Body size scaling of field swim performance was weaker than scaling of lab-measured swim performance. Lab data indicated that larger adult salmon could swim significantly faster than smaller fish (speed increased by 2.53 ± 0.13 cm·s
−1 for each cm FL,
N = 1385, Chi-square = 356.41, df = 1,
p = 2.20E−16;
Table 2, Table S2,
Table 2,
Fig. 8A). In contrast, field-measured aerobic speeds (<250 cm·s
−1) scaled negatively (slope = −0.54 ± 0.52,
N = 266, Chi-square = 1.07, df = 1,
p = 0.3;
Table 2, Table S2,
Table 2,
Fig. 8B). A similar, somewhat surprising, negative relationship was also observed in adult Atlantic salmon traversing a fishway in Norway (
Lindberg et al. 2016). Differences between lab and field size scaling relationships may be driven by fish behaviour, choice, and physical environment (
Castro-Santos 2005;
Peake and Farrell 2006), or by differences in the testing approach (
McDonald et al. 1998). For example, in the lab, the hydraulic conditions are carefully controlled, and fish are confined in swim tunnels swimming against near laminar flows. Furthermore, the selected swimming test can produce different scaling effects; specifically in juvenile salmonids the size effect was stronger using exhaustive TTF than modified
Ucrit (
McDonald et al. 1998). In the field, including at fishways, flows are often turbulent, and fish can freely choose which path to travel, and which flows to avoid (
Standen et al. 2004;
Castro-Santos 2005;
Lindberg et al. 2016;
Bett et al. 2022). Turbulent flows could reduce fish swimming speed (
Ucrit:
Tritico and Cotel 2010), have no measurable effect on swimming (TTF tests:
Nikora et al. 2003), or be exploited by fishes (
Liao 2004;
Hinch and Rand 2000) (physiological, behavioural mechanisms are reviewed by
Liao 2007). The effects of turbulence on swimming depends on the type of turbulence (intensity, periodicity, orientation, scale;
Lacey et al. 2012) as well as the size of the fish (i.e., fish to vortex size ratios). If the turbulent conditions are not beneficial, fish will behaviourally avoid it by searching alternative ways or bursting through turbulent sections to limit transit time (
Odeh et al. 2002;
Li et al. 2022) avoiding high costs of swimming through turbulence (
Hinch and Rand 1998;
Enders et al. 2003). Further, large fish could also prioritize exploring alternate paths avoiding fast flows because it is cumulatively more energy-efficient, not because of their inability to swim fast (
Lindberg et al. 2016, but see
Standen et al. 2004) or limited anaerobic exercise potential (
Casselberry et al. 2023). Lastly, the scaling of the field-measured anaerobic (>250 m·s−1) was weakly negative but not reliable due to low sample size (slope = −0.83 ± 2.34,
N = 63, Chi-square = 0.12, df = 1,
p = 0.72), while the overall field reported field speeds scaled weakly positively (slope = 0.91 ± 1.14,
N = 313, Chi-square = 0.6461, df = 1,
p = 0.42) (
Table 2 and supplementary Table S2; power analysis with recommendations for more robust scaling relationships is provided in Supplementary Fig. S3). The behavioural components of swimming may be particularly important to make ecologically relevant predictions about swim performance across different size adult salmon.
The relationship between individual swim speeds and body size can be nonlinear, which could be partly explained by nonlinear mass-specific tissue-level physiology. Accordingly, mass-specific glycolytic (anaerobic) enzyme activity (lactate dehydrogenase and pyruvate kinase) scaled positively and nonlinearly following a power function in white muscle in various fishes (
Childress and Somero 1990;
Norton et al. 2000;
Patterson et al. 2004a). Additionally, metabolic fuel storage that powers anaerobic swimming scaled positively, but linearly with size in salmonids (i.e., endogenous ATP and glycogen g
−1 in rainbow trout:
Ferguson et al. 1993; fat, endogenous ATP, glycogen stores, and somatic energy in Atlantic salmon
: Lennox et al. 2018a). Worth noting, the mass-scaling of these tissue-level physiological processes is species-specific and non-uniform across tissue types (
Childress and Somero 1990;
Goolish 1991;
Norton et al. 2000;
Norin and Malte 2012). More physiology-informed mechanistic studies (
Cooke et al. 2020;
Eliason et al. 2022), particularly on wild adult salmon, are required to better understand the scaling of swimming performance and field swimming behaviours.
Sex
Swim performance differences between male and female adult salmon tested in the lab remains inconclusive. For example,
Ucrit was lower by ∼0.3 BL·s
−1 in females compared to males in mature Fraser and Tompson River pink salmon (
Williams and Brett 1987), by up to ∼0.5 BL·s
−1 in mature migrating adult Atlantic salmon (
Booth 1998), and by ∼0.2 m·s
−1 in mature Schibetsu River pink salmon (
Makiguchi et al. 2017). However,
Umax was higher by an average of 5% in female Harrison River pink salmon (
Clark et al. 2011), and
Ucrit was similar in male and Fraser River female sockeye (
Farrell et al. 2003;
Steinhausen et al. 2008;
Wilson et al. 2013;
Eliason et al. 2013b), Chilliwack lake coho salmon (
Kraskura et al. 2021), and Shibetsu River chum salmon (
Makiguchi et al. 2008). Here, we were unable to detect systematic differences in lab-reported swim performance between the sexes (Supplementary Fig. S4). Across all species, females’ lab absolute swim speeds ranged between 37.00 and 237.60 cm·s
−1, and males swam between 65.07 and 237.52 cm·s
−1 (
N(unspecified) = 729,
N(females) = 298,
N(males) = 397). Additionally, we found no correlation between gonad mass, or gonadosomatic index (GSI) and individual swim speed (Supplementary Fig. S5BC). The next step will be to specifically examine if sex-specific differences in adult salmon swimming are related to species, population-specific, or individual variation in maturity state (i.e., degree of sexual secondary characteristics, such as kype and dorsal hump, that can alter hydrodynamics).
In the field, there is evidence that female swimming capacity may be lower compared to males, which could further contribute to female-biased mortality rates (
Bowerman et al. 2018;
Hinch et al. 2021). However, we could not confirm any consistent trends from the field data (Supplementary Fig. S4). Across all species, female field swimming speeds ranged between 35.99 and 749.26 cm·s
−1 (
N = 61), and male field swimming speeds varied between 6.46 and 631.36 cm·s
−1 (
N = 143; Supplementary Fig. S4; deficient data at species or population levels). However, others have demonstrated sex-specific migration behaviours and capacities. Specifically, male Atlantic salmon in Sweden were more likely to choose optimal flows compared to females (53% females, 63% males;
Lindberg et al. 2016), while female Gates Creek sockeye salmon were 9%–13% less likely to pass fishway compared to males (
Roscoe et al. 2011;
Burnett et al. 2014b, reviewed by
Hinch et al. 2022). However, there is no clear evidence that bursting speeds in female adult salmon are lower than in males (e.g.,
Booth 1998;
Hinch et al. 2002); Supplementary Fig. S4), though anaerobic recruitment and capacities may differ between sexes (
Burnett et al. 2014b). Additionally, females appear to be more energy conservative compared to males traversing fast flows (e.g., Fraser River pink and sockeye salmon;
Rand and Hinch 1998;
Standen et al. 2002). Swim performance can diverge between male and female adult salmon as they mature
en route to their spawning grounds. Specifically, in Exploits River, NL, Canada, Atlantic Salmon
Ucrit was lower by 0.5 ∼ BL·s
−1 in fully mature females compared to fully mature males, while burst and sustained swimming did not differ between sexes at any maturity state (
Booth 1998). Alternatively, in Fraser River pink salmon,
Ucrit differed based on the fish’s maturity state, but inconsistently across sexes (
Williams and Brett 1987). Possibly, swim performance changes interactively with sex and maturity (
Plaut 2002;
Cooke 2004;
Svendsen et al. 2009) or emerges only under multi-stressor conditions (reviewed by
Hinch et al. 2022).
Various factors could drive lower swim performance in females compared to males. For example, migrating females and males undergo different morphological changes that alter their hydrodynamics (e.g., females change in circumference, males develop dorsal hump and kypes;
Booth 1998;
Conradsen and McGuigan 2015); females may have a lower aerobic metabolic scope (
Clark et al. 2011) and divert more of their aerobic capacity towards gonad development and maintenance (
Fenkes et al. 2016). Additionally, migrating females have elevated cortisol levels that can impair their recovery from strenuous swimming (
Pagnotta et al. 1994;
Hruska et al. 2010;
Eliason et al. 2020; section
Recovery). The extent to which female and male swimming performance differs and “why” are unresolved questions (e.g., Supplementary Figs. S4 and S5). However, they are crucial to predict population stability and dynamics of salmon species that have a once-in-a-lifetime chance to spawn (
Hanson et al. 2008b).