Habitat Selection and Overlap of Atlantic Salmon and Smallmouth Bass Juveniles in Nursery Streams

Abstract

Introduced smallmouth bass Micropterus dolomieu have invaded much of the historic freshwater habitat of Atlantic salmon Salmo salar in North America, yet little is known about the ecological interactions between the two species. We investigated the possibility of competition for habitat between age‐0 Atlantic salmon and age‐0 and age‐1 smallmouth bass by means of in situ observations and a mesocosm experiment. We used snorkel observation to identify the degree and timing of overlap in habitat use in our in situ observations and to describe habitat shifts by Atlantic salmon in the presence of smallmouth bass in our mesocosm experiments. In late July 2008, we observed substantial overlap in the depths and mean water column velocities used by both species in sympatric in situ conditions and an apparent shift by age‐0 Atlantic salmon to shallower water that coincided with the period of high overlap. In the mesocosm experiments, we detected no overlap or habitat shifts by age‐0 Atlantic salmon in the presence age‐1 smallmouth bass and low overlap and no habitat shifts of Atlantic salmon and age‐0 smallmouth bass in fall 2009. In 2009, summer floods with sustained high flows and low temperatures resulted in the nearly complete reproductive failure of the smallmouth bass in our study streams, and we did not observe a midsummer habitat shift by Atlantic salmon similar to that seen in 2008. Although this prevented us from replicating our 2008 experiments under similar conditions, the virtual year‐class failure of smallmouth bass itself is enlightening. We suggest that future studies incorporate the effects of varying temperature and discharge to determine how abiotic factors affect the interactions between these species and thus mediate the outcomes of potential competition.

Effects of Smallmouth Bass on Atlantic Salmon Habitat Use and Diel Movements in an Artificial Stream

“…Thus, under alternative scenarios, results may differ. In a complementary field study (Wathen et al 2011), we observed a period of high habitat overlap between age-0 Atlantic salmon and age-0 smallmouth bass during the summer of 2008. This overlap occurred in mid-to late summer, when water levels were low, water temperatures were high (about 22 • C), and fish were similar in size.…”

“…The similarity of energetically profitable habitats for juvenile Atlantic salmon and smallmouth bass may result in habitat overlap and competition for habitat between juveniles of both species. Wathen et al (2011) found a high degree of habitat overlap between sympatric age-0 smallmouth bass and age-0 Atlantic salmon in warm, low-water conditions. Because Atlantic salmon and smallmouth bass have not coevolved niche partitioning mechanisms, competitive interactions may intensify (Fausch 1998).…”

“…Ideal studies combine research conducted in natural settings with laboratory research to form a morecomprehensive understanding of the mechanisms of competition (Fausch 1998). Our study was designed to be a complement to research conducted in natural conditions during the spring, summer, and fall in 2008(Wathen et al 2011.…”

Environmental effects on survival rates: robust regression, recovery planning and endangered Atlantic salmon

“…These results do not preclude the possibility that a relationship between egg to age 0 survival and alternate hydrological predictors exist, nor that additional data collection and a longer time series would enable a relatively weak relationship to be described. However, in terms of guiding recovery planning, alternate threats that are not as strongly linked to hydrology, such as the effects of invasive smallmouth bass and chain pickerel (Wathen et al 2011) lights the implications that these assumptions can have for future recovery planning. Given the declining trends in a large number of freshwater fish species (e.g., Dudgeon et al 2006) as well as the limited time and resources available for remediation, the efficient identification of priorities for recovery planning is a pressing ecological issue.…”

“…However, in terms of guiding recovery planning, alternate threats that are not as strongly linked to hydrology, such as the effects of invasive smallmouth bass and chain pickerel (Wathen et al. ) or changes to water quality (Paul and Meyer ) should be investigated.…”