Eight-fjords shallow underwater videos

Study Extent

At each site, an average of 4 recordings were made, with each camera recording video for 1 hour, resulting in a total recording time of 4 hours per site. Records in this data are per video. Each video is fitted with a unique identifier.

Sampling

Sampling with video cameras is a relatively common method for investigating flora and fauna in marine environments. When it comes to fish census, baited camera systems are often used (abbreviated as BRUV after the English "Baited Remote Underwater Video"), which can be placed on the seabed or freely suspended in the water column with the help of a buoy on the surface (see e.g., Sherman et al., 2020, and Cambra et al., 2021). An advantage of these systems compared to, for example, ROVs (remotely operated underwater vehicles) or drop-video (a type of "camera sled" dragged across the seabed) is that they are stationary and do not scare away fish through movements and sounds. However, this means that the camera covers a smaller area of water, and therefore bait is used to attract the fish nearby to move in front of the camera. The bait used is often scented food such as fish scraps and shrimp. For the detection of cryptobenthic fish (small, bottom-dwelling species that often hide in crevices or among vegetation), the BRUV method is still in the developmental stage. Because such small fish (especially bullheads) can be difficult to distinguish and identify on video, it is valuable to design camera systems that visualize their characteristics as effectively as possible. In this study, a system with neutral-colored "background boards" has been used, which the fish need to swim in front of to reach the bait. This way, characteristics such as color, pattern, and fin shape are visualized, improving the possibility of species identification. Video cameras baited with frozen shrimp (4 per camera, approximately 45 grams each in wet weight) were placed on the seabed either directly from a pier/dock when possible, or with the help of snorkelling. Each camera system was placed at a minimum distance of 30 meters from each other to avoid fish moving between the cameras during filming. The depth at which the cameras were placed varied between 0.6 – 3.2 meters. To control for lighting conditions, the video rig was always oriented so that the camera filmed northward (and thus received ample light against the background).

Method steps

1. The team followed the methodology described in the SUBSIM software to analyse and publish the occurrences

All fish were identified to species or family level, Brachyuran crabs were identified to species level.

1. Carcinus maenas
   common name: European Green Crab rank: species
2. Ctenolabrus rupestris
   common name: Goldsinny Wrasse rank: species
3. Gobius niger
   common name: Black Goby rank: species
4. Neogobius melanostomus
   common name: Round Goby rank: species

These records cover shallow bays in Sweden's first test-bed for marine ecosystem-based management: The 8-fjords+ area. This area is a freshwater-influenced fjord system containing multiple Natura-2000 sites and a range of habitats including shallow and deep soft and hard bottoms. Records in the dataset are from 3-1 meters in depth, mainly centred around small marinas.