Distribution and Abundance of the
Migratory Bull Trout Population in the
Castle River Drainage, 2011 – 2014


Brad Hurkett and Jason Blackburn


The Castle River bull trout population is one of 38 populations in Alberta identified as
at High Risk as a result of the effects from anthropogenic activities in the drainage.
Factors affecting the Castle bull trout population include habitat degradation,
fragmentation, migration barriers, illegal harvest, and the introduction of non-native
fish species. The increase in industrial, recreational and agricultural activities is posing
great risk to fisheries in the Castle River drainage.

Few studies have been completed on the Castle River drainage bull trout population,
and managers are challenged with making decisions using insufficient data. Alberta
Conservation Association completed a four-year (2011 – 2014) migratory bull trout
population and spawning assessment in the Castle River drainage to collect data to
supplement existing data and help conserve the species. The objectives of the study
were to determine the abundance of the migratory bull trout population in each major
spawning stream and to document the distribution of bull trout spawning areas
throughout the drainage.

Post-spawn bull trout were captured with fish traps installed in the fall season in four
major spawning tributaries throughout the drainage: Mill Creek and South Castle,
Carbondale and West Castle rivers. Adult bull trout (≥300 mm fork length) were
individually marked using passive internal radio frequency transponder tags to
identify recapture events. We supplemented the number of tagged fish by angling.
Spawning surveys were conducted in all bull trout spawning streams throughout the
drainage. We enumerated redds and assessed spawning habitat in all spawning areas.
Between 2011 and 2014, 459 bull trout were captured in the drainage, with fish catch
being highest in Carbondale River (n = 185) followed by South Castle River (n = 149)
and Mill Creek (n = 109); we experienced the lowest catch rates in the West Castle and
Castle rivers.

Fish size and recapture data indicate that bull trout populations in the Carbondale and
South Castle rivers and in Mill Creek are primarily migratory. These data also suggest
that resident components are present in the South Castle River and Mill Creek
populations. Our data suggest the West Castle bull trout population primarily consists
of resident fish and may include a small component of migratory fish. Our recapture
data highlight that migratory fish exhibit strong stream fidelity as most of the tagged
fish were recaptured in the same stream as their initial capture. Recapture data also
revealed migratory behaviours between different streams inside and even outside the
Castle River drainage. Fish from Carbondale River were documented to have the most
widespread migratory tendencies. We detected instances of trap avoidance behaviour
and calculated the annual migratory bull trout population in the main spawning area in
upper Mill Creek.

We documented 68 km of active bull trout spawning habitat throughout the drainage.
South Castle River was found to be the most extensive spawning stream in the entire
study area. We identified three primary spawning streams in the Carbondale River subdrainage.
All spawning areas were influenced by groundwater. Most of the major
spawning areas were immediately downstream of seasonal subsurface stream flow
breaks. The remaining subsidiary spawning areas were influenced by bank seeps and
other types of stream upwellings.

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