2012, Thomas, Gregor, Eckmann, Reiner

The five species of coregonids that inhabit the lower Mackenzie River, broad whitefish, Coregonus nasus, inconnu, Stenodus leucuicthys, lake whitefish, C. clupeaformis, Arctic cisco, C. autumnalis, and least cisco, C. sardinella, have unique life cycles wherein they undertake extensive migrations to and from spawning grounds, overwintering areas and feeding areas. We present analysis of these migratory patterns based on a study in the Arctic Red River situated in the Northwest Territories, Canada. The time of upstream and downstream migration as part of the spawning cycle was observed to be species-specific. We correlate the timing of migration to the seasonal development of gonads to determine the relative timing of spawning. Common garden experiments comparing the two most seasonally distinct species, broad whitefish and inconnu, showed that there was a difference between these species in the number of degree-days to hatch to compensate for the timing of spawning. Emergence of all species is under a strong stabilizing selection corresponding to the spring ice break-up on the Lower Mackenzie River. Due to this environmental constraint shared among all coregonid species in the system, we suggest that spawning timing is under disruptive selection to reduce hybridization among species. The common garden experiments above support a genetic alteration of the rate of embryonic development to allow for separation in timing of spawning among coregonids and therefore reinforce reproductive isolation among species.

2010, Thomas, Gregor, Rösch, Roland, Eckmann, Reiner

The ecosystem of Lake Constance in central Europe has undergone profound modifications over the last six decades. Seasonal and inter-annual changes in the vertical distribution patterns of whitefish were examined and related to changes in biotic and abiotic gradients. Between 1958 and 2007, the average fishing depth in late summer and autumn was related to two factors influencing food supply of whitefish lake productivity and standing stock biomass. In years with low food supply, whitefish were harvested from greater depths, where temperatures were up to 4° C lower. The whitefish's distribution towards colder water might be a bioenergetic optimisation behaviour whereby fish reduce metabolic losses at lower temperatures, or it may result from a reassessment of habitat preference under conditions of limited food supply, according to the ideal free distribution theory.