Shellfish species that are sessile as adults rely on their larval stage in order to self-recruit and connect with neighbouring populations. Wide-spread ‘connectivity’ with many populations increases the likelihood of recovery following a mass-mortality event, e.g., caused by disease or over-fishing, as has occurred for the south Wales cockle populations which has devastated the Burry cockle industry. Using the oceanographically interesting case study of the Celtic/ Irish Seas, larval transport pathways of native shellfish populations are explored, specifically characterising their seasonal variability. Here, summer warming generates a thermally-driven front that controls the transport of larvae. When strongly formed, this front acts as a barrier to northwards larval transport and, hence, limits population spread from southern UK/ France into the Irish Sea. Instead, larvae become entrained in the front-generated currents that are directed westwards around Ireland. Output from different three-dimensional ocean models, is used to characterise the position and strength of the Celtic Sea front throughout a year, and used to track ‘larvae’ particles in the region.