Article
Spatio-temporal variation of the higher trophic level community structure in the western North Pacific Ocean
Takehiro OKUDA, National Research Institute of Far Seas Fisheries, Fisheries Research Agency
Understanding the community structure of oceanic higher trophic level (HTL) organisms (e.g., sharks, tunas, salmons, squids) is fundamental to describing the oceanographic, biogeochemical and ecological features of new ocean provinces with ecosystem services and socio-economic values. However, our understanding of marine ecosystems has lagged behind that of terrestrial ecosystems because of limited data availability for open ocean ecosystems. Scientists in Group A03-1 have analyzed long-term data sets derived from fishery-independent surveys to explore the spatio-temporal variations in HTL community structures and to evaluate the impacts of environmental factors and/or anthropogenic activities such as commercial fisheries on open ocean ecosystems.
Fig. 1. Map of the sampling stations along four transect
lines (cited from Okuda et al. 2014).
Long-term scientific data from the Japanese driftnet survey of Hokkaido University was used to analyze the spatio-temporal variations in the HTL community structure in the western North Pacific Ocean (Okuda et al. 2014). The analyses were conducted using a subset of the data collected along four transect lines (155°E, 170°E, 175°E and 180°E; spanning from 36 to 48°N) that have been regularly surveyed from 1982 to 1996 (Fig. 1). Non-size-selective multiple mesh driftnets were used in the survey.
1）Latitudinal variation
Fig. 2. Latitudinal patterns of the HTL community
structure in the western North Pacific Ocean.
(a) Numerical abundance, (b) Species richness,
and (c) Simpson diversity index.
Modified from Okuda et al. (2014).
Numerical abundance (Fig. 2a) showed increasing trend along latitude, whereas no clear trend appeared for species richness (Fig. 2b) and Simpson diversity index (Fig. 2c), the latter takes into account both species richness and an evenness of abundance among component species. This latitudinal trend of species richness did not follow the well-known general pattern, “latitudinal gradient of species diversity”, but showed higher numerical abundance at higher latitude. These inverse latitudinal patterns would reflect the high productivity at subarctic water of North Pacific Ocean during summer, and corresponding migration and/or aggregation of highly migratory species. Group A03-1 is promoting focused studies on seasonal migration of salmon and neon flying squid and evaluation of their feeding environment.
2）Longitudinal variation
Fig. 3. Longitudinal patterns of the HTL community
structure in the western North Pacific Ocean.
(a) Numerical abundance, (b) Species richness,
and (c) Simpson diversity index.
Modified from Okuda et al. (2014).
The total number of individuals (Fig. 3a), species richness (Fig. 3b) and Simpson diversity index (Fig. 3c) showed similar longitudinal gradients with lower abundance and diversity in the eastern area. These unitary lower biodiversity patterns on the easternmost transect line at 180°E were possibly caused by low productivity in the central part of the North Pacific. Other NEOPS studies suggest that the longitudinal gradient of productivity in the North Pacific is caused by the decrease in the Kuroshio extension transports and mesoscale oceanic gyre (A01: new ocean province from physical oceanography), or by iron deficiency (A2: Material cycling regulating marine biological production).
3）Temporal variation
Fig. 4. Inter-annual patterns of the HTL community
structure in the western North Pacific Ocean.
(a) Numerical abundance, (b) Species richness,
(c) species richness per drift net panels, and
(d) Simpson diversity index.
Modified from Okuda et al. (2014).
Inter-annual patterns in the HTL community structure were not consistent across the three metrics: numerical abundance was relatively constant, but exhibited increased fluctuations in later years (Fig. 4a). Species richness, as its nominal value, decreased over the years (Fig. 4b), but species richness per net panel showed increasing trends (Fig. 4c). The Simpson index did not show any clear temporal trends (Fig. 4d). The inconsistency in the temporal patterns of community metrics was considered to arise from some ecological processes such as regime shifts and/or from changes in the sampling scheme (e.g., sampling effort and mesh size composition of the sampling gear). To evaluate and compensate for the effect of sampling gear change, an invited study project was launched that focuses on the standardization of driftnet selectivity. By promoting these study projects, we aim to reveal the long term dynamics of open ocean ecosystems (i.e., from the 1950's to present day) with proper consideration for environmental and anthropogenic impacts and sampling biases.
