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.

2009, Thomas, Gregor, Quoß, Helmut, Hartmann, Jürgen, Eckmann, Reiner

Size-selective fishery harvest leads to phenotypic changes in fish reproductive traits. When these changes represent an evolutionary response of a stock, they may have severe consequences for future stock dynamics and yields. In freshwater ecosystems, reproductive traits may also be affected by other human impacts such as changes in system productivity. The present study uses regression analysis to evaluate the impacts of changes in lake trophy and of an intensive size-selective harvest over several decades on the reproductive traits of common whitefish in Lake Constance between 1963 and 1999. Fecundity was strongly linked to lake trophy but was also related to the calendar year, suggesting an evolutionary response to size-selective harvest and to massive stocking of the lake with hatchery-reared larvae. The present study is an example of how fish reproductive traits are influenced by the combined action of various human impacts: changes in system productivity, size-selective harvest and massive stocking.

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.

2007, Straile, Dietmar, Eckmann, Reiner, Jüngling, Tobias, Thomas, Gregor, Löffler, Herbert

The year-class strength (YCS) of Blaufelchen (Coregonus lavaretus) in deep Upper Lake Constance was analysed for a 52-year period, from 1947 to 1998. Despite strong anthropogenic influences on the species population dynamics due to cultural eutrophication and oligotrophication, intense fishing, and large-scale stocking, the influence of climate variability associated with the North Atlantic Oscillation (NAO) is apparent in the data set. This influence is significant although large-scale stocking of cold-bred larvae was performed to avoid a mismatch of larvae with their food. The importance of stocking on YCS, however, is unclear and was only detectable when analysing a subset of the data. In addition to climate variability a yet unidentified factor related to zooplankton suitability as food for fish larvae, and density-dependent mortality probably related to cannibalism do significantly influence YCS. The NAO seemed to influence YCS twofold, through temperature effects on egg development time and on larval growth rate. The first of these two mechanisms is related to the NAO via a time lag of 1 year due to the specific mixing dynamics of warm monomictic Lake Constance. Hence, a warm winter in the year before spawning results in earlier hatching of larvae, that is, hatching is decoupled from the actual meteorological conditions. This should make the larvae very prone to mismatch the dynamics of their food. However, we found no evidence for such a mismatch in this 52-year data set.

2009, Probst, Wolfgang Nikolaus, Thomas, Gregor, Eckmann, Reiner

The near-surface distribution of young-of-the-year perch was observed with a towed upward beaming echosounder system (SIMRAD EK60 with a circular 7◦ transducer). Perch aggregated densely in the epilimnion during daytime and dispersed evenly below the surface at night. Shoaling commenced in late June when perch metamorphosed from the larval to juvenile stage. Average shoal width was 6.6 m and average shoal height was 2.35 m in July, when perch were observed for the last time in the pelagic zone of Lake Constance.
Upward echosounding revealed the presence and near-surface distribution of pelagic juvenile perch and therefore this method can be used as a complementary survey tool to get more precise information about the distribution, behaviour and abundance of near-surface fish.

2007, Thomas, Gregor, Eckmann, Reiner

Accelerated growth of freshwater fish during anthropogenic eutrophication has been attributed almost exclusively to the increased nutrient content, while density-dependent effects have been largely neglected. We evaluated the relative importance of these factors by studying the growth of 43 consecutive year classes of common whitefish (Coregonus lavaretus) from Upper Lake Constance. This prealpine lake underwent eutrophication from the 1950s to 1970s, followed by reoligotrophication. Because whitefish are harvested with gill nets in a strongly size-selective way, we used back-calculated lengths of average fast-growing fish to compare growth among cohorts. Standing stock biomass was estimated based upon virtual year-class strengths. Multiple linear regression analysis revealed that growth of whitefish during their second year was most strongly related to standing stock biomass followed by PO4-P content during spring turnover and by calendar year, which was incorporated as a third independent variable (adjusted R2 = 0.84). The negative correlation between whitefish growth rate and calendar year is interpreted as evidence of an evolutionary response to the highly size-selective fishery during at least four decades. We conclude that density-dependent effects on whitefish growth are more important than had been realized previously and that the impact of eutro- phication on growthof whitefish needs to be reconsidered.