Determining spatiotemporal trends in hatch and metamorphosis timing of young-of-year southern flounder Paralichthys lethostigma) in Texas bays

Abstract

It is important to monitor spatiotemporal shifts in the timing of key life history events (i.e., hatch and metamorphosis) since these can influence larval survival and juvenile recruitment success. We use otolith microstructure analysis to examine spatiotemporal trends in hatch and metamorphosis timing of a widespread and economically-valuable finfish, southern flounder Paralichthys lethostigma, in Texas. We use linear regressions that predict otolith-based age from length for young-of-year (YOY) individuals caught coastwide from 2019 to 2021 to calculate hatch and metamorphosis dates of YOY individuals caught coastwide across a 39-yr time series from 1980 to 2018. Also, we use YOY hatchery-reared individuals to indirectly validate the association between otolith accessory growth centers and larval metamorphosis. Our analysis reveals that on the middle coast average hatch timing is shifting toward earlier in the year across the 39-yr time series. In contrast, we find that coastwide, and especially on the middle coast, average metamorphosis timing is shifting toward later in the year across the 39-yr time series. On average, YOY individuals show similar hatch and metamorphosis timing coastwide. We find that on the middle coast and across the 39-yr time series, higher maximum water temperature correlates with earlier average hatch timing. In contrast, we find that coastwide, and especially on the lower and middle coast, higher average water temperature (later years in time series) correlates with later average metamorphosis timing across the 39-yr time series. Fishery-independent monitoring shows no concurrent evidence of individuals emigrating earlier in the fall to spawn offshore. We interpret the spatiotemporal trends in average hatch and metamorphosis timing as evidence of a narrowing survival window where (1) individuals caught on the middle coast that hatch earlier experience higher survival during years with higher maximum water temperature and (2) individuals caught coastwide that metamorphose later experience higher survival during years with higher average water temperature (later years in time series). We highlight the ecological and management implications of the impact of water temperature on hatch and metamorphosis timing and in turn, larval survival and juvenile recruitment success.