Scottish Marine and Freshwater Science Vol 6 No 7: Collection of bycatch data for spurdog (Squalus acanthias) in the North Minch fishery: Report of Fishing Industry Science Alliance (FISA) Project 02/13

Materials and Methods

Catches of spurdog are unpredictable rendering the use of observers on vessels as impractical. A fisher self-sampling scheme was therefore devised with the industry representatives. Derogations were obtained allowing skippers of interested vessels to retain spurdog on board for the purpose of sampling. Derogated vessels were issued with instructions, logsheets and asked to record tow details, amounts of spurdog caught and each week to retain a random sample to be shipped to Barratlantic for subsequent measurements ( Appendix I ). If only small quantities of spurdog were being caught then all the fish were to be returned to Barratlantic (up to around 50 fish per vessel per week). Tows to the north of latitude 57.5 ^oN were nominated as being in the North Minch and to the south of latitude 57.5 ^oN as being in the Sea of Hebrides. The lead scientist visited Barratlantic periodically to measure the samples. The frozen samples were defrosted and individuals examined to determine sex (based on presence or absence of male claspers) and stretched fish lengths (tip of snout to end caudal fin) recorded to the nearest cm. Female fish were also examined internally for the presence of pups.

Non-derogated vessels operating in the area were also asked, via their local associations, to record any spurdog caught using the e-log system.

Age determination was not included in the original project proposal but was added as an additional parameter which could be sampled relatively easily. Dorsal spines were taken from around 30 fish per sample landed from the winter 2013/2014 season. These were air dried, cleaned with ethanol and examined under a low-power microscope with top illumination (Olympus SZX-16). Spine marks were interpreted following Holden and Meadows ( 1962). Interpretation of age from dogfish spines is complicated by the fact that these structures are external to the fish and can become worn in older animals. Holden and Meadows ( 1962) suggested that wear can begin in fish as small as 45 cm although wear above 5 mm was only noted in fish larger than 90 cm. Several workers have attempted to correct spine readings for wear based on changes in the width of the spine. However, Bubley et al. ( 2011) suggested that such an approach can lead to over-estimation of ages so wear correction was not attempted.