Patterns on a parr: Drivers of long‐term salmon parr length in U.K. and French rivers depend on geographical scale

Abstract

Understanding the geographical scales at which environmental variables affect an individual's body size, and thus their mortality risk, can inform management strategies to help conserve wild populations under climate change. Yet, our current understanding of these relationships is based on studies done at different scales that report inconsistent findings. We predicted that temperature‐related variables (e.g. winter temperature) influence body size at a “regional” scale, that is, affecting individuals in geographically independent catchments similarly, whereas non‐temperature‐related variables (e.g. conspecific competitor density) exert a “local” influence, that is, affecting individuals in geographically independent catchments differently. We developed statistical models to test our predictions using body length measures of a large and long‐term sample of juvenile Atlantic salmon (Salmo salar) from three rivers in the U.K. and France. We developed mixture models to predict the individual juvenile salmon ages objectively from their body length. We then developed linear mixed models to describe inter‐annual changes in mean length of the youngest (age 0) cohort of juvenile salmon from river‐specific seasonal variables, and tested whether they exerted their influence at a “local” or “regional” scale. All models accounted for spatio‐temporal differences in sampling protocols and individual reproductive strategy. We estimated and interpreted coefficients using Bayesian theory. Our findings supported our predictions. Juvenile salmon were longer in years of higher overwinter water temperature and this effect was best parameterised as a single “regional” coefficient applicable to all three rivers. Similarly, spring mean temperature was best parameterised with a single “regional” nonlinear coefficient. In contrast, juvenile salmon were shorter in years of high densities of competing conspecifics and their interaction with total mean discharge and these effects were represented by “local” river‐specific coefficients. Summer droughts had a negative effect on juvenile salmon length but was best parameterised as a single “regional” coefficient, contrary to our expectations. We show that environmental variables affect biological processes at different but predictable geographical scales. Temperature‐related variables affect body sizes of exothermic animals at a regional scale, whereas non‐temperature variables, such as the density of conspecific competitors and water abstraction, exert their influence at a local scale. These findings highlight the importance of integrating local and regional management plans to mitigate the impacts of climate change on the body size, and ultimately the conservation, of exothermic species.

Roles of discharge and temperature in recruitment of a cold‐water fish, the European grayling Thymallus thymallus, near its southern range limit

“…The number of days with low discharge was tested against grayling recruitment to capture the effect of low summer discharge rates as an important stressor of the 0+ fish, given that several studies have demonstrated that juvenile salmonid growth and even survival are depressed when summer discharge rates are reduced and vice versa (Davidson, Letcher, & Nislow, 2010;Gregory et al, 2017;Xu, Letcher, & Nislow, 2010). Both Charles et al (2006) and Letcher et al (2015) revealed that survival of 0+ grayling and brook trout survival was reduced during years of low summer discharges and higher water temperatures.…”

“…For example, Skoglund, Einum, and Robertsen (2011) revealed salmon eggs incubated at a higher temperature produced larger eggs with higher survival rates versus those incubated at lower temperatures. The general pattern is warmer winters can result in the production of larger juveniles, perhaps due to a more prolonged growth season (Gregory et al, 2017), with these larger individuals then potentially having higher survival rates via greater access to better feeding areas than smaller conspecifics that might be more easily displaced by other foraging salmonids (Huntingford, Metcalfe, Thorpe, Graham, & Adams, 1990). The postincubation temperature anomaly was parameterised as a quadratic effect and had significant and positive effect on postincubation survival up to a threshold of 13.5°C; at higher temperatures, the effect became negative.…”

These are not the trends you are looking for: poorly calibrated single‐pass electrofishing data can bias estimates of trends in fish abundance

“…There was a significant linear increase in L F over the monitoring period. This could be due to long-term reductions in juvenile density resulting in larger fish (Bohlin et al, 2002;Gregory et al, 2017;Jonsson et al, 1998;Teichert et al, 2011), decadal changes in temperature that result in faster growth of fish (Gurney et al, 2008), or changes in water quality, quantity or food provision that affect fish size (Nislow et al, 2004). In the Girnock Burn, the bias in the estimates of trends in abundance based on the uncalibrated data reflected both the trend in L F (owing to its consequent effect on p) and the trend in p due to unexplained factors (the year effect).…”

Is bigger really better? Towards improved models for testing how Atlantic salmon Salmo salar smolt size affects marine survival

“…(It is acknowledged that a management strategy designed to maximize both number and size and condition of smolts would have to account for many complicating factors, such as any negative effect of density dependence on body size.) This could be a particularly pertinent message at present, given evidence that body sizes of juvenile S. salar are decreasing in countries such as England and France (Gregory et al, 2017).…”