Snapshot assessments of species which live for a number of years are often misleading if they are conducted at an inappropriately short temporal scale. Densities of the trochid gastropods Steromphala umbilicalis (da Costa, 1778) and S. cineraria (Linnaeus, 1758) fluctuated widely between 1996 and 2014 at 10 rocky-shore sites within Lough Hyne, SW Ireland. Between 2014 and 2018, abundance of S. cineraria decreased to levels not seen in the previous two decades. This significant reduction was possibly in response to extreme dissolved oxygen fluctuations related to eutrophication of the semi-enclosed lough. The congener S. umbilicalis showed low density in 2018, but this was within the limits of inter-annual fluctuations. The Lusitanian trochid Phorcus lineatus (da Costa, 1778) was first recorded by us in the lough in the early 2000s, establishing a flourishing population with densities of ca. 6 snails per 0.25 m2. Although this native trochid survived the cold winters of 2009-2012, the population suffered high adult mortality and little recruitment. Between 2013 and 2018, the population increased and population density stabilised. The increase in P. lineatus reflects its position high on the shore where air temperatures have risen over the last two decades. It may therefore be a sentinel species to detect the effects of climate change. By contrast, the decrease in S. cineraria was related to its shallow subtidal habitat where hypoxia and superoxia have recently become marked in the lough. This trochid, therefore, offers potential as an indicator of eutrophication-associated stress. These temporal variations in trochid species emphasise the need for longer-term assessments when estimating the importance of any one species to the ecosystem.