In British Columbia and the Pacific Northwest, construction of off-channel habitat
including artificial ponds and groundwater-fed channels, figures prominently in
restoration efforts intended to benefit coho salmon (Oncorhynchus kisutch). To examine
how juvenile coho production was effected by autotrophic productivity in these systems,
I estimated the average size and abundance of juvenile coho during late summer and early
spring over two years in three pairs of physically similar channels and one pair of ponds
with one member of each pair having relatively high periphyton biomass and the other
member having low periphyton biomass. Differences in total alkalinity between paired
sites suggested that nutrient availability was a strong determinate of periphyton biomass
on artificial substrate. However, periphyton biomass did not appear limited by
phosphorus alone; phosphorus was relatively abundant in the groundwater channels, and
as a result, nitrogen and possibly other nutrients were also important. Average coho fry
size and biomass per unit area were greater by late summer in the periphyton-rich sites,
but fry density was not. Physical habitat variability also failed to explain high variation
in coho densities among the groundwater channels. Biomass and taxonomic composition
of benthic invertebrates also appeared to be related to periphyton biomass, but a strong
interaction between invertebrate abundance or distribution and current velocity was a
confounding factor.
In the subsequent spring, pre-smolt coho were still larger in the periphyton-rich site
in the majority of cases. However, water temperature during the winter had a much
greater effect on average pre-smolt size than growth rate the previous summer. In the
groundwater channels, consumption of chum salmon (O. keta) eggs, fry, and carcasses
may have also contributed to high coho growth rates during the winter. The abundance
of spring pre-smolts in the channels and ponds was not related to either periphyton
biomass or average fry size the previous summer. Favorable growing conditions during
the winter may have lessened size-dependent overwinter mortality, but the benefit of
nutrient addition to coho production in off-channel habitat remains uncertain. Pre-smolt
abundance in the off-channel sites was strongly influenced by small differences in water
depth and velocity in winter and by fry density the previous summer. Physical habitat
differences between the channels and ponds also influenced abundance: pre-smolts
densities were consistently higher in the deeper, more structurally complex ponds. Large,
density-dependent declines in coho abundance in the relatively warm and hydrologically
stable groundwater channels from September to March contradicts traditional emphasis
on starvation and adverse physical conditions (e.g., freshets) as primary sources of coho
winter mortality. My results suggest that other factors such as predation or energetics
warrant attention.
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:BVAU.2429/9267 |
| Date | 11 1900 |
| Creators | Decker, Andrew Scott |
| Source Sets | Library and Archives Canada ETDs Repository / Centre d'archives des thèses électroniques de Bibliothèque et Archives Canada |
| Language | English |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Relation | UBC Retrospective Theses Digitization Project [http://www.library.ubc.ca/archives/retro_theses/] |
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