Temperature-driven phenological changes within a marine larval fish assemblage

Most marine teleosts have a pelagic phase during their early life history, but few studies have investigated how the timing of events within the planktonic larval fish assemblage is related to environmental variability. We examined this issue using a data series of 534 larval fish samples collected between 1975 and 1987 in the Western English Channel, near Plymouth, UK. Two sets of species were identified: spring spawning (April to July) and summer spawning (July to September). The timing of appearance of the spring-spawning group in the plankton was significantly dependent on sea temperatures the previous November and December, with earlier appearance during cooler years. We suggest that this could be due to colder years triggering earlier winter migration of adults from cool inshore habitat to warmer offshore overwintering grounds, which in turn results in earlier gonad maturation and spawning. In contrast, the timing of appearance of the summer-spawning group was significantly dependent on sea temperatures the preceding March, with earlier spawning during warmer years. This may be due exclusively to more rapid gonad maturation in offshore waters. These data emphasize that marine fish populations do not always respond uniformly to temperature change. Moreover, since appropriate timing of larval fish appearance in the plankton may be critical for the match or mismatch with essential trophic resources, the thermally induced phenological changes identified have potential to influence annual recruitment success.