Growing season degree days and thermal energy as intrinsic potential variables #21

Context

Water temperature drives fish fitness through thermal energy budgets — not just upper lethal limits. The key metric is growing season degree days (GSDD): the cumulative thermal energy available to juvenile fish during the growing season (~April/May to October). Juveniles must grow large enough during the growing season to survive winter. In cold, northern BC watersheds, insufficient GSDD is often the limiting factor — not excessive heat.

This connects directly to habitat eligibility (#19) and intrinsic potential modelling. A stream segment that meets gradient and channel width thresholds for rearing may still be unsuitable if it lacks sufficient thermal energy for juvenile growth. Conversely, a segment that looks marginal on gradient alone may be high-value rearing if it has strong GSDD.

Biological basis

• Each species has minimum GSDD requirements for juveniles to reach overwinter-viable size
• Westslope cutthroat trout (WCT) is well-studied — minimum GSDD thresholds for survival are documented in the literature
• The constraint applies to all species as resident fish — insufficient thermal energy means juveniles can't grow fast enough to survive winter
• This is distinct from thermal tolerance (too hot for spawning/rearing) — both matter but GSDD is the less obvious and often more limiting factor in BC's cold watersheds
• Fitness ties to: can the fish capture enough of the sun's energy during the growing season to grow large enough to overwinter?

Prior work

Stream temperature modelling (Poisson Consulting / New Graph)

• skeena-stream-temp-25/ — Spatial stream temperature modelling, Skeena region
• fish-passage-22b/ — Fish passage reporting with integrated temperature analysis
• @hill_etal2025Spatialstream — Spatial stream temperature model (2025)
• @hill_etal2024Spatialstream — Spatial stream temperature model (2024)

These projects explore GSDD and survival rates, species-specific thermal requirements, and spatial modelling of temperature across stream networks. The memos with Poisson touch on WCT GSDD requirements specifically.

WSF proposal

Temperature and water-quality workflow development is a core component of the Watershed Data Workflow Framework (Poisson Consulting scope). Province-wide temperature model outputs mapped to FWA segments would enable integration into fresh/link.

Two dimensions of temperature as habitat variable

1. Growing season degree days (GSDD) — thermal energy for growth

• Species-specific minimum GSDD thresholds
• Determines whether juveniles can grow to overwinter-viable size
• The primary limiting factor in cold northern watersheds
• Mapped as cumulative thermal units per growing season per stream segment

2. Thermal tolerance — upper limits

• Maximum temperatures for spawning (e.g., BT spawning requires cold water)
• Maximum temperatures for rearing (lethal/sublethal thresholds)
• Climate change dimension — warming streams may cross tolerance thresholds

Implementation path (future)

1. Province-wide temperature model outputs mapped to FWA segments (depends on Poisson/Hillcrest work)
2. Species-specific GSDD thresholds and thermal tolerance ranges added to parameters_habitat.csv
3. frs_habitat_classify() extended to accept temperature ranges alongside gradient and channel width
4. Seasonal dimension — growing season window definition per region/elevation

Zotero collections

• Fisheries/temperature — thermal tolerance, GSDD, stream temperature modelling
• Fisheries/species — species-specific thermal requirements
• hydrology — discharge-temperature relationships

Relates to #20, #19, #18