a proxy of primary production), and oxygen concentrations. Such differences affected the isotopic composition of anchovy, which showed changes in δ15N and δ13C values between the two surveys. Mixing models identified different contributions of food sources in the two sampling periods, i.e., major consumption on large copepods and euphausiids in 2015 with respect to 2011, which was consistent with a greater availability of these preys in the environment in 2015. The obtained results evidenced that E. encrasicolus may be affected by any environmental change that influences the plankton community, which is the basis of the diet of these fishes. Such findings highlighted the importance to further investigate the trophic relationships among different compartments of the food web, as well as the possible environmental influences, in order to obtain a more complete picture of ecosystem functioning and also in the light of an ecosystem-based approach to fisheries management.

Inertial bioluminescence rhythms at the Capo Passero (KM3NeT-Italia) site, Central Mediterranean Sea

In the deep sea, the sense of time is dependent on geophysical fluctuations, such as internal tides and atmospheric-related inertial currents, rather than day-night rhythms. Deep-sea neutrino telescopes instrumented with light detecting Photo-Multiplier Tubes (PMT) can be used to describe the synchronization of bioluminescent activity of abyssopelagic organisms with hydrodynamic cycles. PMT readings at 8 different depths (from 3069 to 3349 m) of the NEMO Phase 2 prototype, deployed offshore Capo Passero (Sicily) at the KM3NeT-Italia site, were used to characterize rhythmic bioluminescence patterns in June 2013, in response to water mass movements. We found a significant (p < 0.05) 20.5 h periodicity in the bioluminescence signal, corresponding to inertial fluctuations. Waveform and Fourier analyses of PMT data and tower orientation were carried out to identify phases (i.e. the timing of peaks) by subdividing time series on the length of detected inertial periodicity. A phase overlap between rhythms and cycles suggests a mechanical stimulation of bioluminescence, as organisms carried by currents collide with the telescope infrastructure, resulting in the emission of light. A bathymetric shift in PMT phases indicated that organisms travelled in discontinuous deep-sea undular vortices consisting of chains of inertially pulsating mesoscale cyclones/anticyclones, which to date remain poorly known.