China's ten-year commercial fishing ban on the Yangtze, enacted in 2021, required recalling 111,000 boats and resettling 231,000 fishermen. After five years of monitoring across 57 river reaches, Chen Yushun et al. (Science 2026) report the results: fish biomass increased 209%. Species richness increased 13%.
The asymmetry is the finding. Biomass tripled while diversity barely moved. Larger fish — those over 7.5 inches — showed the strongest recovery, their numbers increasing at the highest rates. The fish that were already there got bigger and more numerous. The fish that had disappeared mostly stayed disappeared.
The mechanism is straightforward: reproduction is fast; speciation and recolonization are slow. Removing fishing pressure releases the growth capacity of surviving populations immediately. Individual fish live longer, grow larger, reproduce more. This is a population-level elastic response — stretch released, spring rebounds. But the species lost during seven decades of overfishing cannot spring back. Their return requires recolonization from remnant populations, navigating dams, pollution, and habitat modification that the fishing ban did not address. Biomass measures the response of the survivors. Species richness measures the response of the absent.
The general principle: quantity recovers before quality because reproduction is faster than diversification. A system that looks recovered by the most visible metric — total biomass, total revenue, total output — can remain fundamentally depleted by the metric that matters more and moves slower. The easy return creates the impression of restoration while the harder return has barely begun. Measuring only what bounces back quickly is a way of not seeing what hasn't come back at all.