Seasonality can affect ecological interactions between fishes of different thermal guilds

Abstract

Seasonality could play a crucial role in structuring species interactions. For example, many ectotherms alter their activity, habitat, and diet in response to seasonal temperature variation. Species also vary widely in physiological traits, like thermal preference, which may mediate their response to seasonal variation. How behavioral responses to seasonality differ between competing species and alter their overlap along multiple niche axes in space and time, remains understudied. Here, we used bulk carbon and nitrogen stable isotopes combined with stomach content analysis to determine the seasonal diet overlap between a native cold-water species [lake trout (Salvelinus namaycush)] and a range-expanding warm-water species [smallmouth bass (Micropterus dolomieu)] in two north-temperate lakes over 2 years. We coupled these analyses with fine-scale acoustic telemetry from one of the lakes to determine seasonal overlap in habitat use and activity levels. We found that dietary niche overlap was higher in the spring, when both species were active and using more littoral resources, compared to the summer, when the cold-water lake trout increased their reliance on pelagic resources. Telemetry data revealed that activity rates diverged in the winter, when lake trout remained active, but the warm-water smallmouth bass reduced their activity. Combining stable isotopes and stomach contents with acoustic telemetry was a powerful approach for demonstrating that species interactions are temporally and spatially dynamic. In our case, the study species diverged in their diet, habitat, and activity more strongly during certain times of the year than others, in ways that were consistent with their thermal preferences. Despite large differences in thermal preference, however, there were times of year when both species were active and sharing a common habitat and prey source (i.e., resource overlap was greater in spring than summer). Based on our findings, important ecological processes are occurring during all seasons, which would be missed by summer sampling alone. Our study stresses that quantifying multiple niche axes in both space and time is important for understanding the possible outcomes of altered seasonal conditions, including shorter winters, already arising under a changing climate.

Seasonal variation in activity and nearshore habitat use of Lake Trout in a subarctic lake

“…We also note that the equations developed to examine the relationship between swimming speed and accelerometer data were based on lab trials conducted at a single water temperature (12 °C;[ 51 ]) and should be recalibrated across a range of environmentally relevant temperatures that Lake Trout occupy. Our estimates of average winter swim speed from the array (winter1: ~7.5 m min − 1 ; winter2: ~4.5 m min − 1 ) were comparable to, although slightly higher than, those for Lake Trout from a similar telemetry study in a southern lake, and which also demonstrated similar variation in swim speed from one winter to the next (winter1: 3.0 m min − 1 ; winter2: 5.8 m min − 1 ) [ 55 ].…”

“…We also observed significantly lower rates of activity in the second winter of the study, which followed a prolonged fall period that lasted three weeks longer than in the previous year. Differences in Lake Trout winter movement rates were also observed between years in a similar study at a more southern lake [ 55 ], and may reflect the energetic constraints imposed by the demands of the preceding fall spawning season.…”

“…Nearshore use by Lake Trout in small lakes lacking pelagic prey fish has been shown to be variable. In one study, Lake Trout appeared to predominantly occupy the upper few meters of the water column in offshore regions of the lake in winter [ 53 ], whereas in another study, Lake Trout were found to occupy both nearshore and offshore regions [ 54 , 55 ]. In the present study, we expect that ice formation, which is typically much greater than in more southern areas and measured up to 75 cm thick in early winter [ 56 ], would have limited access to a portion of the nearshore area, and therefore winter use of this area is likely underestimated.…”

“…The generally lower rates of activity in winter compared to summer have been broadly observed for fishes [ 61 ]. For Lake Trout, however, there is growing evidence of relatively high rates of winter activity that can be similar to, or even greater than, during summer [ 53 , 55 ]. A steady decline in activity as winter progresses, which we observed in both years of our study, has not been documented for Lake Trout before.…”

“…Differences between studies may also be related to the food web in each lake. Lake Trout in Alexie Lake have access to Cisco, which were absent in the other study lake [ 55 ], and therefore may have more energy reserves to sustain greater rates of movement in winter, albeit gradually declining as winter progresses. We also observed significantly lower rates of activity in the second winter of the study, which followed a prolonged fall period that lasted three weeks longer than in the previous year.…”