Ionian Sea Archipelago - Ecology

The Inner Ionian Sea Archipelago is extremely oligotrophic with values of Chlorophyll a, nutrients and particulate organic carbon among the lowest found in Mediterranean coastal waters (Casotti et al. 2003, Pitta et al. 1998). This reflects the low phytoplankton and zooplankton biomass in the area dominated respectively by dinoflagellates and coccolithophores (Pagou and Gotsis-Skretas 1989, Stergiou et al.1997) and copepods (Siokou-Frangou and Pancucci-Papadopoulou 1989). Most of the area is relatively shallow, ranging in depth between 100 and 200 m. The bottom is covered by seagrass meadows (Posidonia oceanica and Cymodocea nodosa) at depth between 1 and 30m (Haritonidis and Tsekos, 1976; Zenetos et al. 1997). Such benthic macrophytes, despite being an important nursery ground for fish populations, in the last decades have suffered a tremendous decline in the area due to fishing operations (mainly by purse seiners, trawlers and beach seiners;(Gonzalvo et al. 2010)).

The European sardine (Sardina pilcardus) and anchovy (Engraulis encrasicolus) are the most important pelagic fish, in terms of biomass in the area, representing approximately 26% of the total Inner Ionian Sea Archipelago catch (Piroddi et al. 2010). Bogue (Boops boops), hake (Merluccius merluccius) and striped red mullet (Mullus surmuletus) dominate the demersal fish compartment (Tsikliras et al. 2007, Piroddi et al. 2010). The area is an important spawning ground for European pilchard (Somarakis et al. 2000, Somarakis et al. 2006, Machias et al. 2007) and hake (Politou et al. 2006, Politou 2007).

Of the 9 species of marine mammals occurring in the Mediterranean Sea, three regularly inhabit the area, the bottlenose dolphin Tursiops truncatus, the Mediterranean monk seal Monachus monachus and the short-beaked common dolphin Delphinus delphis. A further two, the fin whale Balaenoptera physalus and the striped dolphins Stenella coeruleoalba are occasionally encountered (Bearzi et al. 2006). The loggerhead turtle Caretta caretta is the most common sea turtle, nesting in several beaches of the area (Margaritoulis et al. 2003).

An ecosystem model was built for the Inner Ionian Sea Archipelago to assess temporal changes (1964-2008) in abundance of the major marine organisms (Piroddi et al. 2010). Results of this study highlighted a clear decline of top predators and commercially important species (mainly European anchovy and European pilchard) due to overfishing and changes in ocean productivity (Piroddi et al. 2010). Ecosystem indicators analysed by the model have also shown clear changes in the structure of the Inner Ionian Sea Archipelago from 1964 to 2008. Both Kempton’s biodiversity index (for those organisms with TL?3) and the mean trophic level of the community (excluding TL = 1) have shown consistent declining trends through time , reflecting the decrease of high trophic level organisms (e.g., marine mammals) and the increase of lower trophic level groups (e.g., cephalopods, crustaceans, benthic invertebrates). On the other hand, the demersal/pelagic biomass ratio has increased linearly with time, reflecting the decline of small pelagic fish biomass which are the main target of the local fisheries, and a consequent increase of demersal species.