This nature reserve operated by Arcas (Asociación de Rescate y Conservación de Vida Silvestre) comprises a sea-turtle hatchery and some caimans, 8km east along the beach from Monterrico. The reserve is separate from the neighboring Biotopo Monterrico-Hawaii, but engages in the same line of conservation. Volunteers are welcome year round, but the sea-turtle nesting season is from June to November, with August and September being the peak months.
Volunteers are charged Q1330 a week for food and board onsite at the project. Homestays are available with local families for around the same cost. Jobs for volunteers include hatchery checks and maintenance, mangrove reforestation, basic construction and data collection. See the website for the complete lowdown on volunteering here. Most of the egg collection happens at night. It's a way out of town, but there are usually other volunteers to keep you company, and while you're here you can use the kayaks, go on village trips and go fishing in the sea and mangroves.
A bus (Q5, 30 minutes) leaves the Monterrico jetty every couple of hours during the week and every hour on weekends for the bumpy ride to the reserve. Pick-ups also operate on this route, charging Q35 per person.