Catalan Sea (Southern) - Ecology

The southern Catalan Sea is mostly described as oligotrophic, but temporal enrichment occurs due to regional environmental events, mainly related to wind conditions, the existence of a temporal thermocline and a shelf-slope current and river discharges (Estrada 1996, Salat 1996). These factors greatly influence the productivity and fishing activity of the area, which yields almost half of the total landings of the Catalan coast and is especially relevant for the reproduction of small pelagic fish, mainly the European anchovy Engraulis encrasicolus and sardine Sardina pilchardus (Palomera et al. 2007).

Several studies have highlighted the relation between environmental factors and primary and secondary production dynamics in the Mediterranean Sea (e.g. Estrada 1996, Palomera 1992, Sabatés and Olivar 1996, Agostini and Bakun 2002). In the Catalan Sea, the wind mixing index has been positively related with recruitment of various demersal species, most likely due to enhanced fertilization and local planktonic production (Lloret et al.. 2001). This is also the case for sardine landed in the Ebro River Delta area during the sardine spawning season (November–March). River runoffs from the Rhône and Ebro have been positively related with recruitment of demersal species (Lloret et al.. 2001), as well as with anchovy landings during the anchovy spawning season (April–August) in the southern Catalan Sea (Lloret et al.. 2004). The progressive increase in water temperature in the Mediterranean Sea (Bethoux and Gentilli 1990, Salat and Pascual 2002) has been positively related with an increase of sardinella (Sardinella aurita) landings and its expansion to northern NW Mediterranean areas (Sabatés et al.. 2006), while other species such sardine or sprat (Sprattus sprattus) have declined. In addition, it has been suggested that the Mediterranean Sea has been subjected to increased nutrient loading and eutrophication (Caddy and Bakun 1994, DeLeiva et al.. 2000).

In 1983, 7802 ha of the terrestrial domain of the Ebro Delta River was protected as a National Park due to its international importance for its flora and fauna, mostly birds. Thus, the southern Catalan Sea is a strategic area for marine vertebrate conservation, sheltering three quarters of the world’s breeding population of the Mediterranean endemic Audouin’s Gull (Larus audouinii) and important colonies of other terns and gulls (Zotier et al. 1998, Abelló et al. 2003). Some of these species forage actively on the marine shelf and find a complementary food source in the discards generated by the trawling and purse seine fleets (Oro et al. 1997, Oro and Ruíz 1997, Arcos and Oro 2002). Various species of resident or migratory marine mammals like the bottlenose dolphin Tursiops truncatus, the striped dolphin Stenella coerulealba and the fin whale Balaenoptera physalus, as well as important populations of loggerhead turtle Caretta caretta (e.g. Groombridge 1990, Forcada et al. 2004, Gomez de Segura et al. 2006a, 2006b, Monestiez et al. in press), occur in the ecosystem. Other emblematic species of conservation concern occur in the area, such as the bluefin tuna Thunnus thynnus (Mackenzie et al., 2009; Abdul Malak et al. 2011), as well as several unique, endangered and sensitive habitats, such as the seagrass meadows of the endemic Posidonia oceanica (IUCN-Med, 2009).

Mass-balance ecosystem models developed to represent the southern Catalan Sea in different time periods (late 1970s, mid 1990s and early 2000s) (Coll et al. 2006a, 2006b, 2008, 2009) facilitated characterisation of the structure and functioning of this ecosystem by integrating most of the ecological and biological information available. The models represented the continental shelf and upper slope area associated with the Ebro River Delta and showed that from the 1970s, the ecosystem was dominated by the pelagic food web. Small pelagic fish were identified as important components of the ecosystem, dominating the pelagic fraction in terms of biomasses and catches. European hake (Merluccius merluccius) and medium-sized pelagic fish (horse mackerel Trachurus spp. and mackerel Scomber spp.) also played important ecological roles in terms of biomasses and trophic interactions.

Results from ecological modelling applications showed that a total of 67-74% of the variability in available time series from 1978 to 2003 was explained by the models by directly including in the analysis main trophic interactions (37-53%) and fishing activities (14%), and indirectly considering the environment (6-16%), as driving factors. This suggests that both the environment and fishing activities are key ecosystem drivers of the southern Catalan Sea (Coll et al. 2008).
The proliferation of non-commercial species with lower trophic levels (e.g. jellyfish and shrimps) or higher turnover rates (e.g. cephalopods and benthopelagic fish) and trophic cascades have been predicted with ecological simulations (Coll et al. 2008). Although little data is available to validate these results, they are in line with empirical evidence from other Mediterranean areas (Abad and Giraldez 1990, Caddy 1997, Buecher 1999, Pipitone et al. 2000, Pages 2003, Daskalov et al. 2007).