Evidence of Alternative Tactics in Brook Trout Spawning Site Selection Related to Incubation Microhabitats in a Harsh Winter Environment

Species with distributions that span a broad range of latitudes may have populations that exhibit distinct life history traits associated with environmental gradients.  Although the majority of previous studies indicate a strong association between brook trout (Salvelinus fontinalis) spawning site selection and the presence of upwelling groundwater, the thermal regimes at northern sites suggest that the microhabitat scale association between groundwater upwelling and redd site selection may not span the entire latitudinal range of this species.  We investigated the role of hyporheic flow in brook trout redd site selection in a relatively high latitude boreal system.  Hyporheic flows through streambed substrates can be groundwater or surface water dominant, and be influenced by the presence of morphological features.  For autumn spawners such as brook trout, embryos situated in microhabitats where hyporheic flow in the shallow substrate is groundwater dominant (i.e., warmer in winter) are likely to experience accelerated development rates relative to embryos nested in redds that are under the influence of downwelling surface water (i.e., colder in winter).  To explain observed patterns in trout redd site selection in relation to hyporheic flow patterns we measured vertical hydraulic gradients (VHG) at the microhabitat scale, and the spatial and temporal variation in upwelling/downwelling flow and thermal regimes in brook trout spawning/incubation habitats.  Additionally, we noted the proximity of redd sites to stream morphological features (e.g., riffle-crests).  Our results indicate that upwelling flow was not used as a decisive cue in redd site selection at the microhabitat scale (10⁰m), as an approximately equal number of redds were situated in microhabitats with upward flow as were situated in microhabitats with downward flow.  Redds adjacent to a riffle-crest or log-step bedforms were associated with downward flow, whereas redds not immediately adjacent to these morphological features were associated with upward flow.  Winter streambed temperatures confirmed that both steady upwelling (i.e., warm incubation regime) and downwelling (i.e., cold incubation regime) sites were indeed selected by spawners and thus over two spawning seasons redd sites with distinct thermal regimes were selected. Our observations that spawners utilized both cold regime and warm regime sites suggests the existence of distinct reproductive tactics related to hyporheic flow patterns in this boreal system.  As temperature is the dominant factor controlling rates of embryonic development, the use of spawning microhabitats with distinct thermal regimes implies substantial differences in the timing of hatching and the phenology of emergence from these habitats.