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Ecosystem Monitoring & Assessment

Regional Water Mass Characteristics in the Northern Bering Sea

Research Reports
Apr-May-June 2013
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refer to caption
Figure 1. Bering Sea Integrated Ecosystem Research Program(BSIERP) marine regions in the Bering Sea.

Description of Index
The oceanography and shelf dynamics of the southern eastern Bering Sea (EBS) have been well-studied, while less attention has been given to the northern EBS, although commercially important fisheries are present in both the south and the north. Sea ice extent and duration, and freshwater inputs from the Yukon River are substantially higher in the north compared to the south, resulting in large variations in oceanography between the northern and southern EBS and between regions within the northern EBS. We describe spatial variations in oceanographic characteristics (salinity, temperature, and zooplankton abundance) for pre-defined regions (Fig. 1), and compare these characteristics to juvenile salmon biomass (all species combined) in the northern EBS. Sampling was conducted on a station grid using a conductivity-temperature-depth (CTD) cast (Seabird Electronics (SBE) 19, 25 or 9-11) equipped with a Wet Labs fluorometer, and beam transmissometer. The survey grid (60-km station spacing) encompassed areas between lat. 60 and 65N over the EBS shelf. Zooplankton were collected over the water column: large taxa (>505um) with oblique bongo-net tows (505 m) and small taxa (<505um) with a vertical Juday-net tow (168 m). Samples were preserved in 5% formalin and enumerated at shore-based facilities. Juvenile salmon were caught with a surface rope trawl (Can trawl model 400-580 spread 60 m (width) by 15 m (depth)), towed 30 min at 3.5 to 5 knots. Salmon weights were measured for each species (chum, pink, chinook, coho, sockeye), and the multispecies biomass catch per unit effort (CPUE) was estimated for all species combined. Bering Sea Integrated Ecosystem Research Program (BSIERP) region delineations were drawn by consensus across researchers based on observed oceanography, bathymetry, benthic fauna, fish, seabird and marine mammal distribution. Data were broken out by BSIERP region for primary investigations.

Status and Trends
Norton Sound stands out as a distinct region within the northern EBS characterized by high surface and bottom temperatures, low surface and bottom salinities, and lower than average light transmission (Table 1). The South Bering Strait and North Inner Shelf regions are areas of high juvenile salmon biomass and of high numbers of large zooplankton (S. Bering Strait) and high numbers of small zooplankton (N. Inner Shelf). Highest light transmission values are seen with high bottom and surface salinity in the St. Lawrence region, while low transmission values are found with low bottom and surface salinity in Norton Sound.

Table 1. Oceanographic  parameters, large and small zooplankton abundance and juvenile salmon biomass by BSIERP region. Red indicates high/maximum values and blue indicates minimum values.


BSIERP Region

Temp Top
 (° C)

Temp Bottom
 (° C)

Salinity
 Top

Salinity
Bottom

Transmission
(% light trans)

Large
zoo abund.
(# m-3)

Small
zoo abund.
(# m-3)

Juvenile salmon biomass
(kg  km-2)

North Inner

8.25

6.53

30.63

30.92

82

84

104127

3706

North Middle

7.83

1.26

31.15

31.57

83

90

54969

819

Norton Sound

9.70

8.92 

27.00

28.29

65

41

13037

575

South Bering Strait

7.51

5.15

31.11

31.59

82

2418

10399

2287

St. Lawrence

7.65

2.97

31.80

32.20

89

183

13108

194

St. Matthews

7.61

1.33

31.32

31.74

84

67

5941

930


Initial findings reveal connections between juvenile salmon and bottom temperature, bottom salinity, and large and small zooplankton, depending on the region. Surface temperature and salinity changes over the northern EBS can change considerably from season to season and from near to offshore. Norton Sound has relatively low juvenile salmon biomass during late summer/early fall, while highest juvenile salmon biomass is found in South Bering Strait and North Inner Shelf regions. Future analysis will focus on individual salmon species while investigating their spatial and temporal relationships with oceanographic parameters.

Factors Causing Trends
Ice melt and high fresh water run-off contribute to Norton Sound's low salinities, while shallow depths contribute to higher temperatures in summer/early fall.

Implications
Because highest abundances of large and small zooplankton were seen in the South Bering Strait and North Inner regions, respectively (coinciding with the two highest regions of juvenile salmon CPUE), large zooplankton could be important prey for juvenile salmon in the South Bering Strait region, while small zooplankton could be important prey for juvenile salmon in the North Inner region. By Jeanette Gann and Lisa Eisner

 

By Jeanette Gann, and Lisa Eisner

 

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