Nicholas F. Hughes

Testing the Ability of a Temperature-Based Model to Predict the Growth of Age-0 Arctic Grayling

Abstract We tested the ability of a temperature-based growth model for European grayling (also known as grayling) Thymallus thymallus to predict the growth rate of age-0 Arctic grayling T. arcticus in two reaches of a small Alaskan stream. We compared observed and predicted lengths at weekly intervals in both reaches during the summers of 2000 and 2001, calculating predicted lengths from information on daily mean water temperature and the length of larval fish at the start of the growing season. In one reach, the model accurately predicted growth over the course of both summers and the differences in growth between summers. In the other reach, observed length fell below predicted length at 4– 5 weeks of age in both years. We hypothesize that the model works well when prey abundance does not limit growth but fails when low prey density limits the fishs daily ration.

Relationships between ecosystem metabolism, benthic macroinvertebrate densities, and environmental variables in a sub-arctic Alaskan river

Relationships between environmental variables, ecosystem metabolism, and benthos are not well understood in sub-arctic ecosystems. The goal of this study was to investigate environmental drivers of river ecosystem metabolism and macroinvertebrate density in a sub-arctic river. We estimated primary production and respiration rates, sampled benthic macroinvertebrates, and monitored light intensity, discharge rate, and nutrient concentrations in the Chena River, interior Alaska, over two summers. We employed Random Forests models to identify predictor variables for metabolism rates and benthic macroinvertebrate density and biomass, and calculated Spearman correlations between in-stream nutrient levels and metabolism rates. Models indicated that discharge and length of time between high water events were the most important factors measured for predicting metabolism rates. Discharge was the most important variable for predicting benthic macroinvertebrate density and biomass. Primary production rate peaked at intermediate discharge, respiration rate was lowest at the greatest time since last high water event, and benthic macroinvertebrate density was lowest at high discharge rates. The ratio of dissolved inorganic nitrogen to soluble reactive phosphorus ranged from 27:1 to 172:1. We found that discharge plays a key role in regulating stream ecosystem metabolism, but that low phosphorous levels also likely limit primary production in this sub-arctic stream.