The relative influence of breeding competition and habitat quality on female reproductive success in lacustrine brook trout (Salvelinus fontinalis)

Egg losses for female salmonines primarily occur through competition for egg incubation sites (i.e., redds) and the differences in quality among these sites. Through detailed observations and an experiment linking egg survival to groundwater flow, we estimated the relative influence of redd superimposition and habitat quality on female reproductive success for a population of lake-spawning brook trout (Salvelinus fontinalis). Three quarters of all spawning sites were reused by multiple females; however, brood loss was much less (28%–38%) because large females spawned earlier and constructed deeper nests. The relationship between groundwater flow rate and egg survival was not linear, with consistent egg survival occurring only at sites with flows over 20 mL·m–2·min–1. Varying scenarios of redd superimposition and habitat-related egg survival resulted in an estimated 4%-21% of deposited eggs surviving to emergence and greatly reduced the size-related advantages of larger females owing to fecundity. Limited numbers of high-quality spawning sites and overall low survival of eggs resulted in habitat being the dominant route of egg loss. In the absence of female competition, spawning habitat alone accounted for egg losses of 67%–91% and points to the importance of physical habitat features in the maintenance of brook trout populations.

High rate of redd superimposition by brook trout (Salvelinus fontinalis) and brown trout (Salmo trutta) in a Minnesota stream cannot be explained by habitat availability alone

This study was conducted to test the hypothesis that redd superimposition by salmonine fishes is a consequence of limited habitat availability. We monitored redd site selection by brook trout (Salvelinus fontinalis) and brown trout (Salmo trutta) for two spawning seasons in Valley Creek, Minnesota. Redd superimposition rates were high; over one half of the brook trout and one third of the brown trout superimposed redds. We tested the role of habitat availability in this process by characterizing microhabitat at sites with and without redds in four small sections of this stream and then determined whether superimposition could be explained by random dispersal of fish over available habitat. Brown trout preferred spawning sites with high flows whereas brook trout strongly preferred deep sites with upwelling groundwater. No relationship was observed between fish density and superimposition. Additionally, the observed frequency of superimposition was greater than expected by chance in six of eight instances for brown trout and in one of three instances for brook trout. Finally, a behavioral experiment provided direct evidence that females have a behavioral preference to spawn on existing redd sites, suggesting that factors other than habitat may determine redd site selection and hence superimposition.

Estimation of eggs lost from superimposed pink salmon (Oncorhynchus gorbuscha) redds

This study investigated whether repeated use of limited spawning grounds (i.e., redd superimposition) by pink salmon (Oncorhynchus gorbuscha) can cause density-dependent mortality. Loss of eggs from part of Auke Creek, Alaska, was estimated from a series of mark-recapture experiments and maximum likelihood models. The number of eggs lost per day during the 5-week spawning season and for 2 weeks afterward was roughly proportional to spawner abundance and weakly related to stream discharge. Freshets after cessation of spawning induced negligible egg loss. The maximum daily egg loss estimated by one model was 398 000 eggs (80% CI = 267 000 - 1 581 000) or equivalently a loss of about 300 female spawners when the daily spawner abundance (both male and female) in the study area was at most 1000.

Riverine salmonid egg burial depths: review of published data and implications for scour studies

Published data on salmon, trout, and charr egg burial depths are highly variable and inconsistent. Primary sources of variation include elevation datum and portion of the egg pocket referenced to; differences in spawning behavior and the number, thickness, and location of egg pockets; relationships between egg depth, fish species, and corresponding size of female and spawning substrate and velocity characteristics; sampling method; presence of excavation barriers; redd superimposition; and scour and fill by hydraulic and other mechanical processes. Such sources of variability in the reported data have important implications for studies of scouring processes in salmonid spawning areas that require accurate identification of egg burial depths for predicting and preventing potential scour impacts. Cumulative measurement error and unexplained variation may amount to 5-20 cm or more in published values. The most relevant data for scour impact assessments are depths from the original stream bed elevation down to the top of the main egg pocket. Frequency distribution data are needed for determining probabilities and cumulative levels of scour impacts and for managing genetic diversity as well as population size. Preliminary depth threshold criteria are proposed for use now, pending further research.

Reproductive interactions between sympatric brook and brown trout in a small Minnesota stream

The reproductive activities of sympatric brook (Salvelinus fontinalis) and brown trout (Salmo trutta) in a Minnesota stream were documented on a regular basis for 3 years to determine whether behavioral interactions between these species might influence their spawning activities and success. The spawning seasons of brook and brown trout consistently overlapped by 2–4 weeks, during which time nearly 10% of all sexually active females were simultaneously courted by males of both species. Male brook trout attempted to hybridize most frequently; however, both our behavioral observations and population census suggested that these fish had little success. There was also strong evidence of frequent redd superimposition, particularly by the later spawning and larger brown trout. Analysis of redd site habitat demonstrated that these species had overlapping preferences. Although it seems likely that attempted hybridization and redd superimposition work to the disadvantage of both species, the effects of these activities are likely to be particularly severe for the brook trout, which spawns earlier in the season, is smaller in size, and rarely survives to be old enough to spawn twice. Thus, reproductive interactions may be partially responsible for the displacement of brook trout by brown trout in many regions of North America.