Effective population size ( N e ), census size ( N c ) and their ratio ( N e / N c ) are widely used indicators in conservation genetics, yet long‐term empirical data tracking of all three metrics over long time scales in wild populations remains rare. Using monitoring data from 1995 to 2020, we examined temporal dynamics of N e , N c and their relationship during the demographic collapse of Delta Smelt ( Hypomesus transpacifi...