1995 EC Study Contract Clyde Sea Nephrops Stock Assessment Study

The research project was carried out to determine whether precision stock assessment of Nephrops norvegicus can be performed by using a sledge-mounted underwater television (UWTV). This can be achieved by a method of counting the number of Nephrops burrows within a fishery ground, as established by the Scottish Office Agriculture, Environment and Fisheries Department, Aberdeen.
Nephrops stock is estimated via prior knowledge of the sea bed and of Nephrops grounds. Burrow counts viewed by means of UWTV aids in the process of establishing the number of burrows, and therefore stock, of the whole fishery ground. The number of Nephrops burrow counts is converted to biomass with the incorporation of empty burrows recognized. This method of stock assessment is reliable, however accurate Nephrops burrow identification and counts are required. There must also be a realistic count of unoccupied burrows for the stock assessment to be precise. The report investigates Nephrops burrow morphology for identification purposes during counts. Resin casts of Nephrops burrows were obtained from a sea loch site and from previous collections held at the University Marine Biological Station Millport (UMBSM) and the Scottish Office Agriculture, Environment and Fisheries Department (SOAEFD). Burrow temporal morphology was investigated using SCUBA diver observations at two sea loch sites. In situ studies of Nephrops populations were undertaken at two Scottish (west coast) study areas, Loch Sween and Loch Riddon, by means of SCUBA diving. Two study sites in Loch Sween were established for mapping purposes. Nephrops populations were also examined by observing videotape footage obtained in Scottish, Swedish and Mediterranean waters obtained by UWTV. Data recorded, by DANI (Department of Agriculture, Northern Ireland), from the Irish Sea also supplemented the research project. A third study site, in the Achnamara arm of Loch Sween, was set up for the purpose of burrow resin casting. Resin casts of Nephrops burrows were examined and collated to determine the complexity of burrows. Burrow dynamics and longevity were examined. The Loch Riddon study site was utilized for mapping and resin casting purposes. The report also investigated the burrows of other co-inhabiting species. A key is provided to aid in the identification of species burrows based on the observations of surface features. This would prove valid for those wishing to use the UWTV-based stock assessment method as precision counts of only Nephrops burrows would be achieved. Ground suitability for UWTV based stock assessment for Nephrops is discussed. Recommendations for the UWTV method of stock assessment is provided.

This study was conducted by scientists at the University Marine Biological Station Millport (UMBSM) and the Institute of Marine Biology of Crete (IMBC). Research consultation was established with members of the Scottish Office Agriculture, Environment and Fisheries Department (SOAEFD), the Istituto di Ricerche sulla Pesca Marittima, Ancona, Italy (IRPEM) and the Institute of Marine Research, Lysekil, Sweden (IMRL). The Department of Agriculture, Northern Ireland (DANI) provided the project with data. The study project has been supported by financial assistance of the European Commission. The report does not necessarily reflect the views of the European Commission. The study project is in support of the Common Fisheries Policy. The Norway lobster, Nephrops norvegicus, is a commercially important shellfish species in Scotland and in the rest of Europe. Stock assessment within fisheries relies on the data presented by the fishery which is supplemented by commercial and research vessel catches. Such methods of stock assessment are not entirely reliable as catch estimates vary with the seks and size of the animal, time of day, season and sediment type of the ground. A more reliable, precise method of stock assessment is required. Such a method was devised by SOAEFD and has been adopted by other institutions. The method is based on estimating the number of Nephrops within a fishery ground by counting the occupied burrows. This is achieved by the use of underwater television (UWTV). The method relies heavily on the accurate identification and recognition of Nephrops burrows. Nephrops burrows were cast in resin during the project to enable an insight into burrow morphology, complexity and diversity. One hundred and forty eight resin casts were investigated, which included new casts and those previously collected by UMBSM and SOAEFD. UWTV video tape and still photographs displaying Nephrops burrows and fishing grounds from the UK and Europe were analyzed. The colour cameras used in Scottish investigations were Simrad Osprey OE1362 and OE1364. Both cameras had a fixed focal lens and a wide field of view. A flotation device was attached to the camera cable and the tow reached a speed of less than 1 knot to prevent sediment disturbance. An underwater stills camera, supplied by SOAEFD, was occasionally used in conjunction with the UWTV. A Hasselblad 70mm underwater stills camera, equipped with a 50mm lens, 70 frame film magazine and twin strobe flashes (Simrad Osprey) was used. Both cameras were aligned to view the same area. Particular Nephrops grounds were examined for their suitability to use UWTV as a method of stock assessment. Over populated areas and those which included conspicuous species burrows suggested a high error rate for stock assessment and extra care must be utilised when assessing such Nephrops grounds. Larger burrows were identified as being only partially visible to the UWTV camera. By using divers to assess Nephrops burrows along a previously UWTV tow path, exact numbers could be counted. It was identified that not all burrows could be observed on the corresponding video tape. Allowances into partially sighted burrows and occupancy assessments had to be established for stock assessments to be as precise as possible and not over estimated. UWTV research within Scottish sites was carried out using the UMBSM research vessel RV Aora. The two project partners, UMBSM and IMBC, investigated methods of stock assessment in Nephrops grounds in the Scottish west coast and the Aegean respectively. Loch Sween and Loch Riddon (both in western Argyll) were sites investigated in Scotland. Both are sea lochs which have Nephrops grounds. Loch Sween has a varying and dense range of burrowing megafauna which was useful in establishing the problems for UWTV stock assessments. Methods truncated, contact data provider for more information.