The Effect of Growing Degree Days on Walleye Growth, Reproduction and Mortality Rate

Temperature and food availability are the primary determinants of fish growth and devel­opment.  This paper demonstrates the effect of temperature by contrasting the lifetime growth patterns of walleye across a broad latitudinal gradient.  We show that Growing Degree Days (GDD), an index of ambient thermal energy, accounts for much of the latitudinal variation in growth.  When fish age is measured in thermal units (i.e. age x GDD), average length at age is the same across a range of thermal regimes (GDD = 1000 – 4600 °C).  This transformation of fish age also reveals a consistent pattern of sexually dimorphic growth.  Divergence in growth starts when total length exceeds 350 mm, the mean size of maturity in males, after which males grow slower and attain a smaller asymptotic length (≈ 540 mm).  In contrast, females mature later (≈ 450 mm) and reach a larger asymptotic length (≈ 670 mm).  Application of a biphasic growth model, which accounts for ontogenetic changes in female somatic growth due to investment in reproduction, implies that the reproductive investment of females increases with GDD.  Furthermore, because optimal life history theory predicts reproductive investment increases with mortality rate, we expect mortality rate also increases with GDD.  These predictions are supported by empirical relationships between GDD, mortality rate, and female gonadosomatic index.  Given the support shown for this GDD-corrected model of walleye growth, we recommend its use when exploring how walleye growth and development is influenced by other factors (e.g., food availability).