Dataset: Number and sizes of abalone and urchin recorded during dive surveys at specially designed recruitment modules (RMs) deployed at rocky subtidal reef at Van Damme State Park, Mendocino County, California, USA from August 2001 to August 2023

Data were collected during day-trips aboard the California Department of Fish and Wildlife or UC Davis boats, generally 2-3 days to sample the twelve RMs.

We used specially designed recruitment modules (RMs) bolted into the rocky subtidal reef to sample recruitment of larger juvenile abalone and sea urchins (3 to 20 millimeters (mm)). Standardized RMs were deployed at Van Damme State Park (VDSP) (latitude 39.270686, longitude -123.793250), northern California in August 2000. RMs were made of stainless steel cages filled with concrete cinder blocks cut lengthwise resembling the letter M (rectangles measuring 813 mm x 610 mm x 305 mm high) and stacked three levels tall. Each RM had a surface area of approximately 4.8 square meters (m²), of which 2.6 m² was a sheltered area created by the sides, top, and bottom of each M shape.

Twelve RMs were deployed between 10 and 12 meters (m) depth in three groups of four. The modules were placed 3-6 m apart within the cluster of four. The northern cluster was approximately 140 m from the central cluster. The southern cluster was 710 m from the central cluster.

RMs were deployed and left to season for one year before the first surveys began in August 2001. Each year thereafter in August the RM would be surveyed and if any half cinder blocks were missing they would be replaced. In the year of 2014, half the RM cages were replaced with new cages and in the year 2015, the remaining 6 modules were replaced. In each case, the existing bricks were reused in the same module. In some years fewer than 12 RM were surveyed, due to older RM cages being damaged by winter storms. Each year the numbers of invertebrates are reported per module. During the years 2020 and 2021, no RM surveys were conducted due to field work constraints imposed to reduce the spread of COVID.

The surveys involved one dive team for each RM, opening the lid and carefully removing each block, then counting and measuring all the invertebrates inside the RM. Organisms outside the cage were not sampled. Organisms too numerous to count were gently swept into fine mesh bags (2 mm) to be counted and measured at the surface. Organisms cemented to the brick were measured and quantified underwater. Each block was set to the side while the next block was removed, and the animals quantified. When a survey was completed, divers reassembled the RM and replaced the lid, securing it with wide cable ties. Each RM took two divers approximately 40-50 minutes to survey depending on the contents.