This reserve, administered by Cecon (Centro de Estudios Conservacionistas de la Universidad de San Carlos), is Monterrico's biggest attraction. The 20km-long nature reserve of coast and coastal mangrove swamps is bursting with avian and aquatic life. The reserve's most famous denizens are the endangered leatherback and ridley turtles, which lay their eggs on the beach in many places along the coast. The mangrove swamps are a network of 25 lagoons, all connected by mangrove canals.
Boat tours of the reserve, passing through the mangrove swamps and visiting several lagoons, take 1½ to two hours and cost Q75 to Q100 per person. It's best to go just on sunrise, when you're likely to see the most wildlife. If you have binoculars, bring them along for birdwatching; January and February are the best months. Locals will approach you on the street (some with very impressive-looking ID cards) offering tours, and you can arrange tours through hotels, but if you want to support the tortugario arrange a tour directly through Tortugario Monterrico, which, incidentally, has the most environmentally knowledgeable guides.
Some travelers have griped about the use of motorboats because the sound of the motor scares off the wildlife. If you're under no time pressure, ask about arranging a paddled tour of the canal.