Grassholm Island lies 8 miles off the coast of Pembrokeshire, west of Skomer Island. It is a nature reserve and special protection area owned by the R.S.P.B since 1947. It is most notable for its Northern Gannet population, with approximately 39,000 pairs that have been growing since the mid-1800s, and continues to trend upwards. Early observations noted that the Gannet population periodically struggled largely due to the actions of egg collectors but also due to fishermen, tourists, shipwrecks and bad weather preventing the birds from landing. More recently, the avian flu nearly halved the gannet population, which has since recovered. There was a large fire on the island due to tourist litter, in June 1930. Oil pollution also was also prevalent around this time.

In addition to the large number of Gannets, other birds that can be seen around Grassholm island include Guillemots, Fulmars, Kittiwakes and Razorbills, similar to the bird populations of Skomer island. It is also a popular destination for tourist boat trips to see the Gannet colony and other animals such as Dolphins, Porpoise and Seals.

The island also faces an ongoing and increasing problem with plastic pollution. Plastic may be general litter as well as rope and fishing lines/nets, which float into the island or are brought there by the seabirds to use in their nests, around 80% of which contain plastic. This leads to the birds getting stuck in the plastic or ingesting it to themselves and their young. Despite R.S.P.B conservationist intervention, the plastic problem continues to grow as one of the largest threats to the Gannet colony.