Abstract
AbstractThe hypothesis that size‐selective mortality in the first marine year is a major regulator of recruitment in Pacific salmon Oncorhynchus spp. has led to interest in assessing the recent growth of field‐caught fish. Understanding differences in relative growth across years, regions, habitats, and prey fields may provide insights into factors influencing survival. Plasma insulin‐like growth factor 1 (IGF1), muscle RNA : DNA ratio (RD), and scale circulus spacing have all been used as indices of recent growth in juvenile salmonids. We concurrently assessed these growth rate indices in a laboratory study of postsmolt, young‐of‐the‐year, ocean‐type Chinook Salmon O. tshawytscha. We synthesized results with previous work to inform selection of appropriate growth rate indices for field studies on juvenile salmonids. Muscle samples suitable for RD analysis were obtained nonlethally and without subsequent growth impacts, even for very small juvenile salmon (75–99 mm FL). Plasma IGF1 concentration was strongly correlated with growth rate (R2 = 0.79), while loge(RD) and mean spacing of the outer two circuli were moderately correlated with growth rate (R2 = 0.47 and 0.44, respectively). Relationships between the two biochemical indices and growth rate were independent of body size at the start of the experiment. Conversely, initially larger fish formed wider circuli for a given growth rate. Insulin‐like growth factor 1 and RD responded to a change in growth conditions within 4 and 6 d, respectively. Rate of circulus formation varied positively with growth rate, meaning that outer circulus spacing indexed shorter periods of growth in faster‐growing fish. Our results confirm the value of plasma IGF1 as an index of recent growth in juvenile salmon. Although scales (marginal circulus spacing) and white muscle (RD) can both be sampled nonlethally, scale sampling presents a number of practical advantages.
Highlights
Marine residence is widely considered to be a period of high and variable mortality for juvenile Pacific salmon, with implications for both interannual variation and long-term trends in recruitment (Parker 1965; Beamish and Mahnken 2001; Wells et al 2012; Wells et al 2016)
We found that insulin-like growth factor 1 (IGF1) responded to a change in growth conditions within 4 days; earlier than differences in growth rate could be measured directly
Where fish may be sampled lethally, IGF1 provides a robust index of recent relative growth rates
Summary
Marine residence is widely considered to be a period of high and variable mortality for juvenile Pacific salmon, with implications for both interannual variation and long-term trends in recruitment (Parker 1965; Beamish and Mahnken 2001; Wells et al 2012; Wells et al 2016). Given the aforementioned hope of uncovering generalized mechanisms to explain declines in marine survival, many studies have investigated variation in juvenile growth and potential causal factors at large spatial or temporal scales These studies have included comparisons of feeding ecology, growth, and environmental conditions across years (Duffy and Beauchamp 2011, Tomaro et al 2012, Graham et al 2019), broad marine regions (Ferriss et al 2014, Hertz et al 2015), and general habitat types (e.g. nearshore vs offshore; Duffy et al 2010). Diets of one Coho Salmon and twelve Chinook/Coho Salmon hybrids sampled in 2015 were included in this analysis for a total sample size of 337
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