DOI: 10.5061/dryad.6t1g1jx0n
title: Larval fish abundances off southern California from 1951 to 2016

description: 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.
description: CalCOFI has been collecting
 depth-integrated plankton samples quarterly from the same 66 stations
 since 1951 (Fig. 1) using obliquely towed nets (505- µm mesh, 333 µm mesh
 on the cod end). Net contents were preserved at sea in a sodium
 borate-buffered 5% formalin solution (Kramer, Kalin, Stevens, Thrailkill,
 & Zweifel, 1972; P. Smith, 1974; P. E. Smith & Richardson,
 1977). All larval fishes were then sorted, identified and archived at the
 Ichthyoplankton Ecology Lab at the Southwest Fisheries Science Center in
 La Jolla, CA. Raw larval abundances were divided by the percent of the
 sample that was sorted (samples with very thick zooplankton were split
 using a Folsum Plankton Splitter) and multiplied by a standard haul factor
 that accounted for the depth of a tow and the filtered volume of water
 (Kramer et al., 1972). We ultimately expressed abundances as the number of
 larvae under a circle of water with an area of 10
 m<sup>2</sup>. Although CalCOFI now collects samples in
 winter, spring, summer and fall, historical collections were often sparser
 in fall than other seasons; we thus focused on winter, spring and
 summer. We excluded winter 2014 because the research vessel broke down
 during that cruise and most of the stations were not sampled.
 Our capacity to identify
 larvae to species has improved since 1951, and we can currently identify
 almost all species based on morphology. The most notable exceptions are
 rockfishes (genus <i>Sebastes</i>) where we can
 morphologically distinguish only Aurora rockfihs <i>S. aurora,
 </i>Splitnose rockfish<i> S. diploproa, </i>Chilipepper
 rockfish <i>S. goodei, </i>Shortbelly rockfish<i> S.
 jordani, </i>Cowcod rockfish <i>S. levis, </i>Mexican
 rockfish<i> S. macdonaldi </i>and Bocaccio rockfish
 <i>S. paucispinis</i>; the remainder were grouped as
 rockfishes <i>Sebastes </i>spp. Genetic barcoding of rockfish
 larvae collected in 2005 identified 30 species within the
 <i>Sebastes </i>spp. group and found that shortbelly and
 squarespot <i>S. hopkinsi </i>were numerically dominant
 (Thompson, Hyde, Watson, Chen, & Guo, 2016). We are working back
 chronologically to identify archived specimens to current standards, and
 samples from 1962-present are at present resolved to current
 standards. Prior to 1962, we grouped several more taxa to genus or family:
 bristlemouths Gonostomatidae (primarily Showy Bristlemouth
 <i>Cyclothone signata </i>with fewer Benttooth Bristlemouth
 <i>C. acclinidens </i>and <i>Diplophos </i>spp.),
 Lightfishes <i>Vinciguerria </i>spp. (almost all Panama
 Lightfish <i>Vinciguerria lucetia</i>), Lampfish in the genus
 <i>Nannobrachium </i>(mostly Broadfin Lampfish <i>N.
 ritteri </i>with fewer Pinpoint Lampfish <i>N.
 regalis</i>), and Sanddabs <i>Citharichthys </i>spp.
 (mostly Pacific Sanddab <i>C. sordidus </i>and Speckled
 Sanddab <i>C. stigmaeus </i>at approximately equal
 abundances). In the analyses (see below), we used the full data set with
 fishes at coarser taxonomic resolution (1951-2016) to evaluate major
 changes in assemblage composition and phenology and the shorter set
 (1962-2016) with all taxa identified to current standards to examine
 diversity dynamics.