Benthic studies adjacent to Sakhalin Island, Russia, 2015 I: benthic biomass and community structure in the nearshore gray whale feeding area

Abstract

Okhotsk or western gray whales feed in summer along the northeastern coast of Sakhalin Island, Russia, a region with oil and gas extraction facilities. Seismic surveys increased sound levels in the nearshore feeding area in 2015 for part of the summer, potentially displacing whales from preferred foraging habitat or reducing foraging efficiency. Since lost foraging opportunities might lead to vital rate effects on this endangered species, detailed benthic surveys were conducted to characterize benthic community biomass patterns and spatial and temporal differences. Benthic biomass demonstrated strong spatial-temporal interactions indicating that prey biomass differences among locations were dependent on sampling period. Of greatest interest, Amphipoda biomass declined from June to October in the northern and southern portions of the nearshore study area but increased in the middle and Actinopterygii biomass increased in the northern area in mid-summer. Water depth and sediment type were significant covariates with community structure, and water depth strongly covaried with bivalve biomass. Total average prey biomass was ~ 100 g/m² within the nearshore feeding area with no evidence of reduced biomass among sampling periods or locations, although there were fewer amphipods in the south. Multi-prey investigations provide a stronger basis for inferences than single-prey studies of amphipods when gray whales feed on diverse prey. Benthic community-level variability was moderate to high as would be expected for a shallow-water nearshore area. Overall, spatial and temporal changes in dominant macrofauna biomass reflected small to medium-sized effects that were well within the natural boundaries expected for benthic communities.

Keywords: Benthic ecology; Ecosystem variability; Macrobenthos; Marine ecology; Sea of Okhotsk.

Fig. 1

The 2015 benthic sampling locations within the Sakhalin Island gray whale nearshore feeding area, Russia. Inset a gives the geographic reference of the gray whale feeding area, inset b the design for targeted sampling, and inset c the major circulation patterns. The * marks the mouth of Piltun Bay. In inset c, Amur current, waters from the Amur River; ESC, the East Sakhalin Current; and ESCC, the East Sakhalin counter-current

Fig. 2

Average wet tissue biomass (g/m²) and 95% confidence intervals of Actinopterygii, Amphipoda, Bivalvia, Cumacea, Echinoidea, Isopoda, Polychaeta, and total prey biomass from the 2015 detailed grid and feeding hotspots. North, Middle, and South are the zones in the detailed grid. Feeding hotspots were sampled between Period 1 and 2. The very large confidence interval for Hotspot Middle is due to the small number of sites sampled in that zone (2). Data points for different periods within each zone are staggered for clarity

Fig. 3

Amphipoda wet tissue biomass (g/m²) by sampling period in the 2015 nearshore study area. Bubbles represent replicate values. The maximum observed biomass of ~ 300 g/m² was collected in Period 2 from the middle zone in 12.5 m water depth and medium sand. The Sakhalin Island shoreline is provided for context with * marking the mouth of Piltun Bay. The bar chart of amphipod biomass at the bottom presents the proportions of amphipod wet weight biomass above 60, 100, and 200 g/m²

Fig. 4

Nonmetric multidimensional scaling of wet tissue biomass for the 2015 detailed grid for Periods 1–3 at North, Middle, and South Zones. The stations are symbolized by period and zone. Centroids and 95% confidence intervals for the period by zone combinations are presented in the inset. MDS station groups are indicated in the centroid plot

Fig. 5

Overlays of depth and sediment categories and dominant invertebrate and vertebrate wet tissue biomass on the MDS ordination for the 2015 detailed grid. Bubbles represent the biomass with larger bubbles indicating higher biomass