Opposing growth trends in juvenile and mature Humpback Whitefish within the upper Tanana River drainage, Alaska, revealed using otolith biochronology

Abstract

Objective Within areas of Alaska, Humpback Whitefish Coregonus pidschian are an important subsistence resource for many Alaska Native communities. Recently, Humpback Whitefish in the upper Tanana River drainage were reported to be smaller at a given age and reach smaller maximum sizes than fish sampled 20 years ago. The objective of this study was to identify the factors that influence annual growth of Humpback Whitefish within the upper Tanana River drainage based on an analysis of otolith increments. Methods A series of mixed‐effects models were used to determine the relative importance of biotic (i.e., age, age at capture) and abiotic (i.e., year, sampling time period) factors on the growth of Humpback Whitefish. Juvenile (<5 years of age) and mature (≥5 years of age) growth periods were examined separately due to the unique habitats occupied by each life stage. Climate variables were correlated with the mature Humpback Whitefish biochronology to assess environmental drivers of adult growth. Result Biochronologies spanning over three decades (1982–2018) revealed patterns of variation in Humpback Whitefish otolith growth across years. A negative temporal trend spanning the entire study period was evident for juvenile growth, while a mostly positive trend was observed for mature growth. Between time periods, otolith growth was significantly different from zero for juvenile (estimate: −0.087; 95% CI = −0.151 to −0.023) but not mature (estimate: −0.030; 95% CI: −0.092 to 0.032) growth periods. Several environmental factors were positively (i.e., mean monthly temperature, growth degree days) and negatively (e.g., snow depth) correlated with the mature Humpback Whitefish biochronology. However, the model with mean April snow depth was the best‐performing environmental model. Conclusion These results suggest years with elevated snow depths negatively impact mature Humpback Whitefish growth. Cumulatively, these results illustrate the importance of juvenile growth periods and how early‐in‐life declines in growth may not be offset by above average growth as adults and help further our understanding of how environmental conditions influence growth.