CANCELLED - Why does population structure fluctuate in lentic Arctic char populations while abundance remains stable?

Background/Question/Methods

In closed arctic lakes, we observe a strong and intriguing cycle in population structure in Arctic char (Salvelinus alpinus) while population abundance remains generally stable.  At a frequency of approximately five to seven years, the population fluctuates from a high proportion of small adults (<300 mm), to one where the proportion of small and large adults is near equal, to a population dominated by a small proportion of large adults for a short time.  We hypothesized that this cycle was driven by strong internal density dependence that could operate via: 1) food limitation including cannibalism, 2) size-based niche differentiation, and/or 3) or recruitment pulses initiated by an exogenous factor (e.g., date of ice-off).  We tested these hypotheses using a long time series of population structure and dynamics data, and diet, trophic position, and overlap information based on heavy isotope analysis in two different Arctic lakes, AK.  

Results/Conclusions

Although food is clearly limiting in these oligotrophic lakes, the short growing season results in consumption being limited as much or more by temperature than by food, and lake fertilization has little effect at tertiary trophic levels.  We observed no evidence of cannibalism; however, other work has shown that cannibalism can be extremely influential even at very low levels (< 5%), yet extremely difficult to detect.  We did observe some evidence for size-based niche differentiation; large char appear to switch to a more littoral foraging pattern and incorporate large proportions of snails in their diet relative to the more pelagic small adults.  The role of ice-off and annual temperature regime is temporally complex (e.g., lag effects) but also appears to be extremely influential in determining recruitment dynamics and resulting age structure and cycles in age structure.  A better understanding of what factors and processes regulate these populations will be critical in predicting the effect of climate change as well as for completing synthetic evaluations across arctic ecosystems.