BIM survey on seed mussel in the Blackwater/Cahore Point and Rusk Channel Area shows beds could be under threat from starfish
A BIM survey conducted in relation to seed mussel in the Blackwater/Cahore Point and Rusk Channel Area between 21 July and 07 August 2020 showed that one of the distinct beds could be under threat from starfish.
During the survey three partially distinct beds were found by the researchers who were using a 400 kHz side-scan sonar, 1 m dredge and a Go Pro camera for video transects.
Inshore Survey Officer Nicolas Chopin was head of the project that examined the previously productive mussels beds in the Blackwater Bank and shore, south of Cahore Point and in the Rusk Channel.
On the Blackwater Bank and shore, despite extensive searches, including using=g side-scan along with condusting 19 tows, no mussels seeds were found. Locations of beds from previous years along the west side of the Blackwater were checked, but only old oyster shells and a mix of bryozoans and hydroids were found. Tows were made around the 2013 seed mussel settlement closer to shore, but again only stones and a mix of bryozoans and hydroids were found. More data was collected along the shore next to Ballyvaldon where half-grown mussels were found in 2019, but only starfish and some tunicates were observed.
In the Rusk Channel following reports from the industry, 3 small beds were found in the Rusk Channel over 4 km long. Seed coverage is not consistent over this distance, and there is a large gap between the south bed and the middle one. In total, the seed is covering around 40 hectares.
The south bed has smaller mussels than the other two, and they appear to be in high density; the dredge contained 50 kg of loose seed, a lot of byssus and silt typical of high-density settlements (Liu et al., 2012). The average size in TC 19 was 16 mm (min:7.32 mm, max: 23.76 mm) representing 2900 pieces per kg. The waste represented 24% of the total weight, and it was mainly composed of byssus. 49% of the sampled mussels were between 18 and 20 mm at this location. The size range in TC19 could indicate multiple settlement times. The seed in this area has a light brittle shell. Few starfish were found in the area. No ground coverage assessment was carried out due to high turbidity and strong tidal current.
The middle bed is composed of slightly larger mussels, and again density appears to be high (patterns from the sonar, the content of the dredges and video transect). The average size in TC16 was 21 mm (min: 5.38 mm, max: 32.4 mm) representing 985 Pieces per kg. The waste represented 24% of the total weight (70 kg in the dredge) and mainly composed of few starfish and coarse sediment. The video transect from the centre of the bed (close to TR30) showed near-uniform coverage and could be estimated to 80%. A significant quantity of starfish was observed through the transect, although this was not replicated in the various tows carried out in this bed. 66% of the sampled mussel were comprised between 16 and 20 mm.
The North bed appears to be an extension of the main centre bed but separated by a sand ridge. This extension doesn’t appear to be as dense as the other beds. The results from TC28 were as followed: average size 19.83 mm (min: 6.88, max: 31.78mm), representing 1375 Pieces per kg. 44% of the sampled mussels were comprised between 24 and 28 mm, and the range of sizes observed could also indicate multiple settlement times. The waste was slightly higher than the other two beds, reaching 29 %, mainly due to stones and coarse sediment. No video transect was carried out due to strong tidal current. However, patterns observed on the side-scan sonar data appeared similar to the ones seen on the south bed.
On Cahore Point Shore Extensive side-scan sonar data was collected along the shore on previously known settlement area and few features were investigated, but no mussels were found. Only bryozoans, hydroids and stones were observed in the dredges.
Samples indicated a wide size range through the three beds. The centre bed could potentially be under threat from starfish, although, little to no predation was observed throughout the samples. At the time of the survey, it was too early to carry out robust and reliable biomass or tonnage estimation. However, considering the beds already found outside Wexford and Rosslare, BIM is confident that the required tonnage for the fishery to open is already available. Tonnage estimation and alien species survey will be carried out on the known beds starting on the week of the 17th of August 2020. Considering the current situation (Covid-19 pandemic and potential expanding predation on the seed beds), the fishery should be opening as soon as possible.
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