Settlement of Pomatoceros triqueter (L.) in two Scottish lochs, and factors determining its abundance on mussels grown in suspended culture.

In Scotland, some 2,000 tons of mussels (Mytilus edulis) are produced each year from suspended rope cultures. Mussels can act as a settlement substrate for tubeworm, Pomatoceros spp, and badly fouled mussels are devalued and may be discarded. The estimated cost to the Scottish rope-grown mussel industry is between pound300,000 and pound500,000 per annum. Established growers believe the problem is worsening. Collector plates were deployed at two sites in two lochs on the West Coast of Scotland to monitor Pomatoceros spp. settlement. In conjunction, in situ trials at a mussel farm site assessed tubeworm settlement on rope-grown mussels. The tubeworm P. triqueter was found to be the prevalent species settling on the collector plates and on the mussels. The timing of peak tubeworm settlement differed between lochs but was synchronous between sites and different depths within the same loch. Peak settlement in both lochs occurred after the highest seawater temperatures were recorded. Settlement intensity differed between lochs, sites, and depths, indicating the scale of variation in settlement within lochs. Mussel shell size was distinguished as a significant factor influencing Pomatoceros spp. settlement as higher numbers of tubeworm, settled on larger mussels. In large mussels (mean shell length 60.8 mm +/- 0.70 SE) tubeworm settlement was greatest in treatments where conspecific adults were already present and higher levels of settlement were found in mussels stocked at lower densities. In small mussels (mean shell length 33.5 mm +/- 1.03 SE) the initial stocking density and tubing system had no effect on tubeworm settlement. Differences in the fouling intensity between mussel stocks were attributed to variation in the abundance of tubeworm larvae in the water column and the size of the mussels at the time of retubing. A 100% mortality could be expected in adult P. triqueter after 24.1 h and 35.4 h when exposed to air at 7degreesC and 13degreesC, respectively. In areas where tubeworm is a persistent problem consistent annual monitoring of the Pontatoceros spp. settlement is recommended as part of a management strategy to avoid heavy fouling on mussel stock. Grow-out strategies to alleviate tubeworm fouling on rope-grown mussels are discussed.