Larval fish abundances off southern California from 1951 to 2016

The 2014-2016 Northeast Pacific Marine Heatwave (MHW) induced the warmest
3-year period on record in the California Current Ecosystem. We
tested whether larval fish assemblage structure, phenology and diversity
dynamics were comparable to past warming events from
1951-2013. First, we hypothesized, based on past observations of
biological effect of warming, that mesopelagic species with southern
distributions relative to southern California and Pacific sardine
Sardinops sagax (a coastal pelagic species) would increase during the MHW
while northern mesopelagics and northern anchovy Engraulis mordax (coastal
pelagic) abundances would decline. Similar to past warming,
southern mesopelagics increased and northern mesopelagics
decreased. Unexpectedly, however, a common southern mesopelagic,
Mexican lampfish Triphoturus mexicanus, was approximately three times more
abundant than the previous annual high. Further, whereas sardine
abundance did not increase, larval anchovy abundance rose to near-record
highs in summer 2016. Second, we hypothesized that fishes would
spawn earlier during the MHW. Fishes did not spawn in an earlier
season within a year, but five of six southern mesopelagic taxa spawned
earlier than typical within winter and spring. Third, we predicted that
species richness would increase moderately due to an influx of southern
and exodus of northern species. Richness, however, was very high
in all seasons and the highest ever during the summer as multiple species
with primarily southern distributions were recorded spawning for the first
time in southern California. The richness of northern species was
also unexpectedly high during the MHW. Northern species likely persisted
in the study area because in addition to the warm water, pockets of cold
water were consistently present. If, as predicted, conditions
similar to the MHW become more common as oceans warm, this unique and
largely unexpected combination of fishes may reflect future biological
conditions.