Volume22/Number7/July 1991 Marine Pollution Bulletin, Volume22, No. 7, pp. 331-333, 1991. Printed in Great Britain. 0025-326X/91 S3.00+0.00 © 1991 Pergamon Press plc Recovery in Populations of Dogwhelks Nucella lapillus (L.) Suffering from Imposex S. M. EVANS, A. HUTTON, M. A. KENDALL* and A. M. SAMOSIR Dove Marine Laboratory, Cullercoats, Tyne and Wear, NE30 4PZ, UK *Present address: Plymouth Marine Laboratory, Prospect Place, West Hoe, Plymouth PL1 3DH, UK Populations of dogwhelks Nucella lapillus in areas of high boating activity on the Northumbrian coast had severe symptoms of imposex when they were sampled in 1986. They had high RPSI scores, male-biased sex ratios (probably due to high female mortality) and poor reproductive performance. There has however been marked recovery of these populations since the partial ban on the use of anti-fouling paints containing tri- organotin compounds in 1987. RPSI values are lower, recruitment has improved and there is evidence of better survivorship of females. small numbers of juveniles and adult females. The con- dition can eventually lead to the local extinction of the species (Bryan et al., 1986; Bailey & Davies, 1989). In view of the high toxicity of TBT, Britain banned the use of anti-fouling paints containing it on small boats (less than 25 m) and aquaculture cages under the Food and Environment Protection Act (1985) in July 1987. The effectiveness of these measures should now be monitored. The object of the present study was therefore to assess the status of populations of N. lapillus along the Northumbrian coast, north of the River Tyne, and com- pare them with assessments made previously in 1986, before the ban came into force. The term imposex describes the superimposition of male characters on female animals. It is known to occur" in a number of gastropod species (see e.g. Gibbs et al., 1990) but has been most thoroughly studied in the dog- whelk Nucella lapillus, which is of particular importance as a dominant carnivore on rocky shores. The severity of the symptoms vary but the female develops a penis and vas deferens and, in the most extreme cases, the latter may occlude the oviduct, resulting in sterility and even death (Gibbs & Bryan, 1986). Smith (1981, a,b)showed that imposex can be induced in Ilyanassa (=Hinia) obsoleta by tributyltin (TBT), a biocide which is used in anti-fouling paints to prevent the settlement and growth of aquatic organisms on boats and fish cages. Extensive field and laboratory studies have shown that very low concentrations of TBT have the same effect on N. lapillus, (e.g. Bryan et aL, 1987; Gibbs et al., 1987) and N. lima (Stickle et al., 1990). The incidence of imposex has increased in natural populations of N. lapillus since paints containing TBT were introduced in the 1960s, and the condition is now widespread around the British coast. It is particu- larly severe on shores adjacent to marinas and other areas of high boating activity. The worst affected popula- tions suffer from reproductive failure and high female mortality, and are characterized therefore by relatively Methods There were seven study sites. Samples were taken at each of them between October and December in 1986 and again, over the same period of the year, in 1989. There are different levels of boating activity at these sites. Six of them, Tynemouth, Cullercoats, Brown's Bay, St. Mary's Island, Amble and Seahouses are in areas where there is extensive use of pleasure and small fishing boats. There is no anchorage or slip at Bamburgh. Consequently no boats are kept in the water there, and it is generally an area of low boat use. Two samples were taken at each site in both years: First, samples of approximately 100 mature N. lapillus in 1986, and 60-80 in 1989, were collected for measure- ments of imposex. Mature dogwhelks were distinguished from juveniles using criteria described by Feare (1970), but primarily on the presence of a row of teeth along the inner margin of the aperture of mature individuals. Animals were anaesthetised in 7% magnesium chloride and their shells cracked open by using a bench vice. Each individual was examined under a binocular micro- scope and sexed (see Fretter & Graham, 1962; Bryan et al., 1986). If a penis was present, it was measured, using a micrometer eyepiece. The relative penis size index (RPSI) (Gibbs et al., 1987) was calculated as a measure 331