Mediterranean Sea - Large Marine Ecosystem

Brief Description

The Mediterranean is a semi-enclosed sea, with a turnover period of approximately 80 years for water entering through the Straits of Gibraltar. (UNEP, 1989) The Mediterranean Sea contains a narrow shelf, with wider shelves and areas of higher productivity in the Adriatic and the Gulfs of Lions and Gabes. (Caddy, 1993)

Several distinct biogeographical districts can be identified: the Alboran Sea; the Western Mediterranean; the Eastern Mediterranean; and the Adriatic. (Bombace, 1993) The Black Sea, included in biogeographical discussions of the Mediterranean, is discussed separately under LME 26. Through much of this summary, the Adriatic is discussed separately.

The Mediterranean is bordered by arid or semi-arid lands to the south and east. The major inflow to the Mediterranean is nutrient-poor, oxygenated Atlantic surface water through the Straits of Gibraltar, resulting in generally well-oxygenated bottom waters. Nutrient levels are relatively low down to approximately 3,000 m, decreasing from west to east and from north to south, with low levels in the Levant area (Cuddy, 1993; Murdoch and Onuf, 1972). The outflow of highly saline bottom water through the Straits of Gibraltar is richer in nutrients. Freshwater inflows are relatively low, and only approximately 10 percent, including the Nile, enters along the southern shores. Evaporation approximately equals inputs, especially in the Levant Basin. Freshwater inputs, especially from the Nile, have been substantially reduced in the past decades by increased fresh water use, resulting in a decline in nutrient enrichment in the nutrient-poor eastern Mediterranean, reducing freshwater inputs, and perhaps facilitating colonization of the waters by species adapted to higher salinities. Nutrient levels in the remaining flows have increased. (Caddy, 1993; Por, 1978)

The Mediterranean Sea coast has a large population, estimated at 132 million inhabitants and a very large transient population. With heavy use of its coastal and marine resources and severe threats from pollution, the Mediterranean was the first region to be addressed in the United Nations Environment Program (UNEP) Regional Seas Program, beginning in 1974, moving to an Action Program in 1975. (Caddy, 1993; UNEP, 1989)

I. Productivity

Overall, the Mediterranean Sea LME is considered a Class III, low (<150 gC/m2-yr) productivity ecosystem, based on SeaWiFS global primary productivity estimates.

Adriatic

The Adriatic trends northwest/southeast for more than 700 km, with a width between Pescara and Split of approximately 200 km. The Adriatic has a surface area of 138,000 km2, equal to five percent of the entire Mediterranean, with a volume of 35,000 km3, or 1/125 of the Mediterranean. Depths in the northern and central Adriatic are generally less than 75 m, with a large shelf; depths in the southern Adriatic average 100m, extending to 300 m in the Pomo Pit. Approximately one-third of the Mediterranean continental waters flow into the northern and central Adriatic, with total volumes of river flows into the Adriatic varying from 3,000 to 10,000 m3/s, half of which comes from the Po River. The northern Adriatic is characterized by salinity of less than 25 parts per thousand and eutrophic areas along the coast. (Bombace, 1993)

The northern and central Adriatic are subject to strong annual thermal variation, especially in the surficial layers of the water, ranging from 5 to 28 degrees C at the surface and 12 to 17 degrees C at the bottom. During winter, the coastal water is vertically uniform, with low temperatures of five to six degrees C and a salinity below 37 ppt. In the open sea, temperatures range from 10 to 12 degrees C, and the salinity exceeds 38 ppt.(Franco, 1970,1972,1983; Artegiani, 1984,1987; Bombace, 1993) A vertical thermohaline density front separates the coastal from open sea waters, restricting the flow of terrigenous materials (nutrients and other river-borne substances) out of the coastal area, resulting in favorable conditions for spring blooms. In the summer, stratification may occur that vertically separates the warmer, less saline surficial water from the deeper and colder higher salinity water, favoring autumnal algal blooms and extended hypoxia or anoxia. This stratification only occurs with extended periods of calm seas, strong insolation, high temperatures, and inflows of fresh water. Exchange of water masses occurs during a few months in the northern Adriatic, with stormy seas resuspending sediment and nutrients, extending nutrient circulation three to four miles offshore to a depth of 10-15 m. (Bombace,. 1993)

The Mediterranean is a nutrient-limited system compared to the Atlantic coastal waters, with phosphorous, rather than nitrogen the limiting nutrient. (Caddy, 1993) Chlorophyll concentrations off the Israeli coast are between one-half and one-tenth of those for the Sargasso Sea, an area of low primary productivity. (Azov, 1990) A moderate degree of enrichment may increase the biological productivity, as indicated by satellite imagery taken in May 1980 that shows chlorophyll concentrations of 0.15-0.6 mg/m3 over much of the Mediterranean and concentrations greater than 0.6 mg/m3 along the coast and in certain enclosed areas. (UNEP, 1989; Caddy, 1993; NSF/NASA 1989; Caddy, 1990)

The highest levels of productivity occur along the coasts, near population centers (e.g., upper Adriatic, Gulf of Lions, Gulf of Gabes, Genoa, Rome, Tripoli, Benghazi, and Alexandria), rivers (especially the Rhone, the Po, and the rivers along the Catalonian coast) and along the boundaries of the Atlantic surface water entering the Mediterranean. Productivity appears to be enhanced by turbulence and nutrients near the coast and by the influx of nutrients from rivers, densely populated areas, and phosphate industries near population centers and rivers. Riverine flow along the Spanish coast introduces pollution from 40 percent of the coastal population of the Mediterranean, which is likely to affect productivity. High productivity near the Gulf of Taranto may be influenced by outflows of the nutrient-rich Adriatic. Gyres, upwellings, and the influx of Atlantic water appear to contribute to high productivity in the Sea of Alboran, the Gulf of Sirte, eastern, and western Mediterranean. Levels of production decline rapidly with distance offshore from riverine sources and are low in the southeastern Mediterranean and Levant regions. (Caddy, 1993; Darmouli, 1988)

Adriatic

The shallow depth and nutrient input contributes to high productivity. Benthic and demersal species are close to their pelagic prey. For example, Adriatic hake sampled contained 50-80 percent clupeiforms, which would not be likely if the hake lived in deeper waters. (Bombace, 1993) Because of the large influx of fresh water and nutrients, productivity is high in the spring (from several thousands of cells per liter to 600-700,000), especially in the northern and central Adriatic, with higher cell counts during periods of algal blooms. (Boni, 1985)

High productivity occurs in the northern Aegaen, reflecting eutrophication in coastal bays and nutrient-rich discharges of surface water from the Dardanelles, with enhanced phytoplankton production and noxious blooms of phytoplankton and benthic diatoms. (Friligos, 1989; Caddy, 1993) Nutrient enrichment has increased in the central Adriatic. The band of high productivity along the western Adriatic and extending into the Gulf of Taranto may reflect, in part, coastal runoff and the southerly drift of enriched water from the upper Adriatic. (Caddy, 1993; Vucetic, 1988) The phytoplankton community appears to have changed in the last decade in favor of microflagellates. (Bombace, 1993)

II. Fish and Fisheries

The inshore fishery of the Mediterranean has long been an important nutrition source for coastal residents. (Caddy, 1993) Fisheries in the Mediterranean are generally characterized by a short average life span, permitting rapid renewal of resources. Fisheries are multispecies in nature, presenting difficulties for using selective gear types. The use of particular fishing gear on a single boat depends on the season and opportunities. There are a large number of ports for landing catch, dispersed over an extensive coastline, resulting in difficulty in obtaining accurate fishing statistics. (Bombace, 1993)

Technological improvements in the fishing fleet increased fishing capabilities beginning in the 1920s in the northern Mediterranean, resulting in a decline in catch rate, exemplified by the decline in the catch per boat of red mullets (Mullus spp.) in the port of Castellon from 75 to 10 kg/boat/day between1943 and 1961. Similar technological advances occurred in the southern Mediterranean later. (Caddy, 1993; Doumenge, 1968) By the 1970s, a substantial proportion of the less productive southern shelves were being harvested, even by distant-water trawlers, resulting in harvest of demersal resources at close to an overall maximum sustainable yield. The northern stocks were being overfished, possibly reflected in the migration of northern trawlers to southern and deeper waters. Excessive fishing effort out of proportion to available resources has resulted in smaller catch per unit effort (CPUE) and size at recruitment for demersal resources. (Bombace, 127) The rising prices paid for Mediterranean demersal fish, crustaceans, and molluscs are among the highest in the world for these species. Relatively lower prices for pelagic fish, except anchovies, has resulted in more moderate exploitation rates of these species. (Caddy, 1990, 1993; Josupeit, 1987)

National landing data for the Mediterranean (combined with the Black Sea as Statistical Area 37 in the FAO Yearbook of Statistics) show a rising trend in reported landings for both demersals and pelagics, except estuarine species, beginning in the mid-1970s for all areas of the Mediterranean except the Levant in the eastern Mediterranean. Although it is likely that fishing effort in the Mediterranean increased in response to a rise in fish prices, especially for demersals, most fisheries were probably operating close to MSY in the early 1970s. The increase in landings, in part, might reflect a transition from the original oligotrophic to a more highly nutrified or eutrophic situation, reflected by the concentrations of phytoplankton greater than 0.6 mg/m³ in coastal and certain enclosed areas and 0.15-0.6 mg/m³ in much of the rest of the Mediterranean, shown in the 1980 satellite data. (Caddy, 1990, 1993; Josupeit, 1987; NSF/NASA, 1989) Many of the highest levels of fisheries production are associated with areas of high primary productivity. Sardines appear to be associated with the gyres, where primary productivity is high, and areas around riverine flows (e.g., Rhone, Po, and Catalonian rivers) have shown increases in fisheries production. Mussel culture, which was at a relatively low level in the early to mid 1970s had reached 92,000 metric tons (mt) in 1986, especially in the Gulf of Lions, apparently associated with nutrient enrichment from the Rhone outflow. (Caddy, 1993)

The effects of the Aswan Dam on the Nile outflow changed the hydrography of the Levant area in the late 1960s and was considered responsible for the decline in Sardinella production. There is no evidence of increased phytoplankton productivity in the Nile plume, although nutrient enrichment occurs offshore of the delta. Fisheries production in the vicinity of the Nile delta has increased over the last few decades in coastal lagoons that receive drainage water from agriculture and densely populated areas. (Caddy, 1993; Kapetsky, 1984) Egypt's marine fish catch had dropped significantly following the opening of the Aswan Dam in the mid- to-late 1960s, and its annual Mediterranean catch of small pelagics declined from 25,000 mt to 3,000 mt between 1964 and the 1970s. (GFCM, 1989) Recent increases in fisheries production appear to reflect an increase in nutrient outflow in drainage waters from urban growth. (Caddy, 1993; Savini and Caddy, 1989)

In the Mediterranean, areas of high nutrient enrichment are geographically associated with increased fisheries yield, at least in the early stages of enrichment. However, there is evidence of growing negative impacts of overfishing and pollution on productive coastal systems. (Caddy, 1993)

Adriatic

The northern Adriatic produces 250,000 mt of catch per year of the Mediterranean's 1,000,000 mt. The northern Adriatic's shallow shelf favors the use of "grazing" gear (e.g., trawling, mid-water trawling, dredging for bivalve molluscs, and beam-trawling for flatfish) primarily in international waters and along the Italian coast. During autumn and early winter, a seaward migration of exploitable demersal species from the coast leaves only a number of small eurythermic and euryhaline species (e.g., Gobiidae and Atherinidae) remain inshore. The sectors of the northern Adriatic only are allowed to trawl within three miles of shore from October to March, with limitations on mesh size. (Bombace, 1993)

The northern Adriatic contains several endemic species, including Acipenser naccarii, Syngnathus taenionotus, and several Gobides, such as Knipowitschia panizzae and Pomatoschistus canestrini. (Tortonese, 1983) Merlangius merlangus (whiting), which travels to the Norwegian and Icelandic coasts in the Atlantic, is found in the Lion Gulf, northern and central Adriatic, Northern Aegean, the Sea of Marmara. Sprat (Sprattus sprattus) which is an important stock in northern and central Adriatic, even if not fully exploited, can be distinguished in an Atlantic form, which never travels further than Gibraltar; a Baltic form; and a Mediterranean form, which lives in the cooler areas. Other pontic or ponto-caspian species in the northern Adriatic are Huso huso, Syngnathus tenuirostris, Knipowitschia caucasica and Platyichthys flesus luscus. Knipowitschia is well represented in the northern Adriatic, typically found in lagoon and estuarine brackish-water environments. (Bombace, 1993) Suspended organic material is the primary source of nutrients for the large biomass of sestonophagous and detritivorous molluscs (including razor shells, clams, striped venus, mussels, and oysters), which are of great economic importance in the Adriatic. (Bombace, 1993)

The Adriatic experiences the highest phytoplankton productivity in the Mediterranean. Fish kills have occurred in the northern Adriatic as a result of noxious phytoplankton blooms and anoxic conditions. The central Adriatic is becoming increasingly nutrient-enriched, with increased recruitment and production of some demersal and small pelagic fish. (Caddy, 1993;Vucetic, 1988; Bombace, 1993) Following institution by Italian authorities of a closed season for trawling, recruitment of mullet (Mullus spp.) has occurred at above-average levels. (Caddy, 1993) There are links between eutrophication, energy source, nature of the bottom, and type and amount of catch. The large extent of soft substrate and the richness of particulate matter, phytoplankton, and zooplankton, favor plankton, seston, and detritus feeders. (Bombace, 1993)

Fifty-five percent of the total Italian catch of marine fish comes from the Adriatic, of which 40 percent comes from the northern and central Adriatic. The "small pelagic" fisheries and molluscs, particularly bivalves, are very important, with reports of 10-20,000 mt of each per year. (Bombace, 1993; Vusetic, 1988, 1993) The contribution of the Adriatic to the Italian total catch dropped from 62.5% in 1982 to 52% in 1987, primarily caused by a decrease of CPUE (a decrease of one-half from 1982-87), largely in the northern Adriatic. There has been a large decrease in the catch of demersal species, probably due in large part to the effects of anoxia on benthic species and pre-recruitment stages of fish (eggs and larvae). There have been intense and vast occurrences of anoxia in the northern Adriatic bottom. (Bombace, 1993)

Trends in fisheries species include overfishing of:

• Baby clams or striped venus (Chamelea gallina L.), with increased harvest of juveniles in the northern Adriatic threatening a stock that is stressed by anoxia. Fishing restrictions have not slowed the decline of this resource. While fishing boats are now also catching Golden carpet shells ( Tapes aureus), the combined resource will not be able to sustain the growing fishing effort and demand. (Bombace, 1993)

• Anchovies, the stock collapsed within a 10-year period, from a biomass of more than 640,000 mt to just several mt between 1978 and 1986. Many midwater pair trawlers have ceased activities, driving prices up ten times. The fishing effort in the Northern and Central Adriatic for pelagic stocks decreased from 1981 to 1988. (Bombace 1993)

• Sprat stocks fluctuate regularly, with a maximum peak density in 1987 of 32 mt/nm2, decreasing to a few mt/nm2 in 1988, with current density of 14 mt/nm2. Sprat are underutilized. (Bombace, 1993)

Species that appear to be stable or to fluctuate include: • Other molluscs, with mussel and oyster harvests increasing due to mariculture.

• Crustacean catches, which declined from 1983-85 but have returned to 1982 percentages of total crustacean catches.

• Other small pelagic species - Sardine and horse mackerel (Trachurus spp.) production trends are opposite to anchovies, with both increasing from 1981 to 1984 and in 1987, and decreasing in 1988. (Bombace, 1993)

Biomass and catch of small pelagic species in the Northern Adriatic declined following 1986, with pelagic biomass approximately 283,000 mt in 1988, or half of the 1987 estimate. Concentrations in the Gulf of Trieste, the Istria peninsula, the Po delta and the coastal region between Emilia-Romagna and Marche either disappeared or broke down. The average pelagic biomass is high (77 mt/nm²) but fluctuates widely. The average pelagic biomass per year is 596,312 mt/yr, ranging from 249,096 mt/yr to 1,025,087 mt/yr. (Bombace, 1993)

The major factors apparently contributing to the depletion of pelagic resources include the following: break-up of the large density population around the Po delta; below-average biomass trend from 1981-85 and the collapse of the anchovy stock from 1986 to1988; decrease of the sardine stock in just one year, during which the competitor was at a minimum; rapid decrease of the sprat stock in five years; and continued decline of the pelagic biomass in spite of the decrease in fishing effort . (Bombace, 1993) Contributing factors to the decline during the 1970s and 1980s include overfishing, the refunds for unsold products permitted by the European Union regulations, and fishing for juveniles. Possible environmental factors include the recent dominance of microflagellages in the phytoplankton community and the decreased riverine input, reducing nutrient input and raising average temperatures on the sea bottom, producing unfavorable conditions for the sprat stocks that normally live in the colder part of the water mass. (Bombace, 1993; Grego, in press; Sansebastiano and Sansebastiano, 1989)

Artificial reef experiments in several areas indicate the following benefits can be provided: protection of sea bottom from trawling, allowing native species to grow; and protection and refuge for many organisms, including eggs, juveniles,and adults of pelagic, nekto-,and benthic fish, crustaceans, and cephalopods. Reefs appear to increase productivity, as indicated by the increase in fishing yield near the reefs and occurring after construction of a reef. Benthic reef populations, such as mussels and oysters, represent new colonies, increasing their biomass. (Bombace, 1993, 1982; Bombace and Rossi, 1986; Fabi and Fiorentini, 1989)

The University of British Columbia Fisheries Center has detailed fish catch statistics for this LME. A graphical representation is provided below.

Mediterranean Sea Fish Catch Data

III. Pollution and Ecosystem Health

Roughly half of the organic inputs to the Mediterranean are industrial in origin. The remaining half derives from human sewage and agricultural sources. Approximately 60 to 65 percent of organics entering the Mediterranean come from runoff and discharge, and the rest is transported by rivers. Direct atmospheric inputs contribute nitrogenous compounds, contaminants, and heavy metals. (Caddy, 1993; UNEP, 1989) Close to discharge points, nutrient inputs have a locally deleterious effect on the diversity of Mediterranean fauna and flora. However, the effects of the nutrients on the entire Mediterranean may be to increase fishery productivity. (Caddy, 1993, 1990; Degobbis, 1989) Satellite imagery indicates high phytoplankton productivity near riverine plumes and population centers, with associated higher levels of fisheries productivity. (Caddy, 1993; Darmoali, 1988)

Effects of anthropogenic nutrient enrichment are becoming a major cause of concern in the upper Adriatic, caused by runoff, polluted discharges of the River Po, and extensive coastal development, which is augmented by a large influx of tourists in the summer months. Blooms of phytoplankton and benthic diatoms and their mucous rafts have adversely affected the tourism industry and have resulted in local "die offs" of fish and invertebrates caused by anoxia. Mucilage rafts foul fishing nets. Planktonic blooms and sewage contamination of coastal waters have also caused health problems associated with ingestion of contaminated shellfish. (UNEP/FAO, 1990; Caddy, 1993; UNEP, 1989) Concern has been expressed, especially in the northern Mediterranean, over declines in other species or losses in species diversity, but quantifying these effects and the respective contributions from overfishing or eutrophication may be problematic. (Caddy, 1993)

A moderate, controlled, level of nutrient enrichment in oligotrophic systems, may increase production of some economically important species. However, such discharges raise concerns, i.e., discharge of non-biodegradable contaminants and the occurrence of noxious and toxic microorganism blooms close to the discharge point. (Caddy, 1993)

IV. Socioeconomic conditions

Nutrient enrichment related to population centers, industrial activities (e.g., phosphate industries), and riverine inputs appears to increase primary productivity in many areas throughout the Mediterranean. (Caddy, 1993; Darmouli, 1988) While many areas are experiencing increased fisheries production, serious effects of anthropogenic nutrient enrichment in the upper Adriatic are a cause of concern, caused by runoff and polluted discharges of the River Po and the extensive development of coastal areas, which is augmented by a heavy tourist influx in summer months. Impacts to the tourism industry are of great concern, because the tourism industry is generally conceded to be of higher economic value than fisheries in many Mediterranean countries. (Caddy, 1993)

Nutrient enrichment has increased biological productivity and fishery yield, but it may not have increased the value of Mediterranean fishery landings, which may be small compared to the potential and actual losses in tourism revenue resulting from eutrophication of coastal waters. Even apart from the tourism industry, economic benefits of the increased fishery are not clear, because unregulated fisheries result in overcapitalization of the industry.(Caddy, 1993; Hannesson, 1989) In addition, other negative aspects, including health hazards from untreated sewage, greater risk of shellfish contamination through sewage discharges or heavy metals, and increased incidence of toxic blooms have increased recently, imposing extra costs. (Caddy, 1993)

Molluscs such as razor shells, clams, striped venus, mussels, and oysters are of great economic importance in the Adriatic. (Bombace, 1993)

Fisheries declines in the Adriatic relate to harvesting juveniles, overfishing, anthropogenic and other environmental effects. Reduced riverine input due to drought conditions and higher average temperatures lowers nutrient levels and productivity. (Bombace, 1993; Sansebastiano and Sansebastiano, 1990)

Artificial reef experiments indicate that fishing production increases near the reefs. The return on the investment tripled from 1977 to 1980, and the gross income for a reef fisherman doubled from 1982 to 1984. These efforts may aid in the recovery of the small-scale fixed-gear fishing industry (about 7,000 boats in Italy) and may allow small trawlers to be converted for use in reef management and mariculture. (Bombace, 1993, 1982; Bombace and Rossi, 1986)

V. Governance

Italian authorities imposed a new closed season for trawling in 1988-89. (Caddy, 1993) During the winter (October to March) when there is a seaward migration of exploitable demersal species from the coast, the sectors of the northern Adriatic only are allowed to trawl within three miles of shore. Fishing restrictions have been applied to various Adriatic species, including: harvest restrictions on baby clams based on a minimum size of 2.5 cm (2 years old) which is rarely respected; reducing fishing quotas; closing the fishing season for two months (up from one); and reducing weekly fishing activities. (Bombace, 1993)

The European Union (EU) allows refunds for unsold products. (Bombace, 1993)

The Regional Seas Program emphasizes the importance of environmentally sound management practices to safeguard the marine environment. (UNEP, 1982) There is a general realization that land-based activities can affect the marine environment in ways that may harm human uses of the marine ecosystem, but that marine resources should be managed, rather than preserved unchanged. (Caddy, 1993) Managing the coastal zone from an ecosystem perspective can resolve a number of fishing problems and can facilitate the renewal of impoverished resources. The fishing effort must be regulated, through measures such as closing fisheries, establishing and enforcing fishing seasons and areal regulations, and establishing biotechnological means to reduce pre-recruitment mortality. (Bombace, 1993)

A comprehensive plan for the coastal areas is needed that considers an integrated system of artificial reefs, hatcheries, mariculture, research, focusing on specific habitat areas to protect nurseries and damaged stocks and rebuild resources. Water parcels would be allocated based on management decisions involving various species and user groups. Cooperation among management, research, and harvesting groups will be required. Legal, administrative, financial, and institutional requirements to expedite artificial reefs include simplification of the procedure for state concession of coastal marine waters; consideration of an institutional liaison with the coastal regions; consideration of the coastal areas management and marine culture as a distinct problem when revising the law on fishing; modification, at the EU level, of administrative laws regarding initiatives funded with EU funds, and the introduction of a new chapter for planned general initiatives. (Bombace, 1993)

An intensification and greater coordination of scientific studies of fish resources and the biological and oceanographic environment is necessary. (Caddy, 1993) 

Articles Reviewed:

  • Bombace, Giovanni. 1993. "Ecological and Fishing Features of the Adriatic Sea," in Kenneth Sherman, et al. (eds.), Large Marine Ecosystems: Stress, Mitigation, and Sustainability (Washington, D.C.: American Association for the Advancement of Science, 1993) pp. 119-136
  • Caddy, John F. 1993. "Contrast Between recent Fishery Trends and Evidence from Nutrient Enrichment in Two Large Marine ecosystems: The Mediterranean and the Black Seas," in Kenneth Sherman, et al. (eds.), Large Marine Ecosystems: Stress, Mitigation, and Sustainability (Washington, D.C.: American Association for the Advancement of Science, 1993) pp. 137-147.

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Source:

  • NOAA Fisheries, Northeast Fisheries Science Center, Narragansett Laboratory - LME 26: Mediterranean Sea - https://www.na.nmfs.gov/lme/text/lme26.htm

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