Modelling potential effects of petroleum exploration drilling on northeastern Georges Bank scallop stocks

A set of numerical models was used along with laboratory and field observations to evaluate the potential risk of exploratory oil and gas drilling on northeastern Georges Bank to sea scallop (Placopecten magellanicus) stocks. The models were used to predict the drilling waste zone of influence and the impact of chronic exposure on scallop growth and reproduction. Growth and reproduction are generally considered to be the most important sublethal effects of chronic contaminant exposure. The highest near-bottom concentrations of drilling waste (water-based mud) from a hypothetical 92-day exploration well was predicted to occur along the side of the bank (>100m depth). Laboratory information on drilling mud toxicity threshold concentrations indicated a potential for 0–48 days of growth inhibition depending upon the site, settling velocity of the mud, and area over which results are averaged. Scallop stocks on the side of the bank are relatively sparse, but dense aggregations are found in some areas and it is possible that changes in reproductive output could have detectable effects at the population level. Growth inhibition in the tidal front region, which has the densest scallop stocks, was predicted to be more localised and confined to a range of 0–15 days. Growth loss in the central mixed region (<65m) was predicted to be negligible (<2 days). These results illustrate the importance of site location and waste settling velocity on potential effects. The magnitude of effects predicted at each site was closely related to bottom stress (u∗) as this determines how rapidly drilling mud reaching the seabed are redistributed and diluted both horizontally and vertically.