The dynamics and impacts of a fishery on sociocultural aspects of fishing communities are rarely studied. Here, we seek to describe how perceived changes in a fishery system – such as declining fish stocks, market failures and the loss of important species – may strengthen or weaken sociocultural patterns, specifically social networks and cohesion, identity, and traditional institutions. These findings are based on 103 semi-structured interviews, 10 focus group discussions, and nine life history interviews with individuals selected randomly from five coastal villages in the Mtwara district of Tanzania. Data were also complemented by a literature review and participant observation. Over 90% of people interviewed reported that the roles of informal village institutions, such as kinship and neighbourhood, have in recent years become loose and changeable due to the perceived ecological changes in the fishery and the influx of people without a fishing culture background. Nearly three-quarters of respondents indicated that the increased monetisation of fisheries products has led to unfettered private actions that have negative impacts, including degradation of resources, creating front groups in order to conceal private intentions, and social tensions like theft of fishing gear. The findings suggest that approaches to understanding and promoting the development of fishing communities in developing countries should include objectives that recognise how sociocultural systems alter and adapt in the face of extreme conditions within the fishery.