Fish and underwater species
The second largest natural lake in France, Lake Cazaux-Sanguinet is home to an interesting aquatic and piscicultural biodiversity despite a relatively low primary production.
Formation, geology and lake featuresLike its neighbors on the Aquitaine coast, Lake Cazaux – Sanguinet was born following the formation of coastal dunes. The combined action of currents in the Bay of Biscay, westerly winds and the supply of alluvium by the rivers contributed to the obstruction of the mouths. In the case of Lake Cazaux – Sanguinet, the water of the Gourgue was trapped in these dune dams resulting in the accumulation of fresh water and the formation of the lake. The formation of the dune cord occurs gradually from the end of the last ice age (-8000 years).
On the Aquitaine coast, only the Arcachon basin is not cut off from the waters following these geological phenomena, notably due to the higher flow of the Leyre river.
The gradual formation of the Landes lakes explains the nature of the sandy soil found on the surface. This sand, characteristic of the Landes, would come from the erosion of the Pyrenean massif which was mixed with other minerals from the volcanoes of Auvergne and transported by the Dordogne.
Alios and garluche are also strongly present in the Landes subsoil. Although of different formation, these are sandstones formed by sand and iron oxide. The garluche having a higher iron level. The latter has for a long time served as a building material in the Landes.
Lake Cazaux – Sanguinet is today fed by two main tributary streams: the Gourgue and the Larreillet canal. It has two outlets. To the north, via the canal des Landes (or “transaquitain”) which connects it to the Bassin d’Arcachon, and whose management of the hydraulic works is performed by the Air Base 120. To the south, always via the channel transaquitain, where the lock is managed by the Great Lakes Community of Communes. To the south, it feeds in turn lakes of Parentis – Biscarrosse and Aureilhan.
Unlike other large lakes in the Landes, Lake Cazaux – Sanguinet is oligotrophic, which means that primary production is low and the lake is therefore relatively poor in nutrients. This particular state is often favorable to the development of a specific biodiversity (especially plants), and it does not prevent a significant fish fauna.
Food chainsWithin an ecosystem, the diversity of living things and the interactions between these living things are largely influenced by their position within the food web, which describes nutrition relationships.
The food web is broken down into food chains, that is, sequences of living organisms where everyone feeds on lower trophic level organisms. Example of a food chain
At the base of this network, producers are represented by algae and phytoplankton. Through photosynthesis, these organisms are able to produce organic matter from inorganic matter.
Primary consumers will eat algae and phytoplankton before being prey for secondary consumers . At the end of the chain, the decomposers will transform the dead organic matter to return it in mineral form.
FishDespite its oligotrophic nature, the lake of Cazaux-Sanguinet benefits from a non-negligible fish fauna with a large diversity. The majority of species correspond to the species present in the “bream zone”, characterized by calm and deep waters, with significant vegetation.
For forage fish, it is mainly roach, rudd or bream to which is added the presence of ruffles and dowels. Tench and carp are also present.
Concerning predators, perch and pike are strongly present. Zander, eel and, to a lesser extent, black bass also swim in the lake waters.
A large number of species come from more or less recent introductions. Of these, some are considered invasivep pumpkinseed, catfish, gambusie.
Other species have been observed more occasionally: loach, stickleback or stickleback in particular.
Here is a video underwater presenting some species of the lake.
Among predators, pike is probably the most emblematic. On the one hand, it is highly sought after by fishermen who catch large specimens on the lake, and on the other hand, it is the carnivorous naturally present in these waters …
The pikeThe pike is undoubtedly the carnivorous most emblematic of our waters. Its measurements (regularly able to exceed 1 meter and 10kg), its characteristics (about 700 teeth) as well as its behavior of hunter fish generally solitary contributed to feed the imagination of the fishermen and men in general … Its capacity to attack preys of consistent size (ducklings, small rodents) have even forged a reputation of “voracious monster” which we know today is very exaggerated.
This elongated fish, with greenish-yellow flanks and white-tipped under the belly, likes calm or low-current, grassy waters, and can also acclimatize to brackish water. This is particularly the case in the Baltic Sea where the pike population is important.
Breeding occurs between February and April, when the temperature approaches 10 to 12 degrees Celcius. Eggs are deposited on grass beds near the banks in depths between 20 and 80cm. This mode of reproduction makes it particularly vulnerable since nesting areas must remain immersed beyond hatching. Thus, variations in water levels (related to human activities or global warming) and the artificialisation of riverbanks can have serious consequences for pike populations.
If it has long been considered the existence of a single species of pike (Esox lucius), a recent study has shown the existence of several species . Among them, an endemic species in South West France: Esox aquitanicus! This Aquitaine pike, very close to Esox lucius, can be difficult to identify at first glance: marbled dress, shorter snout, less scales on the lateral line.
Its presence on the Lac de Cazaux Sanguinet is however not established and remains uncertain. The many fingerlings from the Esox lucius strain could have harmed the Aquitaine pike populations. The maintenance of Esox aquitanicus populations seems more likely in small streams, especially in the first category. His presence is particularly proven in the Ciron river.