How Mature are Nephrops?
September 4, 2018 by Marine Directorate Communications 2 Comments | Category Collaborations, Fisheries, Marine Directorate general, Marine Directorate Science, Marine Directorate Surveys, Research Vessel Surveys
Duration: 25 August – 10 September 2018
- Obtain estimates of the distribution and abundance of Nephrops burrows in the Firth of Forth and the Moray Firth using underwater cameras.
- Use the TV footage to record the occurrence of other benthic fauna and evidence of commercial trawling activity.
- Collect trawl caught samples of Nephrops for comparison of reproductive condition and morphometrics in each of the different survey areas.
- Collect samples of Nephrops for a PHD student from the National University of Ireland (NUI) Galway for a study on density-dependent effects on Nephrops maturity.
- If time permits, deployments of the sledge followed by the drop-frame will be carried out on the same ground to compare Nephrops burrow density estimates obtained by using the two different methods.
Where possible, a random stratified approach will be adopted to investigate Nephrops burrow density in different regions of the study areas. A list of proposed stations for the survey will be made available to the ship prior to sailing.
- TV Observations: At each station a video camera mounted on the TV sledge will be towed across the seabed, into the tide and for approximately 10 minutes at approximately one knot. Nephrops burrow abundance, other benthic fauna and signs of human activity will be recorded on to DVD. Distance traveled by the sledge, the depth at which the sledge is at and camera height from the seabed will be monitored and recorded automatically.
- Trawling: Fishing trawls of approximately 60 minutes duration will be made within each sediment type and within each survey area. A range of biological and morphometric data will be collected on Nephrops caught.
- Drop Frame: The drop frame will be used where conditions are not suitable for using the TV sledge, recording similar data as to that of the TV sledge.
- Comparative Work: Following on from work carried out on previous surveys, on known Nephrops grounds, the sledge will be towed along parallel tracks approximately 200m in length (10 minutes towing time) and 100m apart. Video footage and all observed data will be recorded as usual. Following this the drop-frame will then be drifted across the same area at 90o to the sledge tracks. The frequency of this operation will depend on the weather and available time.
- Nephrops Sample Collection: Once all data needed by Marine Scotland Science (MSS) has been collected from the fishing trawls five Nephrops individuals per mm carapace length, ranging from the smallest to the largest, will be collected. Samples will be stored in plastic bags labeled with the length and haul information then frozen. Samples will be sent to Ireland for inclusion in a PHD study.
TV work will normally take place during daylight hours whilst trawling will take place in the evening. It is proposed that work will initially commence in the Moray Firth and then the Firth of Forth. The exact date of the half landing will be weather, location, and work dependent.
Gear to be used:
- 80 mm prawn trawl BT 201;
- 2 x Day grabs and 1 x sieving table;
- Towed TV sledge;
- 2 x 600m umbilical towing cable and cameras;
- TV drop frame (large version);
- Lasers and large bracket for drop frame;
- Prawn sorting table; and
- Go-Pro housing.