The present study aimed to address the issue of pressure on the remaining populations of the critical endangered species, fan mussel, Pinna nobilis, in the Eastern Mediterranean. Marine professional (shell/fish farmers, divers, fishers, administrators, etc.) stakeholders’ knowledge (n = 151) in Greece reports that there was a reduction (81.6%) of the P. nobilis individuals during the last 15 years, especially in the years 2010–2012 and 2017–2018. Species’ abundance decline is significantly (ρ = 0.293, p < 0.05) correlated over the last 5 years with the observed natural ecosystem degradation. Participants also stated that the main cause was pathogens alone (28.8%) or in combination with (illegal) fishing (17.1%) or pollution (14.4%). Additionally, 88% of them supported a total prohibition on the use of fan mussels for commercial purposes in order to restore stock levels.; 72.4% stated claimed that no appropriate control was in place and that they (>59.4%) were unsure if this monitoring control is carried out by the competent authorities. Marine stakeholders consider the importance of the species for biodiversity preservation (56.1%), environmental education (35.1%), and diving parks (29.7%). They declared that it is important to record-observe P. nobilis conservation reserves regions (42.6%), monitor areas of responsibility (39.9%), participate in the information society/use of social networks (38.5%), and participate in informational meetings (37.9%). The lack of interest among shell/fish farmers to contribute to P. nobilis on-growing farming indicates the ignorance to the potential benefits of the valuable ecosystem services provided by aquaculture through biodiversity conservation. Nevertheless, there is a demand for promoting the “conservation aquaculture” concept through its incorporation into marine farming activities supplementary to their core business in “production”.