The Strait of Gibraltar is a narrow passage of water that connects the Atlantic Ocean to the Mediterranean Sea and separates Gibraltar from Ceuta in Spain. It is an important migratory route for many species of birds, fish, and marine mammals. Over 300 species of birds migrate across the Strait of Gibraltar each year, along with numerous species of tuna, sharks, whales and dolphins. But why exactly is this relatively small passage of water so critical for these migratory species? There are several key reasons:
Location between Atlantic and Mediterranean
The Strait of Gibraltar represents the shortest distance between the Atlantic Ocean and Mediterranean Sea. It is only 8 miles wide at its narrowest point, providing a direct migratory pathway between these two large bodies of water. Species such as bluefin tuna and humpback whales utilize the strait during their Atlantic-Mediterranean migrations. The strategic geographic location of the strait along major latitudinal and longitudinal migratory routes makes it a convenient “bottleneck” for many migratory marine species.
Abundant food resources
The mixing of waters from the Atlantic and Mediterranean via the Strait of Gibraltar results in nutrient-rich upwellings along the strait. This stimulates phytoplankton growth, which in turn attracts large numbers of zooplankton, squid, sardines, anchovies and other small fish. These abundant food resources are a major draw for migratory species crossing the strait. Species such as sperm whales, loggerhead sea turtles and shearwaters rely on the strait’s productive waters to restock energy reserves and feed intensively before continuing their lengthy migratory journeys.
Presence of major migratory highways
The Strait of Gibraltar overlaps with several major migratory highways used by birds, sea turtles, mammals and fish. The East Atlantic Flyway overlaps the strait, transporting millions of European and African migratory birds between arctic Europe and sub-Saharan Africa annually. The Strait of Gibraltar also overlaps with parts of the Black Sea-Mediterranean Flyway used by soaring birds like raptors and storks. For marine mammals, the strait crosses paths with the northern routes of blue whale and fin whale migrations. The combination of abundant food and its strategic position along these migratory highways make the Strait of Gibraltar a vital stopover point.
Shorter crossing distance
The Strait of Gibraltar is the shortest distance between the Atlantic and Mediterranean basins at only about 9 miles wide. While other passageways between the Atlantic and Mediterranean exist further east, these are significantly longer crossings. For example, the distance from Morocco to Spain is over 15 miles wide. For species like long-distance migratory birds, the shortest direct crossing possible at Gibraltar conserves valuable energy reserves needed to sustain lengthy migrations. This is especially critical for small songbirds migrating across the Sahara desert. Minimizing distance travelled by using the Strait of Gibraltar can mean the difference between surviving the journey or perishing.
Sheltered habitat
Despite its relatively narrow width, the Strait of Gibraltar is deep, ranging from over 900 meters to 3000 meters at its deepest point. The surrounding coastlines provide shelter from wind and waves for migratory species crossing between the Atlantic and Mediterranean. Dolphins, whales, and seabirds can rest in the calm sheltered waters of the strait before traversing the open oceans on either side. The strait’s sheltered channels and coves also serve as temporary feeding grounds for sharks, tuna and sea turtles where they can conserve energy. Having this sheltered aquatic habitat along the migratory route provides valuable respite from the elements.
Key Migratory Species
A diverse array of species rely on the Strait of Gibraltar during migrations between the Atlantic Ocean and Mediterranean Sea. Here are some of the key migratory species that use the strait and why it is so critical to their life cycles:
European Eel
The European eel (Anguilla anguilla) spawns in the Sargasso Sea in the Atlantic Ocean but spends its adult life in freshwater and estuarine habitats across Europe and North Africa. After spawning in the Atlantic, the eel larvae drift on ocean currents for 1-3 years back towards Europe. The Strait of Gibraltar represents the entry point for larvae back to coastal feeding grounds in the Mediterranean Sea. Without this passage, eels would not be able to migrate between their spawning and feeding grounds. Declining eel populations have highlighted the importance of the strait for this species.
Tunas
Multiple tuna species like Atlantic bluefin tuna (Thunnus thynnus), albacore (Thunnus alalunga) and bullet tuna (Auxis rochei) utilize the Strait of Gibraltar during annual spawning migrations between the Mediterranean and Atlantic. Although tuna are highly migratory and have many spawning areas, the Strait of Gibraltar passage allows access to important Mediterranean spawning sites and feeding areas in the Atlantic. Maintaining migratory connectivity through the strait is vital for sustaining Atlantic and Mediterranean tuna populations.
Loggerhead Turtle
The loggerhead turtle (Caretta caretta) undertakes one of the longest turtle migrations in the world, travelling between feeding areas off North America and breeding areas in the Mediterranean. While some follow longer routes around Africa, major loggerhead migratory routes pass through the Strait of Gibraltar to reach critical Mediterranean nesting beaches. Threats in the strait like marine traffic, fishing gear entanglement and coastal development can impact loggerhead migrations between the Atlantic and Mediterranean.
Osprey
The osprey (Pandion haliaetus) is a large fish-eating bird of prey that breeds across Europe before migrating down the African coast to wintering grounds in West Africa. Like many migratory raptors, the osprey utilizes thermals and updrafts to conserve energy during long distance flights. The Strait of Gibraltar is a critical crossing point where ospreys concentrate in large numbers to take advantage of updrafts near the rocky coastlines that provide free lift for gliding over the water.
Whales and Dolphins
Many whales and dolphins migrate through the Strait of Gibraltar including sperm whales, pilot whales, false killer whales and common dolphins. For some species like the endangered sperm whale, the strait represents one of the last viable migratory passages linking Atlantic and Mediterranean populations. Vessel strikes, underwater noise pollution and entanglement in fishing gear pose major risks in the strait that can disrupt whale and dolphin migrations between critical feeding and breeding habitats.
Threats to Migratory Species
While the Strait of Gibraltar provides vital migratory passage, there are also substantial threats in this relatively small area that impact migrating species:
Marine Traffic
Over 100,000 ships including large commercial vessels transit the Strait of Gibraltar each year, making it one of the busiest shipping channels in the world. The narrow passage means ships must follow established lanes, increasing risks of vessel strikes with whales and collisions with migrating seabirds. Underwater noise from ships traversing the strait can disrupt marine mammal communications and migration patterns. Anchoring and accidental oil spills from vessels also degrade strait habitats.
Overfishing
Intense fishing pressure occurs in the Strait of Gibraltar from both commercial and artisanal fleets. Declines in prey species like sardines due to overfishing impacts food availability for migrating tuna, sharks, whales and seabirds crossing the strait. Drift nets and longlines used in the strait to catch swordfish and tuna entangle turtles, dolphins, and seabirds. Illegal fishing for the endangered bluefin tuna also occurs within the strait.
Pollution
Pollution from industry, urban areas and shipping contaminates areas of the Strait of Gibraltar. Oil spills are a major hazard, while inputs of industrial chemicals, sewage discharge and plastic pollution from two densely populated coastlines also degrade water quality and habitats. High concentrations of pollutants are consumed by fish and zooplankton, which are then eaten by migratory predators. Toxins can accumulate up food webs impacting migration, health and survival.
Coastal Development
Expanding ports, industrial facilities, tourism resorts and infrastructure along the Strait of Gibraltar’s coastlines have destroyed wetlands, beaches and other critical habitats for nesting sea turtles and migratory shorebirds. Coastal barriers also fragment habitat, prevent inland movement of eels, and degrade resting areas used by whales and dolphins. Dredging the seafloor for port facilities and sand mining damages submarine feeding habitats.
Climate Change
Climate change and warming waters in the Mediterranean Sea are impacting migratory timings, distributions and survival for species crossing the Strait of Gibraltar. Changing ocean conditions have caused declines in small pelagic fish like sardines that many migrators depend on for food. Rising sea levels are expected to inundate critical coastal wetlands used during migration. Gibraltarʼs role as a climatic boundary between Atlantic and Mediterranean further highlights its sensitivity to climate change.
Conservation Efforts
A number of conservation initiatives aimed at protecting migratory species focus on habitat protection, reducing threats, and promoting international cooperation in the Strait of Gibraltar:
Strait of Gibraltar Special Area of Conservation
This designated marine protected area covers over 5,000 square kilometers of coastline and ocean habitat around the strait under EU Habitats Directive. It conserves important feeding and resting areas for whales, dolphins, sharks, sea turtles and seabirds. Management aims to reduce threats from ship traffic, construction, fishing and pollution. Monitoring tracks population trends in indicator species.
Vessel Speed Reductions
Speed limits imposed on ships sailing through the strait aim to reduce risks of vessel strikes with whales and noise disturbance. Slower vessel speeds also decrease emissions and chance of spills. Studies show speed restrictions in the strait have significantly reduced collisions with large whales over past decades. Ongoing monitoring helps target key areas and times for speed controls.
Fishing Gear Modifications
Changes to fishing techniques like weighted longlines and breakaway hooks help reduce bycatch and entanglements of dolphins, turtles and seabirds in the strait. Seasonal closures have also been established for driftnet and longline fisheries during peak migration periods. Declining fish stocks have also forced reductions in legal fishing quotas aimed at promoting sustainability.
Pollution Control Policies
Stricter policies regulate pollution, dumping and waste disposal from ships and coastal towns bordering the strait. Improved port waste facilities collect ship garbage and oil to prevent marine dumping. Stormwater runoff treatment, sewage system upgrades, and relocation of heavy industry has also decreased coastal pollution loading into the strait.
Migration Research
Ongoing tracking studies of sea turtles, tuna, whales, sharks and seabirds in the strait improves understanding of migratory routes, timing, threats and population changes. Acoustic monitoring identifies key areas, seasons and environmental conditions during migration. Tagging, genetic studies and surveys also collect critical data to focus conservation efforts.
International and Regional Agreements
The Strait of Gibraltar falls under numerous international conventions aimed at protecting migratory species including MARPOL, UNCLOS, Barcelona Convention, ACCOBAMS, CMS, Bern Convention, CITES and the European Unionʼs Marine Strategy Framework Directive. Cooperation between Spain, UK, Morocco, and the EU coordinates policy and enforcement.
Outlook for the Future
The long-term outlook for migratory species using the Strait of Gibraltar is concerning. Major ongoing threats from shipping, fishing and coastal development will intensify with increasing Mediterranean port expansions and a growing global shipping fleet. However, decades of conservation efforts focused on the strait by governments, NGOs and researchers provide some hope. Continued monitoring, threat reduction policies, habitat conservation, and regional planning can help sustain this critical migratory bottleneck. But protection and political will remain essential to conserving migratory connectivity between the Atlantic and Mediterranean via the Strait of Gibraltar. Ongoing global challenges like climate change and marine pollution will further pressure the straitʼs migratory species. The uniqueness of the Strait of Gibraltar for both terrestrial and marine migrations means the stakes are high should this delicate passageway be lost. As the human footprint expands along its shores, wise stewardship of the strait is crucial for maintaining age-old migratory journeys.
Conclusion
The narrow Strait of Gibraltar is a vital migration bottleneck and habitat for hundreds of bird, fish and marine mammal species migrating between the Atlantic Ocean and Mediterranean Sea. Its strategic geographic location along major east-west migratory flyways and waterways makes it a critical crossing point. Prey-rich waters, shelter from the open ocean, and its short span bridging two major basins make the strait an optimal migration corridor. Iconic species ranging from tuna to whales rely on safe passage through the strait for their annual life cycles. However, rising threats from marine traffic, overfishing, pollution, coastal development and climate change increasingly impact migratory species. Conservation efforts through marine protected areas, fishing limits, vessel speed restrictions, research and regional cooperation aim to safeguard the straitʼs irreplaceable importance for global migrations. But continuing habitat loss, emerging threats and lack of comprehensive protections paint an uncertain future for the Strait of Gibraltar and the multitudes that journey through its waters.
