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

Discussion

The figures in Table 1 were used to make a crude estimate of the possible economic losses from discarding of spurdog. Although the market price for small spurdog tends to be lower than for larger animals, verbal reports suggested that £70 a box ^[2] might be expected ( http://ec.europa.eu/fisheries/cfp/market/proposed-guide-prices_2013_en.pdf give a lower guide price of 1,157 euros tonne ^‑1 in 2013). The amount of spurdog caught but not landed from ‘Cheerful’ in the Sea of Hebrides might therefore represent losses of up to £22,000 to £45,000 (using lower EU guide price and upper anecdotal price) per vessel over a year (winter to autumn inclusive). However, it is not known how typical the data reported from Cheerful are for other vessels fishing in the Sea of Hebrides. The economic losses due to discarding spurdog in the North Minch appear almost negligible (a few hundred pounds per vessel per year) based on reported discards from vessels operating in that area.

A by-catch landing trial for spurdog in the South-west of England gill net fishery is being proposed by Defra. The proposal states that only ‘dead’ spurdog would be retained and be allowed to be landed. An upper landing limit will be calculated based on the reported amounts being discarded * estimated post-discard mortality. For the gill net fishery the post-discard mortality has been estimated to be around 40%. As far as the author is aware post-discard survival of spurdog has not been measured in Nephrops trawl fisheries. Post-trawl discard survival rates have been estimated at more than 50% by Mandelman and Farrington ( 2007) and Rulifson ( 2007) working in the north-western Atlantic but their tows were of relatively short duration (45-80 mins) in comparison with the typical Nephrops tow durations (3.5 to 5 hours). Although spurdog can appear healthy after trawling, increased mortality tends to manifest up to 72 h afterwards as a result of internal organ damage and stress responses. In addition, increased tow lengths and bulk of catch are acknowledged to be likely to increase mortality ( Mandelman & Farrington 2007, Rulifson 2007). The post-trawl discard survival for spurdog discarded from Nephrops trawls is therefore likely to be lower than 50%, perhaps significantly so. A low survival rate would mean there is less conservation value in returning spurdog to the sea after Nephrops trawling and also raises the problem that spurdog could act as a ‘choke’ species under the new landings obligation of the Common Fisheries Policy.

Most papers on age determination using spurdog spines have reported a wide variety of patterning including the presence of double marks, spawning rings and other irregular patterns which can complicate interpretation, or spines with no obvious patterns ( Holden & Meadows 1962, McFarlane & King 2009). Most papers have not reported the overall percentage of spines considered readable but rejection rates of between 30-50% can be inferred. Counts of increments on dorsal spines collected in this study were made attempting to interpret marks as annual marks following Holden & Meadows (1962), no corrections were applied for wear although where this was obvious the spine was excluded from further analysis. The ages determined from readable spines seemed low compared with reported size at age for spurdog from other studies. Although spurdog growth rates from different locations do show some variation (Figure 8) a more likely explanation for this result is that the spine-based age estimates were too low. Given the low numbers of readable spines, and the high uncertainty in interpretation of the spine marks, age determination from the vertebrae should be attempted in line with recommendations in Bubley et al. (2011). Spines and vertebrae were thus collected from the spring 2014 and winter 2014/2015 fish to allow this to be attempted in the future.