A Compact Water‐Heating System for Thermally Marking Salmon Otoliths

We describe a compact water‐heating system for thermally marking salmon otoliths during incubation. The system is portable and can be used to provide heated water for either streamside or instream incubators. The heating system consists of a small, propane‐fired boiler and heat exchanger, a jet pump, a motorized set‐point, a four‐way mixing valve, and two circulating pumps. Warmed stream water exiting the heat exchanger is pumped to a head tank, mixed with ambient water to the desired temperature, and dispersed to the incubators, where some or all of the water can be recirculated or discarded. The mixing valve maintains a constant temperature in water flowing to the head tank based on a sensor feedback loop from the recovery tank. Using this system, we successfully marked approximately 800,000 pink salmon Oncorhynchus gorbuscha with no apparent adverse affects. Depending on incubation techniques, larger numbers of eggs or alevins could be marked.