Underwater Fauna of the Gironde Estuary in France.

The Gironde estuary in the department of Charente-Maritime in West of France consists of a mixture of saltwater and freshwater very beneficial for underwater fauna that is very specific. It is marked by all estuaries physical characteristics (turbidity, salinity) and biological. The waters are very nourishing for the underwater fauna. The Gironde estuary is the largest estuary system in Europe. It is also the most open to marine influences. Biodiversity, even if it is low colonized by species able to withstand high environmental variations, including salinity. Changes in salinity are primarily due to strong tides and currents very important.

An estuary is the portion of the mouth of a river where the effect of the sea or ocean in which it flows is noticeable. For some, it is all part of the river where the water is brackish or salt, for others, it is the presence of the dynamic effect of the tide on the river water that defines it. By convention, we do not speak of estuaries to rivers that flow into inland seas that have no tide. In the case of the Gironde estuary, it is rivers: the Garonne and Dordogne. It is 75 km long for 12 kilometers wide at the widest. It covers 635 km square.

The Garonne and Dordogne rivers bring from 800 to 1000 m3 / s freshwater sediment-laden and at the same time, twice a day the tide brings 15 000 to 25 000 m3 of seawater, which promotes the formation of sandbanks and islands of vasards. Meet the fresh water rich in alluvium, with salt water is flocculated clay particles that form a "turbidity maximum" characteristic of estuarine waters. Gironde carries each year from two to eight million tonnes of suspended particles. A portion of the suspended solids (1.5 to 3 million tons per year) is deposited, forming sandbars, and islands of vasards. The Gironde estuary is highly subject to the ebb and flow of the tides.

Another characteristic of the Gironde estuary north for the presence of a river Seudre. It fills and empties with the tides. The waters are then loaded silt and mud that come further fuel the ecosystem of the region so special.

These photos of fish were taken during the summer of 2012 at the mouth of the Gironde estuary off the city Royan. Diving conditions are very difficult because the currents are very violent. Divers can get water at the stall current. Temperatures hover around 16 degrees Celsius. The number of species even though it is far from reaching the tropical waters is still important with blennies, gobies, rays, crabs, crabs, eels, lobsters ... The main problem for underwater photographer and scuba diver is that visibility is completely unpredictable. It can reach 10 centimeters as 15 meters. Many statistics and measurements were made during the last ten years by divers but for now, it is impossible to predict water conditions.

It is a lottery!

Yet despite these specific conditions, the mouth of the Gironde estuary is one of the most exciting places we've photographed.

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Underwater fauna along the coast of Charente-Maritime in France. Photographs by Amar Guillen.

Mexico. Isla Mujeres. Sailfish Sardine Run

Each year during the winter season, nutrient rich currents flow from South America pushing its way north up onto the shallow shelf off of Isla Mujeres, Mexico drawing in large schools of sardines. Following these baitfish are great numbers of Atlantic sailfish (scientific name: Istiophorus albicans).

It is here in these shallow waters that marine life and predators collide in havoc. Locating the sailfish and sardines can take a combination of a few ingredients: lots of patience, persistence, years of experience, and one last ingredient: frigate birds (scientific name: Fregata Magnificens) swirling in a funnel cloud formation. Approaching their location the water surface looks like it is boiling from all the sardines breaking the surface.

The sailfish cooperate as a team to hold the bait ball together. Every time the bait ball changes direction it encounters another sailfish: that sailfish will through up it's sail-like dorsal fin to scare the bait ball into diverting in another direction. The sailfish herd the bait ball near the surface and try not to let it sink down deep or disperse.

When the bait ball gets close to the surface the frigate birds dive down grapping and gulping sardines, which pushes the mass down into a tighter bait ball. This team of sailfish are not all feeding at the same time: their method is to take turns individually going after the bait ball with astounding speed and precision maneuverability slashing the dense bait ball blindly with their bills knocking hapless individuals unconscious or wounding them enough to separate them from the safety of the bait ball. The isolated sardines are consumed within seconds on a second incredibly fast turn and pass by the sailfish. The next sailfish with a good position has its turn and the cycle starts again. The whole process continues unless the sardines perform enough evasive maneuvers to loose the sailfish or the predators are full and loose interest.

It is truly an amazing experience watching the sailfish work together as a team using precision, accuracy, and teamwork to control the bait ball, then demolish it.

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