Allee (1931) first reported that reproductive success in free-spawning marine invertebrates was dependent on population density. However, little attention was paid to the 'Allee' effect until Pennington's (1985) pivotal paper describing the consequeces of sperm dilution in echinoid fertilization. This thesis is a study of the factors affecting fertilization success in five species of commercial shellfish. Commercial shellfish are particularly prone to the deleterious 'Allee' effect. These shellfish are subject to natural and anthropogenic disturbances and disease, and certain species have been severly over-exploited. Some shellfisheries have declined to such an extent that natural recovery is almost impossible (Roberts and Hawkins, 1999).

This thesis examined pre-larval effects on reproductive success in terms of fertilization kinetics. A series of laboratory experiments were conducted to determine fertilization success in the abalone, Haliotis tuberculata; the oyster, Crassostrea gigas; the clam, Tapes decussatus; the limpet, Patella vulgata and the echinoid, Psammechinus miliaris. For all of these species, fertilization success was found to be reliant upon a number of factors such as sperm dilution, gamete age, sperm-egg ratio, sperm-egg contact time and temperature. These data indicate that sperm limitation may severly compromise reproductive success in commercially exploited populations of all these species.

The laboratory-derived data were used with models of sperm dispersal and field experiments to determine minimum stocking densities that would be required to support successful recruitment in adult populations of these commercial shellfish, in an attempt to combat the 'Allee' effect.