L’ecosistema del Mar Tirreno: aspetti strutturali, funzionali, effetti della pesca e delle interazioni trofiche

The Tyrrhenian Sea is an area of great ecological and biological interest due to its high species and habitat diversity. The marine ecosystem of this area has been alterated in many ways due to the anthropic exploitation of biological resources, direct habitat modification of sea and coastal areas, introduction of exotic species, pollution and climate change. Notable changes in abundance of marine organisms and composition of communities have been described for the Tyrrhenian Sea over the last three decades, and several demersal and pelagic target species are known to be overexploited or subject to overexploitation. Fishing activities, anthropogenic and natural factors interact within a context, the whole ecosystem, where target and non-target species estabilish complex relationships. Therefore, there is a growing need to apply an integrative and multispecific approach to fisheries management in order to understand how fishing activity is impacting complex food web structure and functioning. A mass-balance model was developed using the Ecopath with Ecosim software to characterise the structure and functioning of the Central Tyrrhenian Sea ecosystem in terms of trophic flows and biomasses, to define ecological role and interactions among species, quantify the impacts of fishing, analyse the role of different fishing activity and compare results from other mediterranean ecosystems. The mass-balance modelling approach has been widely used to quantitatively describe aquatic systems and to assess the impact of fishing activities and environmental factors on marine ecosystems. Several Ecopath models have been built in the Mediterranean Sea since 2000 in order to describe coastal and shallow areas, marine protected areas, lagoones, gulfs and continental shelf and slope ecosystems. This work is the first ecosystem modelling approach in the Tyrrhenian Sea, taking into account the entire food web, and the first effort to study continental shelf and slope ecosystem separately using a mass balance model in the Mediterranean basin. The present model describes the exploited ecosystem from the continental shelf and upper slope of the Central Tyrrhenian Sea, between the Elba Island and the Gulf of Gaeta, covering a total area of soft bottom sediments of 13785 km2 (approximately 85% of shelf and 15% of slope), with a bathymetry between 12 and 750 m, and representing an average annual situation over the late-2000s (2007-2010). The coastal area, where the artisanal fleet maily operates was excluded. The model comprises 4 fleets (bottom trawler, purse seine, longline and passive polyvalents) and 62 functional groups, including 1 group of seabirds, 1 of seaturtles, 2 of cetaceans, 4 of cartilaginous fishes, 22 of bony fishes, 6 groups of cephalopods, 8 of crustaceans (shrimps and crabs), 6 of benthic invertebrates, 2 of euphausiids, 5 planktonic groups (phyto-, zoo- and bacterioplankton), 2 detritus groups and 1 group of fishing discard. 13 of the mentioned functional groups are distributed on the entire ecosystem domain, 26 only on the continental shelf and the remaining 23 only on the slope. Input data (biomass, landing an diet) were based on several recurrent scientific surveys, the IREPA and ICCAT datasets for fishing data, stock assessment outputs, stomach content analysis and published information. Input parameters were estimated from empirical equation and models or based on stock assessment outputs and literature. Results reveal a rather high quality of the model in terms of data sources and detail and show that the Tyrrhenian Sea has a complex food web structure, high number of energy pathways, trophic levels and functional groups, and high rates of matter cycling. The food web is organised in five trophic levels with phytoplancton and cyanobacteria as primary producers and tuna, swordfish, dolphins, hake, anglerfish and European conger as top predators. The dominant groups in terms of biomass are those from lower trophic levels: detritus, bacterioplankton, macrobenthic invertebrates (echinoderms, molluscs, tunicates, sponges and anthozoans), microzooplankton, heterotrophic nanoflagellates, polychaetes and phytoplankton, followed by cyanobacteria, euphausiids, mesozooplankton and small suprabenthic crustaceans (amphipods, isopods, cumaceans, mysidaceans). Results show differences in terms of biomass between continental shelf and slope at higher trophic levels. The shelf is dominated by small pelagic fishes (European anchovy and European pilchard), small benthic fishes (mullet, flatfish, dragonet, goby) and demersal fishes, followed by epipelagic and medium pelagic teleosteans (bogue, atlantic horse mackerel, mediterranean horse mackerel, atlantic mackerel); while the slope is dominated by mesopelagic (lightfish and lanternfish) and benthic fishes, followed by macrobenthic crustaceans, demersal bony fishes and sharks. Total consumption dominates the Total System Throughput (TST) with 43,51% of the total flows, followed by flow to detritus (39,22%), respiration (15,23%) and export (2,03%). The most comsumed groups in the food web are bacterioplankton, suspended detritus, bottom detritus, phytoplankton and zooplankton. Among fishes, the most comsumed were small benthic fishes (mullet, flatfish, dragonet, goby) and European anchovy and pilchard, followed by shelf epipelagic fishes, slope mesopelagic teleosteans and shelf demersal bony fishes including hake. Primary producers and bacterioplankton account for the most part of production (85,24%), followed by microzooplankton and hetertrophic nanoflagellates. Most flows are included within trophic level I and II, a small fraction involves TL III and flows from TL > III are pratically insignificant. Main flows thus, involve groups at the base of the food web which are distributed on the entire domain. Excluding these groups, flows of continental shelf exceed those of slope. Flows related to the pelagic groups were much higher than those related to benthic and demersal groups, and evidenced the importance of the pelagic domain. The estimated mean transfer efficiency of the Central Tyrrhenian food web is 9,7%. The low value of TE between TL III and IV shows that there is a high proportion of production which is not consumed within the system (mainly macrobenthic invertebrates). Several pelagic-demersal-benthic trophic interaction were identified, highlighting a coupling between these compartments. Functional groups of the Central Tyrrhenian Sea food web are organised from trophic level 1 to 4,87, and the TL estimates for the groups are generally higher than those from other Ecopath ecosystems and literature. This could be explained by the fact that the bacterioplankton has been represented as a separate group and not included in other detritus group, causing a shift upward of all higher TL. Results shows that euphasiids, small suprabenthic crustaceans and planktonic groups are among the most important groups in the Central Tyrrhenian ecosystem Euphasiids perform diel vertical migrations and contribute substantially to the vertical transport of organic matter and to the energy flow from lower to higher trophic level, being an important food resource for several species. Planktonic groups and suprabenthic crustaceans at the base of the food web have a positive impact on various functional groups, probably because they constitute the prey of many groups and can influence higher trophic level species through bottom-up control. European anchovy and European pilchard are also important species, representing a key link in terms of consumption and flows between pelagic primary producers and consumers from the pelagic and the demersal domain, and impacting both lower and higher trophic level groups (wasp-waist control). Groups with high keystoneness and less relative abundance are also small benthic fishes, octopuses, macrobenthic crustaceans, some demersal fishes, hakes, whales and sharks. Results evidence that fishery in the Central Tyrrhenian Sea is multi-specific and that it is an important component of the ecosystem, especially on continental shelf where exploitation rates and fishing mortality rates are higher than those from continental slope. Primary production required to sustain fishery (PPR) is a considerable part of total primary production within the system (22,2%). In addiction fleets are responsible for an important fraction of consumption (31%) of exploitable production, as also shown in other Mediterranean ecosystems. Fishing impact is higher on the Tyrrhenian continental shelf than on the slope (respectively 40% and 18% of consumption of production of commercial species). Bottom trawlers have the highest landing rates (59,63%), followed by purse seines (19,92%), passive polyvalents (18,46%) and longlines (1,99%). The majority of the catch is from demersal groups (43,72%), and to a lesser extent from benthic and pelagic species (respectively 29,08% and 27,65%). Purse seine and longline are the most selective fishing gear and they generally catch species with narrow trophic level range. The former mainly targets small pelagic fishes (European anchovy and European pilchard), the latter mainly targets swordfish and hake. Conversely, passive polyvalents and bottom trawlers are multispecific and generally catch a large number of demersal and benthopelagic species with wide trophic level range. Passive polyvalents are multispecific because they use different fishing gear depending on the period of the year, the availability of biological resourses at sea and their market price. Bottom trawlers catch a large spectrum of species because they have a low selectivity. Results show that this fleet has the highest and most widespread impact on the ecosystem, and that has important direct and indirect impacts on several ecological groups (rays, demersal sharks, hake, anglerfish, European conger, flatfish, Norway lobster, red shrimps, rose shrimps and mantis shrimp) and fleets. Catches on continental shelf are domineted by European pilchard, European anchovy, hake, red mullet, octopods, mantis shrimp and cuttlefish; while Norway lobster, rose shrimp, hake and red shrimps represent the main species caught on continental slope. Exploitation rates (F/Z) showed particularly high values for rays, adult hake, tuna and swordfish, octopods, Norway lobster, anglerfish, European conger, brills and mantis shrimp. Results reveal that exploitation rates for some groups exceed the reccomended rate for sustainable fishery management. These groups are hake, mantis shrimp, Norway lobster and red shrimps. Results suggested that there is a complex set of direct and indirect interactions among fisheries that should be considered when implementing fisheries plans, and thus, any management scheme based on a single-gear approach will lead to a suboptimal result. Fishing pressure and trophic interactions have a key role in the dynamics of several groups and in structuring the Central Tyrrhenian ecosystem. Comparisons with other Ecopath models developed in the Mediterranean Sea revealed that models from the western and central basin are generally more similar to each others than those from the eastern basin, according to the productivity gradient of the Mediterranean. The present model settles the basis to further develop dynamic simulations in order to explore the impacts of different fishing pressure scenarios and management strategies in an ecosystem contest.

Il Mar Tirreno costituisce un’area di grande importanza per la straordinaria diversità di specie ed habitat presenti al suo interno. Negli ultimi anni sono stati registrati cambiamenti nell’abbondanza degli organismi, nella composizione in specie delle comunità e molte delle specie demersali e pelagiche di interesse commerciale risultano sovrasfruttate o a rischio di sovrasfruttamento. In questo lavoro l’ecosistema del Mar Tirreno centrale è stato descritto mediante un modello a bilancio di massa realizzato con il software Ecopath with Ecosim (EwE) per metterne in evidenza le proprietà strutturali e funzionali, definire i ruoli ecologici e caratterizzare le interazioni trofiche tra le diverse specie e quantificare l’impatto della pesca, a livello non solo specifico ma anche ecosistemico. Questo è il primo modello di rete trofica costruito nel Mar Tirreno ed il primo caso nel bacino del Mediterraneo in cui, in un modello a bilacio di massa, siano stati rappresentati separatamente i gruppi funzionali di scarpata e piattaforma continentale. I risultati sono presentati e discussi anche effettuando confronti con altri modelli ecosistemici a bilancio di massa con struttura e dimensioni paragonabili, realizzati fin’ora con il software EwE in Mediterraneo (Mar Catalano meridionale, Mar Adriatico centrale, Mar Ionio, Golfo del Leone ed Egeo settentrionale). Il modello descrive la piattaforma e parte della scarpata continentale (12-750 m) del Tirreno centrale nell’area compresa tra l’Isola d’Elba ed il Golfo di Gaeta. Sono rappresentati 62 gruppi funzionali che comprendono pesci ossei e cartilaginei, crostacei, cefalopodi, uccelli, tartarughe marine, cetacei, invertebrati bentonici, fito-, zoo- e batterioplancton, detrito di fondo, detrito sospeso e scarto di pesca. 13 dei 62 gruppi funzionali sono presenti su tutto il dominio, 26 solo sulla piattaforma continentale e 23 sulla scarpata. I risultati indicano che l’ecosistema del Tirreno centrale è caratterizzato da una grande quantità, complessità e diversità di interazioni trofiche e si struttura su 5 livelli, con i produttori primari alla base e predatori quali i grandi pesci pelagici, il nasello ed i delfini all’apice. Sono stati individuati collegamenti importanti tra i domini pelagico, mesopelagico, bentonico e demersale (coupling). La maggior parte dei flussi si concentrano nell’ambito del dominio pelagico-mesopelagico, una piccola parte in quello bentonico e solo una frazione trascurabile in quello demersale. La catena del detrito (microbial loop) ha un peso molto importante nell’ecosistema, che presenta alti tassi di riciclo della sostanza organica. Le specie alla base della rete trofica hanno un ruolo fondamentale nell’ecosistema e possono influenzare le dinamiche delle specie fino ai livelli trofici più alti (bottomup). Fra queste gli eufasiacei rivestono un ruolo particolarmente importante poiché sono coinvolti nel trasferimento dell’energia tra i livelli trofici più bassi e quelli più alti della rete trofica e nel trasporto verticale della sostanza organica. Le acciughe e le sardine sono fra i gruppi più importanti in termini di catture, consumi e flussi tra il dominio pelagico e quello bentonico e demersale. I gruppi chiave nel modello del Tirreno centrale sono i piccoli crostacei del suprabenthos, alcuni pesci bentonici (Mullidae, Callyonimidae, Gobidae), i cefalopodi ottopodi, i crostacei reptanti, i naselli e gli quali demersali. La pesca ha un impatto molto forte sull’ecosistema studiato, in particolar modo sulla piattaforma continentale, ed è di tipo multispecifico. La pesca a strascico è il sistema che ha l’impatto maggiore e più diffuso sull’ecosistema e gli effetti diretti e indiretti più importanti sui gruppi funzionali. Il tasso di sfruttamento di diversi stock (nasello, scampo, gamberi rossi, pannocchie) risulta superiore ai limiti suggeriti per uno sfruttamento sostenibile (STECF, 2011). Nonostante la forte pressione di pesca è emerso che le interazioni trofiche hanno un ruolo molto importante nel determinare la struttura dell’ecosistema e le dinamiche di diversi gruppi funzionali. In linea generale si è constatato che i modelli realizzati nel bacino occidentale e centrale del Mediterraneo risultano avere caratteristiche più simili tra loro, e diverse dai modelli sviluppati nel bacino orientale, coerentemente con il gradiente di produttività di questo mare.