AQUACULTURE-RELEVANT POLYSTYRENE MICROPLASTICS TRANSLOCATE TO GAMETES AND DISRUPT REDOX BALANCE WITHOUT IMPAIRING REPRODUCTIVE OUTPUT IN BROWN TROUT Salmo trutta
Introduction and methods
Aquaculture organisms are exposed to microplastics (MPs) via feed formulated from contaminated fishmeal, packaging and the degradation of plastic infrastructure. Reported burdens of MPs in farmed salmonids range from ~1.8 × 103 to 9.4 × 103 MPs fish-1 per production cycle (Wang et al., 2022; Walkinshaw et al., 2022). However, most studies use unrealistically high concentrations and focus on marine species, thereby limiting their relevance to aquaculture. Most importantly, whether MPs translocate to gametes and impair reproductive performance under aquaculture-relevant exposure remains unresolved.
A total of 180 adult (3-year-old) Salmo trutta (570 ± 190 g; 38 ± 3 cm) were equally allocated to three treatments (~30 kg biomass tank-1): control, low MPs (LMP; ~4.8 × 103 MPs fish-1), and high MPs (HMP; ~4.9 × 104 MPs fish-1). Fish were fed polystyrene microplastic (PS-MP)-spiked feed for 4 weeks (0.7–0.8% body mass day-1), followed by commercial feed. Broodstock maturation, PS-MP translocation to gametes, antioxidant responses (superoxide dismutase [SOD], catalase [CAT], peroxidase [POD]), and reproductive outputs (semen volume, fecundity, hatching success) were assessed.
Results and discussion
PS-MPs were detected in both gametes (Fig. A2-3b–c), with higher particle numbers per gram of tissue and detection frequency in eggs than in semen (eggs: 0.53 × 106 ± 1.13 PS-MPs g-1; 4/10 fish; semen: 0.27 × 106 ± 0.59 PS-MPs g-1; 2/10 fish). In semen, only 1 µm particles were detected, whereas eggs also contained 5 µm; no 10 µm particles were observed in either. These patterns indicate size-dependent translocation, likely driven by differences in gamete structure and retention capacity.
At the physiological level, PS-MPs disrupted antioxidant responses (Fig. d–f), with reduced enzyme activities (µmol min-1 g-1 tissue) across tissues: semen SOD (Control: 9.76 ± 3.16; LMP: 6.79 ± 2.08; HMP: 6.59 ± 3.20), and egg CAT (2.35 ± 1.79; 0.21 ± 0.35; 0.03 ± 0.09) and POD (0.19 ± 0.34; 0.11 ± 0.34; 0.004 ± 0.004). As key enzymes that form the primary defence against reactive oxygen species, their suppression significantly reduces the ability of spermatozoa and oocytes to mitigate oxidative stress, potentially leading to increased lipid peroxidation, protein oxidation, and DNA damage.
On the other hand, reproductive parameters showed selective effects, with reduced maturation and spawning females in MP treatments (82.1% control vs 54.8% LMP and 45.9% HMP; Fig. g). While all males were spermiating at each assessment, semen volume was reduced by 40.2% in the LMP group compared to the control (Fig. h). However, fecundity (Control: 3505.3 ± 596.0; LMP: 3140.2 ± 1016.3; HMP: 3532.9 ± 663.6; Fig. i) and hatching success (Fig. j–k) did not differ among treatments, likely due to buffering by aquaculture practices.
These findings indicate that small MPs (≤1 µm in sperm; ≤5 µm in eggs) can translocate into gametes and disrupt antioxidant defences, contributing to oxidative stress and sex-specific reproductive effects, without altering overall reproductive output under aquaculture conditions. However, threshold-based analyses (Fig. 1h–k) reveal a higher proportion of broodfish with suboptimal reproductive outputs under MP exposure. Accordingly, salmonid aquaculture is unlikely to be constrained under standard hatchery management conditions, but these effects may become consequential in less controlled environments. In natural systems, where reproduction is unassisted and subject to multiple environmental stressors, such effects may translate into reduced recruitment. Given the increasing use of hatchery-reared fish for restocking, further work is needed to assess implications for wild populations.
Figure 1. PS-MPs in S. trutta: Gamate translocation, burden, effects and reproductive outcome
Figure 1. Effects of PS-MP exposure in S. trutta. Particle characterisation confirming spherical morphology and example of excluded non-spherical artefacts from counting (a); PS-MP abundance in gametes (b–c); antioxidant responses (d–f; n = 10 per treatment; POD in eggs plotted on a log10 scale for visualisation only; statistical analyses conducted on untransformed data); semen volume (h; n = 10 per treatment); fecundity (i; n = 23, 17, and 15 for Control, LMP, and HMP, respectively); and hatching success following PS-MP exposure in eggs and semen (j–k; n = 10 per treatment). Data are shown as violin plots overlaid with boxplots, individual observations, and means (red diamonds). Dashed lines denote functional failure thresholds (<1 mL semen volume, <2000 eggs per female, <70% hatch rate). For panels h–k, the proportion of individuals below the set threshold per treatment is indicated above each violin plot. Pairwise comparisons were performed among all treatments; only statistically significant differences are shown (*P ≤ 0.05, **P ≤ 0.01).
References
Wang, Q.; Li, J.; Zhu, X.; Sun, C.; Teng, J.; Chen, L.; Shan, E.; Zhao, J. Microplastics in fish meals: An exposure route for aquaculture animals. Sci. Total Environ. 2022, 807, 151049. https://doi.org/10.1016/j.scitotenv.2021.151049
Walkinshaw, C.; Tolhurst, T.J.; Lindeque, P.K.; Thompson, R.; Cole, M. Detection and characterisation of microplastics and microfibres in fishmeal and soybean meal. Mar. Pollut. Bull. 2022, 185, 114189. https://doi.org/10.1016/j.marpolbul.2022.114189