Surf smelt accelerate usage of endogenous energy reserves under climate change

larval yolk size throughout the duration of the experiment (Table 3; Fig 2) and for every 1˚C increase in temperature, the average larval yolk size decreased by 0.011 mm2 (Table 2). The average size of larval yolk sacs decreased by 27.1% at 13˚C and 49.8% at 18˚C over the duration of the experiment. Larval yolk size was not significantly affected by pCO2 (Table 3; Fig 2). Surf smelt oil globule size decreased over time in all treatments with day having a significant negative effect on oil globule size (Table 3; Fig 3). Temperature had a significant negative effect on the size of larval oil globules and for every 1˚C increase in temperature, the average oil glob- ule area decreased by 0.0018 mm2 (Table 3; Fig 3). Over the three-day incubation, oil globule size decreased by 25.7% at 13˚C compared to 43.2% at 18˚C. On average, oil globules at 18˚C were 31.9% smaller than oil globules at 13˚C on the final day of data collection (Fig 3). pCO2 had no significant effect on oil globule size (Table 3). Discussion Increased energy usage due to elevated metabolism across fish life histories can result in trade- offs in growth, performance, and reproduction [34–36]. We found that elevated temperature was a primary abiotic driver affecting early life history stages of surf smelt, accelerating yolk usage in both surf smelt embryos and larvae, and oil globule usage in larvae. Our results show- ing increased usage of endogenous energy in response to increasing temperatures agree with findings from numerous other fish species [e.g. 37–43], including the forage fish Engraulis

japonica [38], Clupea pallasi [40] and Clupea harengus [41], Brevoortia tyrannus [42], and Sar- dinops sagax [43]. However, our results also suggest that surf smelt may be more tolerant to temperature variance than other fish species. Heming and Buddington [37] summarized the findings of 23 species of fish and showed that Q10 values for yolk absorption in fish over a tem- perature span of 1–30˚C averaged 2.9. In this study we observed Q10 values of 1.58 and 2.09 for surf smelt embryos and larvae, respectively. In the broadcast spawning Pacific sardine, Lasker Fig 2. Larvae yolk usage. Yolk sac area of surf smelt larvae (n = 144/ day) that were submerged in treatment water for 3 days. Data are averages representative of the temperature treatment, pCO2 treatment (points offset for visualization), and day with standard error shown as whiskers. Day, and Temperature were significant factors predicting larvae yolk usage. https://doi.org/10.1371/journal.pone.0270491.g002 PLOS ONE Surf smelt response to climate co-stressors PLOS ONE | https://doi.org/10.1371/journal.pone.0270491 June 27, 2022 8 / 14