Benthic studies adjacent to Sakhalin Island, Russia, 2015 II: energy content of the zoobenthos in western gray whale feeding grounds

Abstract

The waters adjacent to the northeastern coast of Sakhalin Island, Russia, are an important feeding ground for the endangered western gray whale. Data on the energy available to foraging whales from their prey resources is required for researchers interested in modeling the bioenergetics of whale foraging, but little energy content information is available for the benthic prey communities of gray whales in this region. In this study, we describe the energy density (ED), biomass, and total energy availability (ED × biomass) of benthic prey sampled from two gray whale foraging areas adjacent to Sakhalin Island: the nearshore and offshore feeding areas. ED varied almost seven-fold among benthic taxa, ranging from 1.11 to 7.62 kJ/g wet mass. Although there was considerable variation within most prey groups, amphipods had the highest mean ED of all of groups examined (5.58 ± 1.44 kJ/g wet mass). Small sample sizes precluded us from detecting any seasonal or spatial differences in mean ED within or among taxa; however, mean biomass in the offshore feeding area was, in some cases, an order of magnitude higher than mean estimates in the nearshore feeding area, resulting in higher mean total energy available to foraging gray whales offshore (958-3313 kJ/m²) compared to nearshore (223-495 kJ/m²). While the proportion of total energy accounted for by amphipods was variable, this prey group generally made up a higher proportion of the total energy available in the benthos of the offshore feeding area than in the benthos of the nearshore feeding area. Data presented here will be used to inform bioenergetics modeling of the vital rates of mature females in an effort to improve understanding of population growth limits for western gray whales.

Keywords: Amphipods; Calorimetry; Eschrichtius robustus; Gray whale prey; Macrobenthos; Russia; Sakhalin Island; Sea of Okhotsk.

Fig. 1

Map showing the 2015 seismic survey areas (gray polygons) and their proximity to western gray whale feeding areas (blue polygons in a) derived from 2001–2014 shore-based, vessel-based, and aerial survey programs. b The locations where caloric samples were taken, and the 11 cells for which energy density estimates were calculated (purple polygons). In 2015, the detailed benthic sampling occurred in cells 1, 3, and 5, and sampling in other cells was less dense

Fig. 2

Averages and 95% confidence intervals of Amphipoda and Bivalvia (soft tissue only and with shell) energy density pooled across all locations in the nearshore Piltun feeding area (grouped by “Early,” “Mid,” and “Late” season) compared to the offshore samples, which were only collected during the “Late” sampling period (see text). Sample sizes: nearshore early: Amphipoda N = 9, Bivalvia N = 2; nearshore mid: Amphipoda N = 3, Bivalvia N = 3; nearshore late: Amphipoda N = 3, Bivalvia N = 3; offshore late: Amphipoda N = 2, Bivalvia N = 1

Fig. 3

Averages and 95% confidence intervals of Amphipoda and Bivalvia (soft tissue only and with shell) energy density pooled across all seasons in the Piltun feeding area (grouped by north (cell 5), middle (cell 3), and south (cell 1)) and the offshore feeding area (cell 11). Sample sizes: nearshore north: Amphipoda N = 6, Bivalvia N = 3; nearshore middle: Amphipoda N = 7, Bivalvia N = 2; nearshore south: Amphipoda N = 7, Bivalvia N = 4; offshore: Amphipoda N = 2, Bivalvia N = 1