Measuring selective mortality from otoliths and similar structures: a practical guide for describing multivariate selection from cross-sectional data

Abstract

Selective mortality is an important process influencing both the dynamics of marine populations and the evolution of their life histories. Despite a large and growing interest in measuring selective mortality, studies of marine species can face some serious methodological and analytical challenges. In particular, many studies of selection in marine environments use a crosssectional approach in which fates of individuals are unknown but the distributions of trait values before and after a period of selective mortality may be compared. This approach is often used because many marine species have morphological structures (e.g. otoliths in fishes, statoliths in some invertebrates) that contain a permanent record of trait values. Although these structures often contain information on multiple, related traits, interpretation of selection measures has been limited because most studies of selection based on cross-sectional data consider selection 1 trait at a time, despite known problems with trait correlations. Here, we detail how cross-sectional data can be analyzed within a multivariate framework and provide a practical guide for conducting these types of analyses. We illustrate these methods by applying them to empirical studies of selective mortality on early life history traits in 2 species of reef fish. These examples demonstrate that analyzing selective mortality in a multivariate framework can vastly improve estimates of selection and yield new insight into how combinations of traits can interact to influence survival. Accompanying the paper are 2 R scripts that can be used to perform the calculations described here and assist with visualizing selection on multiple traits.

Ocean's eleven: a critical evaluation of the role of population, evolutionary and molecular genetics in the management of wild fisheries

“…; Johnson et al . ). However, there remains debate about how best to demonstrate harvest affects while accounting for environmental effects and phenotypic plasticity (Dieckmann and Heino ; Law ).…”

Natural selection by pulsed predation: survival of the thickest

“…By parametric bootstrapping (n ¼ 1000), we calculated significance under the null hypothesis that the estimated coefficients are zero. We calculated linear and nonlinear selection gradients (Lande andArnold 1983, Endler 1986) on length and body composition with multivariate models following Johnson et al (2012). As collinearity between variables can make these multivariate analyses unreliable, we calculated selection gradients for length in a multivariate model with an individual's relative AFDM flesh and DM shell .…”

Strong nonlinear selection against fluctuating asymmetry in wild populations of a marine fish

“…The value of h(z) ranges from 0 to 1 and h(z) can be estimated as a smooth function (Johnson et al. ). In this study, we used a generalized additive model with a logit link to describe h(z) by fitting a regression spline to the data.…”

“…Where S 1 (z) and S 2 (z) are the number of individuals in the first and second samples that had an FA value of z. The value of h(z) ranges from 0 to 1 and h(z) can be estimated as a smooth function (Johnson et al 2012). In this study, we used a generalized additive model with a logit link to describe h(z) by fitting a regression spline to the data.…”

“…At each location, our approach to measuring selection was to take (and keep for analysis) an initial sample of the "young of the year" and then resample the same population later in the year, with collection again focused on members of that same cohort. Distributions of asymmetry values in the initial sample were compared to the distributions of asymmetry values in the sample of survivors to estimate selective mortality associated with FA (Lande and Arnold 1983;Johnson et al 2012). The samples were collected by divers using hand-held nets in late May 2014 and again in the fall (Oct-Nov) of the same year for both populations.…”