Who reproduces and who is caught are not the same fish. The left panel compares each fleet's selectivity-at-length with the maturity ogive; the right panel compares the trend in spawning biomass (SSB, reproductive potential) with the trend in each fleet's vulnerable biomass (the biomass its catches and CPUE come from), both relative to the first year. With dome-shaped selectivity, large spawners escape: a fleet's CPUE can fall faster or slower than SSB, so an index can be a distorted picture of the reproductive stock.