The monitoring program at San Nicolas Island (SNI) was conceived in anticipation of the potential reintroduction of southern sea otters to the island. Subtidal monitoring was initiated in autumn of 1980 at six sampling sites positioned around the island to sample communities exposed to a range of wave exposures and other oceanographic conditions. Three additional sites were subsequently added. Following their reintroduction in 1987, the sea otter population at the island remained low for several years but 154 were counted there in 2019. There are nine permanent sites that are monitored on rocky reefs around San Nicolas Island. Six of these (Nav Fac, West End Urchin, West End Kelp, West Dutch Harbor, East Dutch Harbor, and Daytona) were installed and first sampled in 1980. The two West End sites are physically joined, with the end bolt of the main 50 m transect of the Urchin site being also the beginning of the Kelp site’s main transect. The Dutch Harbor sites are not contiguous but are on adjacent rock reefs separated by approximately 140 m of sand. A seventh site, Sandy Cove, was added in 1986. Data collected from the project’s inception in October 1980 to October 2011 were published in Kenner et al. (2013). In fall 2014, two additional sites were added that are contiguous with two of the original sites. North Nav Fac was installed with the end of its main transect line sharing the beginning of the Nav Fac site and South Daytona was added in a similar relation to the Daytona Site. This yielded eight paired sites or four “supersites” in addition to Sandy Cove. Most of the data is collected separately on the individual sites but can be pooled at the larger supersite level. The original sites consisted of a 50-m main transect with five 10-m x 2-m benthic band transects (swaths), ten 1-m2 random point contact (RPC) quadrats and five 50-m fish transects. The two new sites do not have fish transects associated with them but are otherwise identical. Temperature data and sea urchin and other invertebrate size data are collected at the supersite level. Sites have usually been sampled twice a year, in spring and fall since their installation but have been missed several times due to weather, budgetary constraints, or mechanical issues with boats. References: Kenner, M.C., Estes, J.A., Tinker, M.T., Bodkin, J.L., Cowen, R.K., Harrold, C., Hatfield, B.B., Novak, M., Rassweiler, A. and Reed, D.C., 2013. A multi-decade time series of kelp forest community structure at San Nicolas Island, California. Ecological Archives E, 94(11), p.244. https://doi.org/10.1890/13-0561R.1