Lac Castérau

Lac Castérau is a small mountain lake in the Ossau Valley, in the commune of Laruns (Pyrénées-Atlantiques, France), and one of the six lakes making up the well-known Ayous lakes group. Alongside Lac Gentau, Lac Bersau, Lac du Miey, Lac Paradis and Lac Roumassot, it forms part of a loop that draws hikers throughout the summer season. Castérau sits furthest into the massif, at the foot of the peak that shares its name, and marks one of the last stops on the classic tour of the Ayous lakes.

Where exactly is Lac Castérau?

The lake lies at roughly 1,943 m of elevation, within the French Pyrenees, close to the iconic Pic du Midi d'Ossau. Rising above its eastern edge is Pic Castérau (2,227 m), which lends its name to both the summit and the lake below. The area falls administratively within the commune of Laruns, and the usual starting point for reaching it is the car park at the Bious-Artigues dam, sitting at around 1,422 m.

What does the lake itself look like?

Castérau is an oligotrophic lake: clear water, a rocky, stony bed, and sparse aquatic vegetation, characteristic of the glacial tarns scattered across this part of the range. Its surface area is modest, a little over a hectare, making it one of the smaller lakes in the Ayous group. Despite its size, it holds a population of brown trout (truite fario), though anglers are required to release any catch back into the water and use a barbless hook, or one with the barb flattened.

How do you get to Lac Castérau?

Most visitors reach it on foot, as part of the loop hike around the Ayous lakes that starts from the Bious-Artigues dam car park. The trail passes Lake Roumassot, Lake Miey, Lake Gentau and Lake Bersau before reaching Castérau, which sits closer to the far end of the circuit. Walking from the car park up to the lakes area typically takes around three hours, depending on pace and the exact route chosen. During the summer, access to the Bious-Artigues car park is sometimes restricted, with a shuttle service running from Laruns to ease congestion on the narrow approach road.

What views does the area offer?

The main draw here is the view of the north face of Pic du Midi d'Ossau, the sharp, unmistakable peak visible from most of the lakes in the group. From Castérau, other ridgelines enclosing the valley also come into view, including the more distant Pic des Moines. The surroundings feel rockier and starker than around the lower-lying Gentau or Bersau, giving Castérau the character of the most remote stop along the route.

Is it worth stopping here?

Lac Castérau is often bypassed by hikers rushing toward the postcard view at Lac Gentau, which tends to leave it quieter and less crowded. For those walking the full Ayous loop, it makes a natural last stop before heading back toward the car park — a good spot to rest away from the busiest stretch of the trail.

FAQ

Can you swim in Lac Castérau? The water stays cold year-round, typical of glacial lakes sitting near 2,000 m, and the bed is rocky — this is a spot for walking and taking in the view rather than swimming.

How long does it take to reach the lake? From the Bious-Artigues car park, reaching the Ayous lakes area, including Castérau, usually takes around three hours, depending on fitness and the chosen route.

Is fishing allowed? Yes, the lake holds brown trout, but any catch must be released back into the water, and only barbless hooks (or ones with the barb crushed) are permitted.

Is it close to the other Ayous lakes? Yes, Castérau is one of six lakes in the group, along with Gentau, Bersau, Miey, Paradis and Roumassot, all linked by the same loop trail.

How do you get to the trailhead? The starting point is the car park at the Bious-Artigues dam in the commune of Laruns; access can be regulated in summer, with a local shuttle bus from Laruns offered as an alternative.