Fisheries for razor clams (Ensis siliqua and Ensis arcuatus) have gone through a series of expansions in Scotland associated with changes in markets and the introduction of new harvesting techniques. Electrofishing has been recently introduced as an efficient method of forcing clams to the surface of the seabed where they can be collected by divers. Although electrofishing is illegal under European legislation the available evidence suggests that the method could be less impacting on the benthic habitat than alternate techniques, such as mechanical and hydraulic dredges. Electrofishing is also much more efficient than forcing the clams to emerge by pouring high concentration brine into the burrows (a technique known as ‘salting’). Concerns over the use of electrofishing therefore relate mainly to its efficiency and the health and safety risks of using the equipment itself (potential H&S risks are not dealt with in this report). Electrofishing therefore has the potential to lead to over-harvesting and depletion of razor clam beds, especially considering the slow growth rates of razor clams.
A further concern is that there have been very limited surveys of the stock status of Ensis beds around Scotland so that the impacts of harvesting would be difficult to assess. Historical stock assessments in Ireland and Scotland have mainly used dredges or salting followed by divers counting the emergent clams using quadrats . As mentioned previously dredges cause damage to the habitat, as well as to the clams themselves, whilst counting using divers is slow, expensive and samples relatively small areas. The present project was designed to evaluate whether combining an electrofishing rig with a towed video-camera array could be used to assess the razor clam resources in a bed in an efficient manner.
Eighteen tows of between 30-60 mins duration were conducted at three sites. Cameras mounted on a sled were towed 2 m behind a commercial electrofishing rig using the fishing vessel ‘Lizanna’. Tows were undertaken in the summer and autumn of 2016 at sites close to the Isle of Barra in the Outer Hebrides - south of the Isle of Fuday on 9th August, to the south-east of Fuday on 10th August and 1st October and in the Sound of Eriskay on 2nd October. The videos were subsequently processed using bespoke programs to correct for camera lens distortion and to relate video frame number with the estimated distance that the cameras had moved along the tow-track.
The quality of the videos was generally high and clams and other benthic organisms could be easily identified, counted and measured. The cameras did become obscured on a few tows by seagrass and macroalgae. This problem was more significant at certain sites and led to shorter durations on some tows. Useable data were however obtained from all 18 tows.
Razor clams were identified and measured from the videos using interactive software written for the project. Resulting size frequency data were compared with samples of live clams collected by diver on three of the tows. These comparisons showed good agreement between video-based razor clam lengths and live measurements suggesting that the technique could be used to estimate both the abundance and the lengths of clams within reasonable margins of error.
As well as stationary clams we observed a proportion of the Ensis on the videos moving across the seabed by foot-kicking, or righting themselves and reburying. This suggests that electrofishing does not cause damage to the clams, at least in the short-term. The electrofishing-camera rig appeared to create little habitat impact apart from the shallow tracks generated by the electrodes and some dragging of weed caught on the towed gear.
The video data were also analysed in terms of the spatial clustering of clams along the tow tracks and inferences made regarding the precision of mean density estimates with variations in tow length and replication. These considerations should aid the design of future surveys if the combined electrofishing-video technique were to be more widely deployed as a stock assessment tool.
We conclude that the combination of electrofishing with towed video is a promising tool for assessing the status of razor clam beds without causing excessive damage to the habitat or the clams. However, some predation of recovering organisms may occur as large numbers of shore crabs (Carcinus maenas) were seen on the videos. Two instances of crabs attacking stunned clams were observed but the full recovery of the clams was not monitored in this project so the rarity of observed predation in the videos may underestimate actual predation rates.
|Name||Scottish Marine and Freshwater Science|
- stock assessment