Observations and censi of the echinoid Paracentrotus lividus over 40 y at Lough Hyne marine nature reserve have revealed population fluctuations of >4 orders of magnitude. The P. lividus population has been anomalously low since the mid 1980s. The population age–size structure has also changed considerably and is now dominated by older individuals. Sea temperature is a known and important determinant of spawning (and recruitment) in this species. In certain years sea surface temperatures (SST) may not reach critical values for P. lividus mass-spawning to be triggered. In addition, toxic dinoflagellate blooms may have been responsible for large-scale mortalities within short periods. Years of anomalously low SSTs match the timing of critical decreases in the Lough Hyne P. lividus population. Years with low maximum SSTs coincide with the timing of major El Niňo Southern Oscillation (ENSO) events. We suggest a SST and possibly ENSO link to unfished (Lough Hyne) P. lividus population changes and propose such links as potential aggravating factors in the decline of the fisheries.