Are long-term growth responses to elevated pCO2 sex-specific in fish?

Abstract

Whether marine fish will grow differently in future high pCO2 environments remains surprisingly uncertain. Long-term and whole-life cycle effects are particularly unknown, because such experiments are logistically challenging, space demanding, exclude long-lived species, and require controlled, restricted feeding regimes-otherwise increased consumption could mask potential growth effects. Here, we report on repeated, long-term, food-controlled experiments to rear large populations (>4,000 individuals total) of the experimental model and ecologically important forage fish Menidia menidia (Atlantic silverside) under contrasting temperature (17°, 24°, and 28°C) and pCO2 conditions (450 vs. ~2,200 μatm) from fertilization to ~ a third of this annual species' life span. Quantile analyses of trait distributions showed mostly negative effects of high pCO2 on long-term growth. At 17°C and 28°C, but not at 24°C, high pCO2 fish were significantly shorter [17°C: -5 to -9%; 28°C: -3%] and weighed less [17°C: -6 to -18%; 28°C: -8%] compared to ambient pCO2 fish. Reductions in fish weight were smaller than in length, which is why high pCO2 fish at 17°C consistently exhibited a higher Fulton's k (weight/length ratio). Notably, it took more than 100 days of rearing for statistically significant length differences to emerge between treatment populations, showing that cumulative, long-term CO2 effects could exist elsewhere but are easily missed by short experiments. Long-term rearing had another benefit: it allowed sexing the surviving fish, thereby enabling rare sex-specific analyses of trait distributions under contrasting CO2 environments. We found that female silversides grew faster than males, but there was no interaction between CO2 and sex, indicating that males and females were similarly affected by high pCO2. Because Atlantic silversides are known to exhibit temperature-dependent sex determination, we also analyzed sex ratios, revealing no evidence for CO2-dependent sex determination in this species.

Fig 1. Female sex ratios from trials 1–3.

The mean female sex ratio (F/(F + M)) of juvenile M. menidia reared under 450 and 2,200 μatm pCO₂ at 17° and 24°C. Dotted lines connect treatment means within trials. Horizontal black lines indicate the temperature dependent female sex ratios predicted for the experimental source populations by Conover & Heins (1987).

Fig 2. Shift functions and quantile differentials for trials 1–3.

M. menidia. Shift functions for trials 1 (A-C), 2 (D-F), and 3 (G-I) are denoted by different letters. Upper panels show frequency density distributions as colored dots (blue: 450 μatm; red: 2,200 μatm). Black vertical bars overlaying each distribution indicate the .1, .25, .5, .75, and .9 quantiles. Quantile shifts are indicated by connecting lines where red lines indicate a reduction in trait value and blues denote a positive shift. The lower panels show quantile differentials (high pCO₂ –ambient pCO₂) and bootstrapped 95% CIs. Dots are color coded to indicate a negative (red) or positive effect of high pCO₂ on the trait value. The size of the quantile shift is denoted in color boxes above or below the colored dots.

Fig 3. Shift functions and quantile differentials for trial 4.

M. menidia. Temperature treatments are indicated by differing letters (24°C: A-C; 28°C: D-F). Upper panels show frequency density distributions as colored dots (blue: 450 μatm; red: 2,200 μatm). Black vertical bars overlaying each distribution indicate the .1, .25, .5, .75, and .9 quantiles. Quantile shifts are indicated by connecting lines where red lines indicate a reduction in trait value and blues denote a positive shift. The lower panels show quantile differentials (high pCO₂ –ambient pCO₂) and bootstrapped 95% CIs. Dots are color coded to indicate a negative (red) or positive effect of high pCO₂ on the trait value. The size of the quantile shift is denoted in color boxes above or below the colored dots.

Fig 4. TL of subsampled juveniles.

M. menidia. Mean TL (± s.d.) of all subsampled juveniles reared under two pCO₂ conditions (blue: 450 μatm; red: 2,200 μatm) and three temperatures. Significant differences between pCO₂ treatment within sampled age groups are denoted by black stars (LMM, p < 0.05).