Dataset: Urchin Barren Characterization Data before Installment of Experimental Pycnopodia Cages in the Sitka Sound during February 2023 (High latitude kelp dynamics project)

Surveys

Three urchin barren sites, Ellsworth Cut (57.036, -135.280), Harris Island (57.033, -135.277), and Whale Park (57.033, -135.255), were identified as potential sites for Pycnopodia experiments because they all had transitioned in the last several years from kelp-dominated habitats to barren. Sites were qualitatively similar in rugosity (low rugosity), substrate (bedrock, with some boulder and cobbles), and depth (6-9m). In order to better characterize the shifting state of these subtidal environments before experimental manipulation, we surveyed the density and size structure of dominant grazers (H. kamtschatkana, M. franciscanus and S. droebachiensis) at each site using a swath approach on SCUBA (2 x 30m at Ellsworth Cut and Harris Island, and 2 x 15m at Whale Point). Urchin test diameters and longest abalone shell lengths were measured with calipers.

Gonad Indices

We also compared the gonad indices of randomly collected red sea urchins (M. franciscanus) at our three sites to an intact kelp forest reference site (Magic Island; 57.098, -135.401) about ~12 km away from the nearest experimental site. We measured the gonad index of M. franciscanus individuals from each site (N = 9-11) as: [wet mass of gonadal tissue/ total mass of the organism]*100.

Analysis

Mean and standard deviation of density and size were calculated for each species at all three barren sites with each 10m2 block of the swath surveys acting as a replicate. H. kamtchatkana was dropped from analysis for low replication. Two sample independent t-tests were used to compare means and variances between urchin species groups for density and size across sites. Gonad index was calculated and averaged for each barren site and the singular kelp forest site. T-tests were similarly used to compare average gonad content in barren sites versus the kelp forest site.