Adult size, egg mass, fecundity and mass of gonads are affected by trade-offs between reproductive investment and environmental conditions shaping the evolution of life-history traits among populations for widely distributed species. Coho salmon Oncorhynchus kisutch have a large geographic distribution and different environmental conditions are experienced by populations throughout their range. We examined the effect of environmental variables on female size, egg size, fecundity, and reproductive investment of populations of Coho Salmon from across British Columbia using an information theoretic approach. Female size increased with latitude and decreased with migration distance from the ocean to spawning locations. Egg size decreased with average intragravel temperature during incubation, migration distance, in larger rivers, but increased in rivers that were lake headed. Fecundity increased with latitude, warmer temperature during the spawning period, and river size, but decreased in rivers that were lake headed compared to rivers with tributary sources. Gonadal somatic index increased with latitude and decreased with migration distance. Latitude of spawning grounds, migratory distance and temperatures experienced by a population, but also hydrologic features – river size and headwater source – are influential in shaping patterns of reproductive investment, particularly egg size. The lack of an effect of latitude on egg size suggest that local optima for egg size may drive the positive relationship between egg number and latitude – a pattern that is partially off-set by larger female size and gonadal somatic index with latitude.

Data were obtained from Fisheries and Oceans Canada (FOC) and included mean female size (post orbital hypural length [L_POH] most commonly or fork length [L_F]), mean egg mass (M_egg), and mean fecundity (F) for each year data were available. Additional life-history data on mean F and L_POH were also obtained from Fleming and Gross (1990) for Big Qualicum River, Black Creek, and Quinsam River populations, and L_POH from Meldrum et al. (2017) and references therein for Black Creek.

For female size, measurements for L_F were converted to L_POH according to the relationship established by Fleming and Gross (1990) and L_POH was used in all models (see below). Egg mass was used in all calculations, but only volume measurements were available for the Toboggan Creek population. Volume was transformed to weight by multiplying by the fraction of space occupied by random packed spheres (0.64; Finney 1970) and the average density of salmon eggs (1.076; Bonham 1976). A measure of reproductive investment, gonadal somatic index (I_G), was calculated from the average M_egg multiplied by average F and expressed as a ratio of gonad weight to somatic weight for each population and year that M_egg, F, and L_POH were available. To calculate female weight, L_POH was converted to fork length (L_F) by multiplying by 1.292 · L_POH – 37.21 (Fleming and Gross 1990). Based on a condition factor of 1.06 g · cm^–3 (K; 100 M · L_F^–3), determined from Kitimat River population females, L_F was used to calculate an approximate total weight. A K of 1.06 for mature female Coho Salmon falls within the range reported by Scholz et al. (2011) for spawning female Coho Salmon.

Environmental variables were chosen based on their expected importance for explaining variation in L_POH, M_egg, F, and I_G for anadromous salmon. Environmental data were collected from global positioning system measurements of spawning locations, the Terrain Resource Information Management maps and databases of hatchery operations from FOC. The average latitude (LAT) for each population was used with an accuracy of ± 3 m. Migration distance from the mouth of the Fraser River to the spawning grounds (D_MIG) was also calculated for each population. Years of hatchery operations (YOH) was based on the difference in time between when FOC initiated supplemental stocking for a population and when the data used for the analysis was collected. Stream width (WD) was categorized as either small (< 5 m wetted width) or large (> 5 m wetted width). Headwater (HW) sources were categorized as streams or lakes.

For each location, temperature loggers (HOBO U-22; Onset Computer Corporation, Bourne MA) were placed in three different locations within the river mainstem or tributaries to the mainstem river (Tuor and Shrimpton 2019). A surface temperature logger and two to three intragravel temperature loggers were deployed at each site within a study reach; intragravel loggers within a site were a minimum of three meters apart to capture some of the potential variation in temperatures from different redds within each system. Intragravel temperature loggers were buried at a depth of approximately 25 cm. Loggers recorded temperature hourly for approximately six months throughout incubation from November 2012 to May 2013. Additional temperature data was provided by FOC for the Quinsam River (2000 to 2017) and Big Qualicum River (2005 to 2018) watersheds. Mean surface water temperature was calculated during the peak of spawning from November 16 to 30 (T_NOV). Average intragravel temperature (T_AVG) was calculated from November 16 to May 15 or until 750 ATU had been reached. Average intragravel temperature was calculated during the coldest period of incubation – the month of January (T_JAN). 

References

Fleming, I.A., & Gross, M.R. (1989). Evolution of adult female life history and morphology in Pacific salmon (coho: Oncorhynchus kisutch). Evolution, 43, 141–157.

Meldrum, J., Campbell, K., Van Will, P., Stiff, H.W., Nagtegaal, D., Miyagi, E., & Duncan, K. (2017). Enumeration of juvenile and adult Coho salmon at Black Creek, Vancouver Island, 2013. Canadian Manuscript Report of Fisheries and Aquatic Sciences, 3116: viii + 273 p.

Tuor, K.M.F., & Shrimpton, J.M. (2019). Differences in water temperatures experienced by embryo and larval Coho Salmon (Oncorhynchus kisutch) across their geographic range in British Columbia. Environmental Biology of Fishes, 102, 955-967. DOI: 10.1007/s10641-019-00882-7