Mixed-Effects Modelling of Scale Growth Profiles Predicts the Occurrence of Early and Late Fish Migrants

Abstract

Fish growth is commonly used as a proxy for fitness but this is only valid if individual growth variation can be interpreted in relation to conspecifics' performance. Unfortunately, assessing individual variation in growth rates is problematic under natural conditions because subjects typically need to be marked, repeated measurements of body size are difficult to obtain in the field, and recaptures may be limited to a few time events which will generally vary among individuals. The analysis of consecutive growth rings (circuli) found on scales and other hard structures offers an alternative to mark and recapture for examining individual growth variation in fish and other aquatic vertebrates where growth rings can be visualized, but accounting for autocorrelations and seasonal growth stanzas has proved challenging. Here we show how mixed-effects modelling of scale growth increments (inter-circuli spacing) can be used to reconstruct the growth trajectories of sea trout (Salmo trutta) and correctly classify 89% of individuals into early or late seaward migrants (smolts). Early migrants grew faster than late migrants during their first year of life in freshwater in two natural populations, suggesting that migration into the sea was triggered by ontogenetic (intrinsic) drivers, rather than by competition with conspecifics. Our study highlights the profound effects that early growth can have on age at migration of a paradigmatic fish migrant and illustrates how the analysis of inter-circuli spacing can be used to reconstruct the detailed growth of individuals when these cannot be marked or are only caught once.

Marine growth patterns of southern British Columbia chum salmon explained by interactions between density-dependent competition and changing climate

“…The longitudinal mixed-effects model design accounted for correlations of repeated measures within an individual by treating each individual as a random effect (Singer and Willett 2009;Cheng et al 2010;Marco-Rius et al 2013). We found curvilinear relationships between ages and growth (Fig.…”

“…Although a relationship between salmon scale radius size and fish length has been well established in the literature (e.g., Marr 1943;Ricker 1973Ricker , 1976, in common with many other studies (e.g., McCarthy et al 2008;Hogan and Friedland 2010;Marco-Rius et al 2013), scale sizes were not converted to body length or mass estimates, as it would introduce additional errors as a result of (i) scale reabsorption of the final annuli during the final year reducing accuracy (Seo et al 2006) and (ii) uncertainty in the relationship between scale growth to body growth. Scale radius was correlated with fork length (Pearson r = 0.47, p < 0.001; fish length data available from 1998 to 2010).…”

Contrasting patterns of genetic and phenotypic differentiation in two invasive salmonids in the southern hemisphere

“…), (ii) number of scale growth circuli during the first winter, (iii) scale intercirculi spacing during the first winter, and (iv) scale radius at the end of the first winter, as detailed in Marco‐Rius et al. (, ). Analysis of scale growth circuli can be used to reconstruct and compare growth profiles of individuals of different ages and has previously been used to assess variation in growth performance of invasive trout in the area (Schröder and Garcia de Leaniz ).…”

“…Repeatabilities for the other scale traits were not calculated as we had previously found that these were similarly high in brown trout (Marco‐Rius et al. , ).…”

Inter-cohort growth for three tropical resources: tilapia, octopus and lobster

“…This method has commonly been used for species in temperate regions where the intraannual variations in the environment favour the record of these marks (Marco-Ruis, Caballero, Morán, & García-de Leaniz, 2013).…”