Effects of ambient oxygen and size-selective mortality on growth and maturation in guppies

Beatriz Diaz Pauli,Jeppe Kolding,Mikko Heino,Geetha Jeyakanth

doi:10.1093/conphys/cox010

Abstract

Growth, onset of maturity and investment in reproduction are key traits for understanding variation in life-history strategies. Many environmental factors affect variation in these traits, but for fish, hypoxia and size-dependent mortality have become increasingly important because of human activities, such as increased nutrient enrichment (eutrophication), climate warming and selective fishing. Here, we study experimentally the effect of oxygen availability on maturation and growth in guppies (Poecilia reticulata) from two different selected lines, one subjected to positive and the other negative size-dependent fishing. This is the first study to assess the effects of both reduced ambient oxygen and size-dependent mortality in fish. We show that reduced ambient oxygen led to stunting, early maturation and high reproductive investment. Likewise, lineages that had been exposed to high mortality of larger-sized individuals displayed earlier maturation at smaller size, greater investment in reproduction and faster growth. These life-history changes were particularly evident for males. The widely reported trends towards earlier maturation in wild fish populations are often interpreted as resulting from size-selective fishing. Our results highlight that reduced ambient oxygen, which has received little experimental investigation to date, can lead to similar phenotypic changes. Thus, changes in ambient oxygen levels can be a confounding factor that occurs in parallel with fishing, complicating the causal interpretation of changes in life-history traits. We believe that better disentangling of the effects of these two extrinsic factors, which increasingly affect many freshwater and marine ecosystems, is important for making more informed management decisions.

Highlights

Maturation determines the beginning of the reproductive part of an individual’s life cycle and is costly in terms of survival and energy
This model was superior to the model that did not include any treatment effect (M0; ΔAIC = 25.99, likelihood ratio test statistic = 35.99, P < 0.001)
Males under low ambient oxygen from each selection line reached lower predicted weights at age 210 days than their counterparts with high ambient oxygen (Fig. 1b and Table 1), but their size-specific maximal potential growth rate was higher

Summary

Introduction

Maturation determines the beginning of the reproductive part of an individual’s life cycle and is costly in terms of survival and energy. The age and size at which an individual matures are key life-history traits. Growth determines the relationship between age and size, with the latter being a key determinant of survival and fecundity. Studying the effects of different extrinsic factors on growth and maturity is important for understanding the variation in life-history strategies (Roff, 1992; Stearns, 1992, 2000). Many different environmental factors, such as food availability, temperature, oxygen and presence of predators, affect the acquisition and allocation of resources to growth, maturation and reproduction (Berner and Blanckenhorn, 2007; Enberg et al, 2012). Two factors affecting life-history traits are of particular interest in fishes, namely oxygen and size-dependent mortality. Oxygen demand is proportional to the rate of metabolism and increases with, e.g. body size and stress. Temperature plays a dual role: increasing temperature reduces the solubility of oxygen, while in ectotherms, it increases the metabolic demand for oxygen (Pörtner and Knust, 2007; Holt and Jørgensen, 2015)

Methods

Results

Conclusion