Largemouth bass are used as host fish for temperature trials for the endangered Higgins eye pearlymussel. - FWS photo

In the heart of January, most aquatic biologists can be found keeping warm in front of computer monitors that glow with data collected in the previous field season.  Likewise, biologists at the Genoa National Fish Hatchery and the La Crosse Fishery Resources Office have kept warm this winter while investigating the thermal requirements for early life development of the federally endangered Higgins eye pearlymussel. 

Patterned after similar work conducted in 2003 on the federally endangered winged mapleleaf mussel, Genoa mussel propagation biologist Tony Brady reared Higgins eye glochidia on largemouth bass host-fish that were maintained in two varying temperature regimes at the hatchery during spring 2006. 

Genoa NFH staff recorded the water temperature and enumerated juvenile mussel excystment over metamorphosis periods that ranged from 40 (warmer regime) to 49 days (colder regime).  Using a mathematical model that he developed for the earlier winged mapleleaf work, La Crosse fishery biologist Mark Steingraeber used these Higgins eye data to empirically derive a value of 10.9 C as the minimum (i.e., threshold) temperature required for metamorphosis of largemouth bass-encysted Higgins eye glochidia into juvenile mussels. 

This value was subsequently applied to (i.e., subtracted from) all of the daily water temperature values that exceeded it in each thermal regime.  The ensuing values were then summed for each regime to determine the cumulative temperature units of development (169 C·d) needed for Higgins eye glochidia to complete metamorphosis on largemouth bass from the date of encystment until the start of peak excystment.