The central Sea of Japan around the Noto Peninsula is an important nursery ground for Japanese anchovy. The larvae of Japanese anchovy mainly feed on copepod nauplii. However, little information on the spatial distribution of copepod nauplii has been available in this region. In this study, we studied the species-specific distribution of copepod nauplii in May 2005, which corresponds to the main spawning season of Japanese anchovy. The most dominant species (>8.0 ind. L^-1) were Oithona atlantica N2–N6, Oithona similis N2–N6 and Paracalanus sp. N2–N6. All these species were in high abundance off Niigata. From a comparison with environmental parameters, nauplii of Oithona nana showed positive correlations with temperature and salinity. Conversely, nauplii of O. atlantica, O. similis, Paracalanus sp. and Pseudocalanus spp. had negative correlations with temperature and salinity. Cluster analysis based on the abundance of the eight dominant species classified the 60 stations into four groups (A–D). Occurrence of each group was separated geographically. Group D, which had the largest numbers of stations but the lowest abundance of naupliar, was distributed from Wakasa Bay to Toyama Bay. These areas were warm, saline and low in chlorophyll a, indicating that group D was characterized as an oligotrophic naupliar community. The other three groups (A–C) were mainly concentrated within smaller areas off Niigata, where community diversity was relatively higher. River runoff (ice-melt water from the mountains) from various large rivers in this area may provide a nutrient input, phytoplankton bloom and induce massive reproduction of copepods. This mechanism may maintain a high biological community diversity of copepod nauplii in this region.

We investigated regional differences of zooplankton—especially the community structure of copepods—as an important food for juvenile chum salmon (Oncorhynchus keta) in the coastal waters around Hokkaido, using multivariate statistical techniques. Zooplankton samples were collected by vertical hauls from 20 m depth or, at shallower stations, from near-bottom to the surface using a Norpac net (mesh opening: 0.33 mm) at four sampling sites (regions); Atsuta (JS: Hokkaido Sea of Japan), Shari (OH: Okhotsk), Shiraoi (WP: West Hokkaido Pacific) and Konbumori (EP: East Hokkaido Pacific) in spring and summer 2007.

Copepod community structure was classified into three clusters (F_2,121=65.8, p<0.001) that contained significantly different indicator species compositions: Cluster 1, “cold water–neritic/oceanic group,” was characterized by Neocalanus plumchrus, Eucalanus bungii, Acartia tumida, Pseudocalanus minutus, Pseudocalanus newmani, Calanus glacialis and Neocalanus flemingeri. Cluster 2, “cold/warm water–neritic/oceanic group,” was characterized by Mesocalanus tenuicornis, Oithona atlantica, Tortanus discaudatus, Paracalanus sp., Clausocalanus pergens, Metridia pacifica and Corycaeus affinis. Cluster 3, “cold water–neritic group,” was characterized by Acartia longiremis, Centropages abdominalis and Eurytemora herdmani.

Food conditions of juvenile chum salmon were better in Cluster 1 (“cold water–neritic/oceanic group”) than in the other groups. Cluster 1 copepods had a greater mean copepod abundance (F_2,121=12.11, p<0.001) and were represented by large, lipid-rich N. plumchrus. Furthermore, while copepods in Cluster 2 had small-medium bodies, their mean abundance was substantially lower than that of Cluster 1. Similarly, the relatively abundant Cluster 3 copepods were much smaller in size than those of Cluster 1, which resulted in less available biomass for juvenile chum salmon. Comparing relationships between the spatiotemporal distributions of three clusters and hydrographic conditions (surface temperature and water mass structure) at each site, feeding conditions of juvenile salmon may be regionally evaluated as follows: favorable in EP (mostly Cluster 1 in Konbumori), unfavorable in JS (exclusively Cluster 2 in Atsuta) and seasonally variable in OH/WP (a mixture of three clusters in Shari/Shiraoi).

For Japanese juvenile chum salmon migrating eastwards into the Sea of Okhotsk along the Pacific coast of Hokkaido, summer coastal environmental conditions off Konbumori (EP) on their assumed migration route may play an important role in improving feeding conditions for their growth, which have deteriorated because of changes in the copepod community structure (that is, from “cold water–neritic/oceanic group” to “cold water–neritic group”), the early disappearance of N. plumchrus and higher surface temperatures (>13°C) in Shiraoi (WP).

In marine ecosystems, bacterial interactions with zooplankters are highly complex, and much attention has recently been given to these interactions. Bacteria not only play the role of food and symbionts for zooplankters, but also function as decomposers for their carcasses, exuviae and feces. Free-living bacteria are involved as major producers in microbial loops, and form the diet of nanoplanktonic flagellates, ciliates, appendicularians and thaliaceans. Epibiotic and enteric bacteria use zooplankters as refuges to avoid predation and/or as food sources. However, aggregations of epibiotic bacteria or biofilms may function as “a second skin,” sensu Wahl et al. (2012), to modulate hosts metabolism and behaviors. Because they contain rich nutrients, low pH and low oxygen, copepod guts provide a unique environment for bacteria in which anaerobes can survive. Bacterial communities on copepods vary seasonally and among species, depending on the physiology of the host. The conveyor-belt hypothesis implies that bacteria vertically, and presumably horizontally, hitchhike in different water masses in accordance with the migrations of zooplankters. Bioluminescent bacteria are likely used as biomarkers of detrital foods for some planktonic copepods belonging to the Bradfordian families and as obligate symbionts for bioluminescent ichthyoplankters. Tetrodotoxin-producing bacteria are associated with chaetognaths that may use toxins to capture prey animals. Colonial cyanobacteria provide substrata for miraciid harpacticoid copepods. Hyrdomedusae play a role as vectors of pathogenic bacteria, causing lesions in farmed fish. Modern genetic analysis is a powerful tool that will be the first step in revealing the physiological and functional interactions between bacteria and zooplankton.