November 28, 2009 by David Taylor
Society is right to be very concerned when chemical use leads to deleterious environmental effects, particularly where the impact is on a whole population as opposed to a few individuals. There is, however, a tendency to assume that all such impacts represent irreversible damage caused to the environment by human exploitation. A recent paper by Morton (2009) in Marine Pollution Bulletin (Volume 58 pp 1530 to 1538) on the impact of TBTO on the dogwhelk, demonstrates that populations can recover from very dramatic anthropogenic impacts.
Over a 52-month period beginning in May 2004 and concluding in August 2008, and coinciding with the period over which TBT was banned as a ship
anti-foulant globally, a population of the dogwhelk, Nucella lapillus, was studied for changes in population size and structure, and reproduction. During the study period, the size of the population of N. lapillus on the Mewsbrook Groyne at Littlehampton on the southeastern coast of England grew from 25 individuals to >500, i.e., a 20-fold increase. Similarly, population structure normalised to reveal a maximum age of up to 3 years. The numbers of egg capsules produced by the N. lapillus population also grew over the study, again by a factor of 20, and the length of the breeding season increased from 7 months in 2004-2005 and 2005-2006 to 11 months in 2006-2007 and 2007-2008. Such changes were reflected in the incidence of imposex from Vas Deferens Index Stages 3 and 4 in 2003 to zero commencing in 2008 and continuing into 2009.
Due to a lack of confirmatory chemical data, the changes in population size, structure and reproduction herein reported upon for N. lapillus cannot be correlated positively with changes in ambient TBT levels, but they can and are correlated with freedom from imposex. This is the first time such a dramatic recovery from imposex, following the banning of TBT, has been documented.