A dynamic-bioenergetics model to assess depth selection and reproductive growth by lake trout (Salvelinus namaycush)

Abstract

We coupled dynamic optimization and bioenergetics models to assess the assumption that lake trout (Salvelinus namaycush) depth distribution is structured by temperature, food availability, and predation risk to maximize reproductive mass by autumn spawning. Because the model uses empirical daily thermal-depth profiles recorded in a small boreal shield lake (lake 373 at the Experimental Lakes Area, northwestern Ontario) during 2 years of contrasting thermal stratification patterns, we also assessed how climate-mediated changes in lakes may affect the vertical distribution, growth, and fitness of lake trout, a cold-water top predator. The depths of acoustic-tagged lake trout were recorded concurrently with thermal-depth profiles and were compared to model output, enabling an assessment of model performance in relation to the observed fish behavior and contrasting thermal conditions. The depths and temperatures occupied by simulated fish most closely resembled those of the tagged fish when risk of predation was included in the model, indicating the model may incorporate the most important underlying mechanisms that determine lake trout depth. Annual differences suggest less use of shallow (warm), productive habitats, resulting in markedly less reproductive mass, during the year with the warm stratification pattern. Mass for reproduction may be lower in warmer conditions because of reduced reproductive investment, yet survival may be inadvertently higher because risky surface waters may be avoided more often in warmer, shallower, and metabolically costly conditions. At a minimum our study suggests that lake trout reproductive mass and fitness may be expected to change under the anticipated longer and warmer stratification patterns.

Behavioral responses to annual temperature variation alter the dominant energy pathway, growth, and condition of a cold-water predator

“…Regular, although limited, foraging into waters with temperatures above their thermal preference indicates the energetic importance of acquiring large, energy-dense prey (27). This suggests that capturing prey fish in warm water could be a more efficient strategy than foraging on small prey within the pelagic zone-at least for some parts of summer.…”

“…Further reductions in access to littoral regions with future warming could prevent lake trout from accumulating sufficient energy to spawn in the fall (27), potentially increasing the frequency of skip-spawning (32,33). The prospective smaller postadult body size with warming could also lead to reductions in fecundity, which is positively correlated with body size (34).…”

Climate change alters the quantity and phenology of habitat for lake trout (Salvelinus namaycush) in small Boreal Shield lakes

“…Moreover, King et al (1999) found 435 that in years when thermal stratification was earlier, lake trout had reduced growth, likely due to 436 reduced access to littoral energy without thermal consequence. This is further supported by 437 evidence that lake trout typically occupy the upper portions of the water column until surface 438 waters reach 15 ºC (Plumb and Blanchfield 2009) and achieve lower growth with reduced use of 439 littoral habitat (Plumb et al 2014). Therefore an expansion of the spring period could be 440 important for tempering the effects of future warming in lakes where littoral prey fish are the 441 main source of energy for lake trout.…”

“…In these lakes, lake trout 432 are highly dependent on their ability to access to littoral forage fish (i.e. minnows) and benthic 433 invertebrates when surface water temperatures are cool to achieve sufficient growth for 434 reproduction in fall (Martin 1952, 1970, Plumb et al 2014). Moreover, King et al (1999) found 435 that in years when thermal stratification was earlier, lake trout had reduced growth, likely due to 436 reduced access to littoral energy without thermal consequence.…”

“…Interestingly, minimum 518 optimal oxythermal habitat became more variable over time, while usable oxythermal habitat did 519 not, suggesting that optimal habitat may be more sensitive to climate change. With evidence 520 that lake trout behaviour in these lake types follows usable habitat (Plumb and habitat suggests that lake trout will need to be highly adaptable in their use of littoral habitat and 525 prey to minimize exposure to warm water (Plumb et al 2014 …”