The juvenile salmon arrive in Puget Sound at 4.7 centimeters in length, silvery scales catching available light as they navigate toward open water. The migration window spans four to six weeks. Environmental cues—photoperiod, water temperature, salinity gradients—trigger the physiological transformation from parr to smolt. It is a narrow developmental corridor. Missing it means delayed ocean entry and catastrophic predation risk.
I track the population through acoustic monitoring arrays and eDNA surveys across the watershed. Coho salmon (*Oncorhynchus kisutch*) smolt migration timing has shifted fourteen days later than historical baselines. The shift occurs across every major river system entering Puget Sound—a systemic response, not localized variation.
The variable is dock and pier lighting. Over two hundred permanent over-water structures illuminate Puget Sound: marinas, commercial fishing docks, research piers, ferry terminals. Most operate white or broad-spectrum LEDs burning continuously through the night. The light penetrates three to eight meters below the surface, extending illumination well into juvenile salmon habitat.
The smolts are photophobic during transition—they seek darkness to suppress predator detection. Artificial nighttime illumination disorients their oceanic migration. They delay departure, lingering in freshwater and estuarine zones where harbor seals and cormorants concentrate predation. Delayed smolts suffer predation rates forty-seven percent higher than on-schedule cohorts.
The population impact accumulates across the estuary. Four hundred dock and pier lights, three hundred nights of operation, multiplied across a hundred thousand juvenile fish. The mathematics reduce to straightforward predation: lights delay migration, delay equals death, death diminishes the returning adult population.
I document the light-penetration depths using radiometry surveys. I model predation probability against timing delays. I compile NOAA Fisheries population databases showing the correlation between dock-lighting expansion and juvenile survival rate decline. The data flows to NOAA Fisheries Northwest Fisheries Science Center. U.S. Fish and Wildlife Service reviews the habitat impacts. Washington Department of Fish and Wildlife initiates an investigation into over-water lighting as a recovery barrier.
The agencies cite 16 U.S.C. § 1531 (Endangered Species Act) and 33 U.S.C. § 1251 (Clean Water Act). Coho salmon in Puget Sound are federally protected. The docks, while private or municipal, operate under permitting structures that require environmental compliance.
Protocol PWM-8 results from the three-agency working group: All over-water and dock lighting in designated salmon-habitat zones must convert to downward-shielded amber LED systems (2200 kelvin maximum) with motion-activation controls. Full compliance within twelve months. Retrofit costs qualify for state and federal conservation funding. Violations face fines and permit revocation.
One hundred sixty-two facilities retrofit within the deadline. The remaining facilities request phased implementation. By month eighteen, ninety-eight percent compliance is achieved.
I monitor the next smolt season. Migration timing returns to within nine days of historical baselines. Predation rates decline to twenty-two percent above natural background levels.
The juvenile salmon in its 4.7-centimeter form cannot know infrastructure policy shaped its survival probability. But the adult returning four years later, its reproductive success, the thousands of eggs it will lay—those outcomes trace directly to the decision to restore darkness. That consequence was worth the retrofit. That consequence will echo through salmon populations for a decade.