Brook Trout (Salvelinus fontinalis) are the only trout species native to the state of Maryland. Brook Trout are the top predators in many coldwater streams but their range has been declining in Maryland, as well as elsewhere in the eastern US, due to their sensitivity to environmental disturbance.
Once widespread throughout Maryland west of the fall line, Brook Trout have been increasingly restricted to isolated headwater populations over the last 100 years. Human-induced stressors to Brook Trout populations include: land use change, climate change, and population fragmentation. Climate change effects could be especially pronounced for Brook Trout because they are a coldwater species. Climate change is expected to bring a warming of stream temperatures, as well as an expected increase in frequency of extreme weather events, like drought and floods. The impacts of these stressors are expected to continue or worsen, making the future of the Brook Trout in Maryland uncertain.
In Maryland, Brook Trout are clustered in three areas, roughly corresponding to the Piedmont, Catoctin Mountains, and the Appalachian Plateau, where populations are most robust and the most likely to persist into the future based on climate change predictions. The Savage River watershed within the Appalachian Plateau hosts relatively robust Brook Trout populations and represents a regionally important stronghold for Brook Trout. A large portion of this watershed has been placed under special angling regulations, prohibiting the use of live bait or harvest of Brook Trout, which provides an excellent opportunity to follow large numbers of Brook Trout through space and time while evaluating the effectiveness of these special regulations.
Research Area Description
The Savage River watershed on the Appalachian Plateau of western Maryland contains more than 100 km of interconnected coldwater streams and has been identified as an important stronghold for wild Brook Trout. We primarily focused on the Big Run watershed within Savage River, which is almost entirely on public land. Middle Fork, another tributary flowing directly into Savage River Reservoir served as a complimentary study area to boost our spatial coverage in the watershed and allow for comparisons among tributaries for certain aspects of our work. The aquatic habitats are representative of many Brook Trout streams in the eastern portion of the species range.
Large Scale Mark-Recapture Study and growth variation in a Mid-Atlantic Brook Trout population
Brook trout are a highly plastic and variable species whose life history, habitat use, movement patterns, morphology, and genetics may all vary markedly among and within populations. In turn, growth and longevity in Brook Trout are also highly variable within and among populations, resulting in considerable differences in population size structure and reproductive output. Because size and reproductive output are closely linked, growth rates have a strong influence on population dynamics. This phase of our work focused on trying to identify factors that are important controls of growth in order to identify opportunities for managers to enhance wild trout populations.
We used tagging data from more than 2,200 individually marked fish within the study areas. Fish were collected annually during early summer from 2010-2013 using three-pass backpack electrofishing surveys throughout each of the study sections within the Big Run watershed. Fish were anesthetized and surgically implanted with a passive integrated transponder (PIT) tag. Two stationary antennas were installed within the study area – one on Monroe Run approximately 50 m upstream of its confluence with Big Run (within section 4 on map) and the second at the upstream extent of our study area on Upper Big Run (within section 3 on map). These efforts yielded 681 recapture events during 2011 and 2012. Based on recaptures of individually marked fish, we characterized the physical movement, growth, and survival patterns of Brook Trout within the study area. We described these movement patterns using both the mark-recapture observations and stationary PIT antennas, which would detect fish movements that would have otherwise gone undetected using only a mark-recapture study design. We calculated annual growth increments for any individuals that were recaptured on consecutive summer electrofishing samples. We then generated individual encounter histories for 2,973 fish representative of the two streams based on four years of electrofishing surveys.
Using the recapture data from both Big Run and Middle Fork watersheds, we quantified growth rates. Our results show that size is a poor proxy for age and that many of the drivers of Brook Trout growth in these streams are uncontrollable. Individual growth rates varied greatly among fish, ranging from 0 to 144 mm per year. We also found substantial growth differences among years, resulting in some fish in their second year being as long as four-year-old fish. We also found sex-specific differences in growth with males growing 10.5 mm per year faster on average than females and also documented significant growth differences among stream reaches, highlighting the importance of downstream areas for Brook Trout growth. Our results suggest that population projections and management decisions based on a single year of data may be considerably different than those based on multiple years of data, which are required to capture the inherent variability within a population. Projected increases in environmental variability will only increase the importance of longer-term datasets.
The average relative reproductive value for each stage also varied across the different scenarios. Under conditions of decreased growth and survival, large Brook trout are the most valuable with medium-sized individuals also of enhanced value.
Our study suggests fishing regulations may have some utility to promote population resilience in western Maryland. It also suggests that the aspects of Brook Trout ecology we examined are naturally somewhat resilient to environmental change. Where environmental change significantly jeopardizes Brook Trout, regulatory measures alone are unlikely to be adequate to conserve wild populations. Collectively, these results suggest regulatory approaches may offer some utility in promoting population resilience while enhancing the quality of the fishery, but are likely insufficient to fully offset the impacts of predicted environmental changes.
These tools have been used to improve our understanding of the structure and function of Brook Trout populations in western Maryland in a variety of different areas.