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Many subtropical fishes spawn multiple batches throughout a year. To understand plasticity in their reproductive output, we evaluated variation in oocyte size and batch fecundity for an exploited subtropical cutlassfish, Trichiurus japonicus , between two temperature periods (warm vs. cold) on the NE and SW coasts of Taiwan, northwestern Pacific. Given greater temperature variability on NE compared with SW coast, we hypothesized greater changes between warm and cold periods in oocyte size and fecundity for T. japonicus on the NE coast. We found opposite changes in sizes of ripe oocytes between periods (cold > warm on the NE but warm > cold on the SW coast) but consistent patterns in batch fecundity between coasts (warm > cold). Furthermore, the between‐period patterns in female length were consistent on both coasts (warm > cold). The differential between‐period patterns in oocyte size were related to potential adaptive responses to differential thermal environments between coasts, whereas the changes in fecundity mainly involved plastic processes (e.g., changes in population demography or movement). Together, our findings suggest that variability of temperature and maternal effects underlie recruitment variability of T. japonicus .
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