The Mediterranean Sea

Map of Mediterranean Sea (13)

Mediterranean Region

The Mediterranean Sea is bordered by Southern Europe to the North, the Middle East to the Eastern edges, and North Africa to the South. The Strait of Gibraltar, which is a narrow passage-way on the Western edge of the Mediterranean Sea, connects the Mediterranean Sea with the Atlantic Ocean. The Suez Canal, in Egypt, connects to the Mediterranean Sea, as does the Black Sea by the Bosporus Strait in the Northeast (see map below) (1). 


Due to the narrow passage-way of the Strait of Gibraltar, the Mediterranean Sea has limited tide flow from the Atlantic Ocean. Evaporation greatly exceeds precipitation and river run-off, causing the Mediterranean Sea to be saltier (more saline) than the Atlantic Ocean. The Mediterranean Sea also has a pressure gradient in which cool, low salinity surface waters come in from the Atlantic Ocean and as those waters move through the Mediterranean, it warms and becomes saltier, thus sinking. The saltier water then circulates around the eastern portion of the Mediterranean as bottom waters and heads out toward the Atlantic Ocean (1). 

The Subregional Seas of the Mediterranean

Regional map of the mediterranean Sea (14)
The Mediterranean Sea has several smaller regional seas within it. This often comes in handy when wanting to know more about a specific portion of the Mediterranean Sea. 

Biodiversity: Mediterranean Sea Fauna

Cratena peregrina (15)
The whole of the Mediterranean Sea is derived primarily of Atlantic Ocean species (derived meaning essentially evolved from). However, the subregional Alboran Sea is considered the "transition zone" where present-day Atlantic species and the derived Mediterranean species mix. It's logical that the Alboran Sea is the "transition zone" because the Mediterranean Sea is more saline than the Atlantic (1).  

Biodiversity: the Subregional Aegean Sea Habitats

Seagrass meadow (16)
The Aegean Sea has three significant marine habitats other than rocky shores and sandy beaches. Those habitats are seagrass meadows, coralligenous algal reefs, and upwelling.
The seagrass meadows primarily have three species of grasses; Posidonia oceanica, Cymodocea nodosa, and Halophila stipulacea. These grasses are important because they provide habitats to other species of fauna, especially as a nursery for incubating species (2).
The coralligenous algal reefs also provide habitat for many sessile invertebrates, or suspension feeding organisms. The algal reef is primarily made of coralline algae. And most of the residing inhabitants feed off of the pelagic fauna, rather than directly off of the coralligenous algal reef itself (2). 
Finally, the Aegean Sea is a site of upwelling. Ekman flow, which is the process of surface waters dragging the layers beneath it due to strong winds, occurs both away from the eastern coast and toward the western coast producing an upwelling/down-welling couple. During upwelling, nutrients are brought up from the bottom waters and triggers a bloom of phytoplankton production. This upwelling is weakest in spring, but strongest in early summer (3).

Food Webs of the Subregional Aegean Sea

The Aegean Sea has two food webs; one occurring for the coastal range and the other occurring for the open ocean/benthic communities. The coastal food web starts with the phytoplankton as the primary producers. Also, I have found published work identifying "discards", the scraps or pickings from a boat, as an inserted, but not authentic, form of primary productivity. From tier one follows ciliates and mesoplankton; followed by seabirds and loggerhead turtles; followed by large pelagics and dolphins. Note: This is a highly simplified food web; many more divisions of fauna exist (4).
The open ocean/benthic food web starts with detritus as the primary producers. This is because most of the life present in the open ocean occurs at deep depths where light is absent, halting the ability for photosynthesis to occur. From the consumption of detritus comes benthic invertebrates; followed by Red Mullet; followed by anglerfish and octopus. Note: This is a highly simplified food web; many more divisions of fauna exist. Of this food web, the red mullet is a major organism because it is considered a delicacy in Grecian culture (4).
Coastal range food web for the Aegean Sea (5)
Open ocean food web for the Aegean Sea (5)


What is Fished in the Mediterranean?

Species catch (17)
The European pilchard is a herring-like, small pelagic fish. The European anchovy is an anchovy species that is also a small pelagic. The mixed group of species catch constitutes various crustaceans, such as lobster, crab, and shrimp; cephalopods (squid, possibly octopus), and other bottom-dwelling invertebrates. According to the graph, courtesy of the Sea Around Us Project 2010, the mixed group is what is most caught throughout the Mediterranean Sea. The graph cuts at year 2005, however, it looks as though there was beginning to be another peak in the catch amount of the mixed group. Catches of the European anchovy and pilchard appear to be at steady catch amounts, neither increasing nor decreasing (5).

How is it Caught?

Catching gear (17)

 Net  fishing and the trawling industry are the methods (reportedly) used for fishing in the Mediterranean. Net fishing includes purse seines, gillnets, and long lines, as well as artisanal fishing, or local community fishing. Net fishing is usually concentrated in shallow, near-shore waters. Trawling (mid-water trawls, bottom trawls, dredging) encompasses boats (~15 m or larger) that stay out for several weeks at a time and collect massive amounts of pelagic fish for harvesting. The trawling industry is also considered the true player in commercial fishing as oposed to net fishing of the locals and is generally concentrated around Sicily, Italy (6).

Who Does the Most Fishing?

Country catch (17)
The countries with the highest fish production are Italy, Spain, and Greece. Around 1970 is when fish catches started to increase for these countries. Throughout the years, all three leading countries have seemed to keep relatively steady catch amounts. By the beginning of 2005, however, it appears that Spain's fish catch production decreased compared to Greece's catch amounts (5). 

Problems With Fisheries

Trawling vessel in Mediterranean(18)
In addition to obvious overfishing of many fish species throughout the Mediterranean Sea, trawling produces unnecessary bycatch (retrieval of unwanted fish or organisms). Often, over half of that bycatch is tossed back in the Mediterranean Sea as trash. Trawling in waters immediately off the coast often destroys important marine habitats such as seagrass meadows. Specifically, Posidonia oceanica, one of the main seagrasses found in seagrass meadows, is being threatened by trawling practices (6). 

Fisheries Management in the Mediterranean

Fish crates (18)
Fisheries management in the Mediterranean Sea is governed by the General Fisheries Commission for the Mediterranean (GFCM). The GFCM is an intergovernmental organization. Members include all 21 bordering countries and a few non-bordering countries/entities that have investments in the Mediterranean Sea. Their mission is to promote development, conservation, rational managment and the best utilization of living marine organisms and promote sustainable aquaculture development (7). 
As of 2005, new pressures the GFCM has proposed are monitoring the Capture-based Aquaculture (CBA) industry, banning bottom trawling and towed dredges at depths greater than 1,000m, mandate boats larger than 15m be logged in a central registry to reduce illegal fishing, and implement a 40mm mesh size restriction in trawl nets to reduce bycatch (7).

Invasive Green Algal Species: Caulerpa spp.

Caulerpa taxifolia

Caulerpa taxifolia (19)
Caulerpa taxifolia is native to several regions including the coasts of the Caribbean, the Gulf of Guinea, the Red Sea and the Indian Ocean (8; 9). This particular species was intially introduced into the Mediterranean Sea from waste water from the Oceanographic Museum of Monaco in 1984 (8; 9). Since then it has been spread, as fragments that can reproduce and mainly to port areas, by the fishing industry whereby it gets caught in propellers and netting (8). The alga produces large amounts of a single toxin compound called caulerpenyne instead of producing many toxins in reduced quauntities. It also is found on a variety of substrates such as rocks, sand, mud, and seagrass beds (8; 9). In 2000, there was a Southern California case of invasion by this species. It has since been eradicated using PVC tarpaulin and chlorine on the effected areas (8; 9). Impacts on humans from the effects of this alga are reduced fishing catches due to the elimination of fish habitat, the nuisance of it getting tangled in nets and propellers, toxin accumulation in the flesh of fish that is then consumed by humans, and the costs of controlling or eradicating the invasion (8). Impacts on the ecosystem are competition for food and light in which the invader out-competes the native species P. oceanica and C. nodosa (8). Also, any unforeseen consequences of toxin accumulation in fish that are able to eat the alga, such as Sarpa salpa.  

Caulerpa racemosa

Caulerpa racemosa (20)
Caulerpa racemosa is another species of green algae that has invaded the Mediterranean Sea. This species is native to Australia, New Zealand, the Indian Ocean, the Red Sea and a few other regions (8). It is unclear where exactly it came from or how it was introduced. However, it is quite probable that it was introduced from ballast water of ships travelling through the Mediterranean Sea. Fishing activity also helped facilitate the spread of this algae in the form of fragments that could reproduce (10). This species produces the same single toxin as does its sister species, C. taxifolia. Caulerpa racemosa is considered more aggressive than C. taxifolia due to greater dispersal of invasion in both polluted and unpolluted waters (10). Although, it supposedly exhibits selectivity for substrate habitats, which include rocky habitats and the margins of P. oceanica meadows (10). Impacts of Caulerpa racemosa include competition for food and light, toxin effects on fish and humans, and resistance to sedimentation disturbance. This resistance is said to even enhance growth under those conditions, thus prohibiting any slower growing aquatic plants from establishing (11).   

Seaslugs Found to Graze on Caulerpa spp

Elysia subornata (21)
Sea slugs native to the Mediterranean Sea that were found to graze on Caulerpa, particularly C. taxifolia are the herbivorous sacoglossans Oxynoe olivacea and Lobiger serradifalci. though they graze on this invasive algae, the rate at which they graze can't keep up with the rate at which the plant reproduces and spreads (8). A non-native (Florida, USA native) sacoglossan was found to be able to graze on Caulerpa taxifolia, however, it can't survive in the colder Mediterranean waters compared to the US Florida waters (8; 9). And, if it was able to survive, the question would then be is it ethical or right to introduce this sea slug to the Mediterranean Sea when it has been impacted tremendously by introduced species already.   

Other Ways of Controlling Them

Manual labor using SCUBA (22)
There are a few other ways to control the spread of Caulerpa species. One way is to cover the effected area with PVC tarpaulin and add chlorine to kill the plant. This method, though, also kills anything else in the area (other plants and animals) (9). Another simpler method was using black PVC tarpaulin alone. It had a reasonable success rate (8). One final way is manual labor by using SCUBA (8). This method can be successful if the area is not too big. 

Historical Use of the Mediterranean Sea

The Suez Canal (23)
In November of 1869, the Suez Canal opened and connected the Red Sea to the Mediterranean Sea. Navigationally, it is important because it allows transportation between Europe and Asia instead of going all the way around the African continent (12). Ecologically, it is important because the opening of the canal has impacted the Eastern Mediterranean (and whole Mediterranean) marine ecosystems tremendously. A region within the Red Sea called the Bitter Lakes used to be hypersaline lakes that inhibited the Red Sea species from migrating into the Mediterranean. Unfortunately, the salinity of the Bitter Lakes has leveled with the salinity of the Red Sea, therefore allowing plants and animals from the Red Sea to invade the Eastern Mediterranean, or known as Lessepsian migration. In addition, the Red Sea is more saline than compared to the Mediterranean Sea. This also allows the Red Sea species to adapt better to a less saline environment than the reverse for mediterranean species. Another related impact on the Eastern Mediterranean was the opening of the Aswan High Dam across the Nile River in 1968. This reduced freshwater flow (from the Nile Delta) into the Mediterranean and stopped natural nutrient-rich silt from flowing into the Mediterranean Sea. This reduction in freshwater flow and no silt further makes the environment more pleasing for Red Sea species (12).     

[1] Wikipedia (2010) Mediterranean Sea. < Sea>  December 7, 2010. [2] Archipelago Institute of Marine Conservation (2010) <>  December 7, 2010.
[3] Allsopp, Michelle (2009) State of the World’s Oceans.  Springer. Dordrecht, London.
[4] Tsagaraki, K. et al. (2010) Food-Web traits of the North Aegean Sea Ecosystems: (Eastern Mediterranean) and comparison with other Mediterranean Ecosystems. Estuarine, Coastal, and Shelf Science.  88:233-248.
[5] Sea Around Us.  Mediterranean Sea. <>  December 7, 2010.
[6]Hinrichsen, Don (1998) Coastal Waters of the World. Island Press. Washington, D.C.
[7]FAO (2010) Fisheries. <>  December 7, 2010.
[8] IUCN/SSC (2010) Invasive Species Specialist Group. <>  December 7, 2010.
[9]Wikipedia (2010) Callerpa taxifolia. <>  December 7, 2010.
[10] Sanevakis, Kat et al. (2010) Modeling Distribution Pattern and habitat preference of the invasive green alga Caulerpa racemosa in the Saronikos Gulf (Eastern Mediterranean). Aquatic Biology. (10)1:57-67.
[11] Hendriks, et al. (2010) The Effects of Seagrasses and Algae of the Caulerpa Family on Hydrodynamics and Particle Trapping Rates.  Marine Biology. (157)3:473-481.
[12] Wikipedia (2010) Suez Canal. <>  December 7, 2010.

Created for Your Intellectual Pleasure by Amber Rendleman
Last Modified December 7, 2010