Geographic distribution of micronektonic fish communities in the subtropical North Pacific: The effect of primary productivity and nitrogen fixation

We investigated the geographic variations of micronektonic fish communities along the transects in the subtropical North Pacific (23 degrees N, 138 degrees E-120 degrees W) and California Current (30-45 degrees N, 123-125 degrees W) during August-October 2017 in relation to environmental factors, including primary productivity and nitrogen fixation. Throughout the cruise, 102 species across 31 families were collected at a depth of 0-500 m at night. Clustering analysis indicated that the micronektonic fish were categorized into three communities along the east-west subtropical transect, in addition to the subarctic-transitional water community. Species in the subtropical communities were composed of indicator species, which were characteristically found in their respective community, in addition to the common species (Vinciguerria nimbaria, Notolychnus valdiviae and Bolinichthys longipes), which were regularly found in all communities. Redundancy analysis indicated that the distribution of indicator species in western (Benthosema suborbitale and Lampanyctus alatus) and central (Cyclothone alba and Hygophum proximum) subtropical communities were positively associated with chlorophyll a concentration and nitrogen fixation rate, respectively. Regarding the eastern community indicator species (Ceratoscopelus townsendi, Cyclothone acclinidens, and Diogenichthys laternatus), various factors such as chlorophyll a concentration, dissolved oxygen, temperature and salinity were negatively related to their specific occurrences. In addition, specifically abundant occurrence of V. nimbaria was observed at a station where the thermocline became shallower and the chlorophyll a concentration was highest. Our results suggest that the productivity of the lower trophic level affects the geographic distribution of micronektonic fish in the subtropical North Pacific in addition to physicochemical parameters.