A core part of our program is a large-scale recruitment monitoring effort. Recruitment monitoring quantifies the onshore consequence of patterns of larval transport in the nearshore coastal environment. Long-term quantification of recruitment is essential if we are to distinguish long-term trends from natural annual and seasonal variability. To quantify rates of input of new individuals and detect temporal trends in these rates, we measure recruitment of key shallow-water species (including mussels, barnacles, sea stars, crabs, sea urchins, fishes, limpets, hermit crabs and macroalgae). We use a variety of tried-and-tested methods successfully employed by PISCO PIs and their students and post-docs. At all of our core sites in the rocky intertidal we have recruitment collectors, which are typically sampled on a monthly to seasonal time scale.
PISCO has documented large spatial and temporal patterns in the recruitment of invertebrates to rocky shores along the west coast. In general, recruitment of barnacles and mussels is positively associated with warm waters and is notably elevated during warm-water El Niño periods, and declines in cold-water La Niña periods. The generally positive association between recruitment and temperature is also reflected in the seasonal signature of recruitment, with peaks in late summer-early fall in Oregon, early spring in northern-central California, and summer in southern California. The spatial patterns of recruitment along the west coast are extremely sharp for some species -- The barnacle Balanus glandula and mussels (Mytilus spp.) showed a decline of two orders of magnitude south of Oregon. In contrast, recruitment rates of barnacles of the genus Chthamalus showed a variable pattern across the west coast. In Oregon, periodic current reversals are associated with wind reversals. We have documented that these reversals are usually associated with barnacle recruitment events on the shore. In the Santa Barbara Channel, a persistent mid-channel eddy retains fish larvae, and may also prevent pre-settlement invertebrate larvae from moving beyond Point Conception. Such eddies may play an important role in transporting larvae across large scales and preventing retention.