DOI: 10.20350/digitalcsic/17059
title: Habitat complexity and fish data from restored and natural Eelgrass meadows from Sweden

description: [Description of methods used for collection/generation of data] The structural complexity of he natural and restored eelgrass (Zostera marina) meadows was quantified through three metrics: shoot density (shoots/m2), dry shoot biomass (g/m2), and canopy height (cm). Samples were collected in-situ at the planted areas and the interior part of the reference meadows by snorkeling divers in June 2023. Ten and five replicates of 452 cm2 were sampled in Askerön and Gåsö, respectively, in natural and restored Z. marina meadows by placing a 24 cm diameter bucket over the sediment and collecting by hand all the eelgrass shoots inside. Shoots were then placed in a plastic bag and transported to the laboratory. All shoots were counted, and the length of each leaf within a shoot was measured in ten shoots per each habitat and site. Subsequently, canopy height was estimated as 80% of the average maximum leaf length per shoot. All collected shoots were dried at 60ºC for 48 hours and weighed to quantify dry shoot biomass. Fish communities were surveyed on transects by snorkeling divers using a stereo-video system (Stereo Diver Operated Video, Stereo-DOV). Simultaneously, the divers used the traditional method of Underwater Visual Census (UVC) to record all fish observed on an underwater pad. Each transect lasted 7 minutes, covering an average of 140 m2 (70 x 2 m), and was separated to the next transect by at least 10 m. The samplings took place during daylight hours within the two study areas in June 2023. In Askerön, a total of 6 transects for each method were performed on each habitat (i.e. edge, meadow and sand) except for the replanted habitat, where 10 transects were performed. In Gåsö, 9 transects for each method were performed on both the meadow and edge habitats and 13 on each of the replanted and sand habitats. Each observed fish with both methods was identified to the lowest taxonomic level (species level when possible). With Stereo-DOV fish were measured for total length (TL) to enable subsequent biomass estimations. However, with UVC fish were only counted, and their TL was not estimated with this method.
description: [Methods for processing the data] The stereo pairs of recorded videos were analyzed with EventMeasure software, where fish TL was measured by marking each fish from the tip of the snout to the tip of the tail in both videos (left and right). To ensure accuracy and precision to the millimeter level, fish lengths were only measured when individuals were at less than 7 m from the cameras and exhibited one of their body sides straight to the cameras. Fish individuals that did not meet these criteria were excluded from measurement and only identified and counted.
description: This dataset includes structural complexity and fish data from June 2023 from two areas in the West coast of Sweden (Skagerrak): 1) Bay of Askerön; and 2) Bay in the island of Gåsö; where two eelgrass (Zostera marina) plantations were conducted in 2021 and 2019 respectively. The samplings took place during daylight hours within the two study areas in June 2023. The structural complexity of the restored and natural (reference) eelgrass meadows was quantified through three metrics: shoot density (shoots/m2), dry shoot biomass (g/m2), and canopy height (cm). Fish communities were surveyed on transects by snorkeling divers using a stereo-video system (Stereo-DOV) and Underwater Visual Census (UVC) simultaneously. Each transect lasted 7 minutes, covering an average of 140 m2 (70 x 2 m). Each observed fish with both methods was identified to the species level when possible. With Stereo-DOV fish were also measured for total length (TL) to enable subsequent biomass estimations.
description: Castro-Fernández_etal_2025_DATASET_EELGRASS.xlsx