Effects of climate change on growth and otolith thermometry of sand whiting (Sillago ciliata)

Increased water temperature and ocean acidification, due to climate change, is predicted to impact aquatic species’ growth, physiology and calcification rates. The present study investigated how a high-emissions scenario of future water temperature and ocean acidification could influence somatic and otolithic growth, and oxygen isotope fractionation in accreted otolith material, of an important fisheries species, sand whiting (Sillago ciliata) (family Sillaginidae), using an outdoor mesocosm system. The experiment included four treatments with an orthogonal combination of current [∼22 °C], and future [∼25.0 °C] predictions of water temperature and current [∼8.13] and future [∼7.83] pH. Fish somatic and otolithic growth demonstrated a positive response to warmer water temperatures, but were not significantly influenced by increased ocean acidification. Stable oxygen isotopes within otolith material (δ¹⁸O_otolith) deposited during the 3-month experimental period, micro-milled from thin-sections and analysed via Isotope Ratio Mass Spectrometry, displayed a negative relationship with water temperature and also varied between acidification treatments. Although acidification increased the intercept of the linear relationship between temperature and oxygen isotope values, the effect was not significant, and the pooled relationship was determined to be δ¹⁸O_otolith – δ¹⁸O_water = – 0.22*T + 34.34 (R² = 0.522, p < 0.001). This model demonstrated a similar slope to that for inorganic aragonite and other fish species, but a noticeably higher intercept. This species-specific relationship has fundamental applications for determining water temperature estimates using δ¹⁸O_otolith values from wild-caught whiting which may elucidate dispersal and movement patterns, thus offering essential information for informed decision-making amidst a changing climate.