2019 年 66 巻 2 号 p. 72-85
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 (F2,121=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 (F2,121=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).
