The Islas Medas marine reserve is one of the strongest arguments for a trip to the Costa Brava. The catch is that the destination really peaks during a narrow window: September. Summer brings crowds, October drops the temperature fast. Between the two, two or three weeks when the water holds its warmth, dusky groupers leave their caves and serious divers book their slots early.
The Islas Medas are seven rocky islets off L'Estartit, designated a fully protected marine reserve in 1990. That means over 30 years without fishing inside the perimeter, and the effect on marine life is obvious within minutes of the first dive. Fish density that most parts of the Mediterranean have not seen for half a century. It is, by most accounts, the finest marine reserve in the western Mediterranean.
Why September. In July and August the water is warm — 24–25 °C — but there are so many divers that descending feels like queuing underwater. By October the thermocline drops to 14 °C within days and the fauna retreats. September is the sweet spot: surface temperatures still at 22–23 °C, around 20 °C on the bottom, visibility typically 15–25 m, and dive centres with open slots because family tourism has wound down.
The dusky grouper (*Epinephelus marginatus*) is the headline. More than 1,000 individuals have been counted at the Medas, some exceeding 30 kg. A dive on La Vaca or Tasco Petit at 18–22 m makes an encounter virtually certain. The large territorial males hold their ground: they approach to within centimetres and occasionally seem to pose for the camera. Divers who have only dived outside marine reserves find the contrast stark — here the fish do not run.
Five sites account for most of the dive day traffic. La Vaca and La Vaca Gegant: walls with caves and outsized groupers. El Reconcó: ideal for beginners, with abundant life across sand and posidonia meadows. La Cova de la Vaca: a through-cave at 16 m with extraordinary overhead light. Tasco Gros: a platform holding big meagre. Centres allocate spots by certification level; if you have the experience, ask for La Vaca or skip the trip.
Logistics matter here: the Medas have a daily cap. The Generalitat de Catalunya limits the site to 450 divers per day in high season, shared among three main centres in L'Estartit. Booking two to three weeks ahead in September is the sensible move. Guided dives with kit run roughly €50–65, with four-dive packages over two days around €200.
What most trip reports leave out: the Medas can have terrible visibility. After a heavy tramuntana (northerly) or sustained rainfall, sediment from the mouth of the río Ter — just 3 km away — clouds the water. You descend to 18 m in 5 m visibility and the magic disappears. Checking forecasts before travelling avoids the disappointment. Ideally, three consecutive days of southerly wind or flat calm.
For visiting divers looking for accommodation: skip L'Estartit. La Bisbal d'Empordà, 15 minutes away, or Pals, 20 minutes out, are beautiful medieval towns with better restaurants at half the price. L'Estartit itself is a functional family-holiday resort from the 1980s, pleasant but unremarkable. The dive centres are there, so you drive in, dive, and leave.
After eight trips to the Medas, the conclusion is consistent: for three September days in the Spanish Mediterranean, this is the one. Cabo de Palos competes; Columbretes delivers more pelagic action; but the Medas at warm-water temperatures with good visibility have no real equal. Range of sites, easy access, solid infrastructure, and groupers that close the distance on their own terms. For a first visit to a Mediterranean marine reserve, this is the reliable choice.