Diel changes in vertical distribution patterns of zooplankton and walleye pollock (Theragra chalcogramma) off the Pacific coast of eastern Hokkaido, Japan, estimated by the volume back scattering strength (Sv) differen

ce method

在北海道東部太平洋近海 , 利用 Sv 差值的方法 , 估計鱈魚與浮游動物

日夜間垂直分佈的變化

KAZUSHI MIYASHITA,1,* KOUTAROU TETSUMURA,2 SATOSHI HONDA,3 TATSUKI OSHIMA,4 RYO KAWABE1 AND KEI SASAKI1

Fish. Oceanogr. 13 (Suppl. 1), 99–110, 2004

Reporter: Chao-Yi Yu

INTRODUCTION

Euphausia pacifica and Neocalanus copepods are the main components of the zooplankton.

In the Doto area Hokkaido, walleye pollock prey especially heavily on E. pacifica.

Acoustical methods are very useful for estimating the abundance and spatial distribution patterns of fish and zooplankton.

Distribution of the walleye pollock

walleye pollock ; Alaska pollock(Theragra chalcogramma)

MATERIALS AND METHODS

acoustic data collection

• EK500, SIMRAD, Norway

• at two frequencies (38 and 120 kHz).

• A total of 19 parallel transect lines perpendicular to the isobathswere (A~S) set at 8-nauticalmile intervals.

• sea depths range : 50 ~ 500 m

• ship speed : 8 knots

Figure 1. Map of the transect lines andstations sampled by the RV Kaiyo-maru No. 3, in June 2001.

Theoretical scattering mode

ae :the equivalent sphere radius (ESR)

k : the wave number in water

TS : target strength 單體標物反射強度

Sv : volume back scattering strength 體積散亂反射強度

LH : body height LW : body width (a zooplankter)

L : total body length

g : density contrast h : sound speed contrast

Values of g and h were taken from Mikami et al. (2000) (g =1.030, h =1.034)

以水平距離 0.005 海哩 , 深度 1m 為一積分單位 ( 網狀方格 )

其中每積分單位以 Sv120KHz - Sv38KHz = ΔSv 可用來推演對象生物之等價求半徑 , 再與浮游生物網樣

本大小作為比較

當兩頻率任一小於 -75dB 時 , 假定為 no echo

皆大於 -75dB 時需去除背景雜音獲得對象生物聲探資料

Sv difference method (ΔSv)

sv is the linear value of SVn : biomass density

Sv, which depends on the frequency because the TS is frequency dependent, can be written as follows:

Fish samples (mainly walleye pollock)

bottom trawls (19)

mid-water trawls (10)

Zooplankton samples bongo nets (BN 01~07)

XCTD

投棄式電導率、溫度、深度剖面測量儀

CTD

Results

(1) Species composition and ΔSv

bongo nets and trawls

(2) Distribution patterns of zooplankton and

walleye pollock

during day and night

Table 2. Summary of the zooplankton compositions in the bongo net samples.

Copepods (>50% Neocalanus copepods) and krill (>70% E. pacifica) were predominant based on both number and wet weight .

bongo nets samples

For Neocalanus copepods, there was one size class with a mean total body length of 2.0 mm (SD =±1.1, N =1044080).

For E. pacifica, there were two size classes with mean total body lengths of 6.1 mm (SD =±2.2, N =13 099) and 15.4 mm (SD =±3.6 , N =7628).

The mean ESRs of E. pacifica were 1.0 mm (SD =±0.3, N =13 099) and 2.8 mm (SD =±0.9 , N =7628),

The mean ESR of Neocalanus copepods was 0.5 mm (SD =±0.3, N =1044080)

The ESR distribution of small E. pacifica is similar to that ofNeocalanus copepods.

the ESR distribution of Neocalanus copepods and E. pacifica calculated from Eqn (3).

These ΔSv ranges of the small E. pacifica + Neocalanus copepods and large E. pacifica were estimated from ESRs and Eqn (6).

• small:19.0 dB < ΔSv < 21.0 dB

• large: 8.0 dB < ΔSv < 16.0 dB

ΔSv

The large size class was distributed mainly in the eastern part of the survey area (Line A–K, depth range: 150–200 m).

The small size class was distributed mainly in the western part of survey area (Line F, and Line I to Line S; depth range: 50–150 m).

Both small andlarge walleye pollock were distributed in the middlepart of the survey area (Line F, and Line I–K).

Trawl sampls

＜ age 2: mean SL = 187.1 mm (SD =±32.1, N =1,279) ＞ age 3: mean SL = 399.6 mm (SD =±30.4, N

=1343)

Typical echograms (from Lines C, I and L) at 38 and 120 kHz during day and night. Lines C, I and L were in the eastern, central, and western part of the survey area, respectively.

Typical echograms (Lines C, I and L) of the distributions of zooplankton and walleye pollock overlaid with water temperature during day (left) and night (right).

The end

Thanks