Ecosystem modeling in the western North Pacific using Ecopath, with a focus on small pelagic fishes

Small pelagic fishes like sardine, anchovy, and mackerel play important commercial and ecological roles in the western North Pacific. We present a static, mass-balance 'Ecopath' model for this region, focusing on small pelagic fish species, as an initial step to evaluate the role of these fishes in this ecosystem. Our quasi sub-model structure has 3 blocks (coastal Oyashio, coastal Kuroshio, and offshore) that were established to take sub-regional differences of bottom topography and oceanography into consideration. This model consists of 41 functional groups and assumes that some species are endemic to a single block, while some migrating species occur in 2 or 3 blocks. We evaluated the quality of our model using pedigree and pre-balance diagnostics. The impact of fisheries on the marine ecosystem assessed by both the L-index, i.e. the index of loss in secondary production due to fisheries exploitation, and the impact of fisheries targeting small pelagic fishes on the total production of small pelagic fishes, are compared with other ecosystems. Both ecological indices indicate that the western North Pacific ecosystem is not overexploited. Our static mass-balanced Ecopath model will contribute to expanding ecological knowledge of the western North Pacific.