Pacific Science, vol. 68, no. 2 October 24, 2013 (Early view)

Ecological role of common minke whales in the southwestern East Sea (Sea of Japan) ecosystem during the post-commercial whaling moratorium

period

By Kyung-Jun Song* and Chang Ik Zhang Abstract The structure of the southwestern East Sea ecosystem and the role of common minke whales (Balaenoptera acutorostrata) in this ecosystem during the post-commercial whaling moratorium (post-CWM) period (1986-2007) were examined using an Ecopath model. Results showed that catch and biomass of common squid was the highest in this ecosystem (catch=0.302 t/km2, biomass=1.031 t/km2). Although this ecosystem consists of primary producers, primary consumers, secondary consumers and terminal consumers, most taxonomic groups were classified as secondary consumers. Common minke whales were classified as terminal consumers along with apex predators. The trophic level of common minke whales in this ecosystem was estimated at 3.34, and the mean trophic level of this ecosystem was estimated as 2.91. The relative contribution of common squid to the total energy flow at trophic level III, which includes common minke whales, was the highest (72.0%). However, the relative contribution of common minke whales at this trophic level was low (1.3%) compared to other taxonomic groups. This study is one of the first to address anthropogenic impacts (commercial whaling) on the trophic role of common minke whales in the East Sea, and it could potentially be used to understand the importance of common minke whales to this ecosystem within a context of potential biological removal (PBR). *Corresponding Author E-mail: kjsong329@ulsan.ac.kr

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Introduction

Common minke whales (Balaenoptera acutorostrata) are widely distributed from the equator

to the polar regions (Jefferson et al. 2008). In the western North Pacific, two stocks are

recognized, the East Sea-Yellow Sea-East China Sea stock (J stock) and the Okhotsk Sea-West

Pacific stock (O stock) (IWC 1983). Common minke whales in Korean waters, which are from J

stock, are the most abundant baleen whale in this area, with a population size of 5,841

individuals (95% confidence interval: 2,835-12,032) during August and September of 1989 and

1990 (Buckland et al. 1992). Their locations of highest abundance correspond with the

continental shelves parallel to shore (Cho et al. 2003), with southward migrations to breeding

areas occurring in winter, and return migrations occurring to northern feeding areas in the

summer (Gong 1988).

Common minke whales were extensively hunted until the moratorium on commercial whaling

began in 1986 (Kim 1999). During the commercial whaling period, the total recorded catch

numbers from 1962-1986 were 13,734 whales, with a peak of 1,033 in 1973 (Kim, 1999).

Although common minke whales in Korean waters were protected for more than 23 years after

this moratorium, they continued to be anthropogenically threatened by the fishing industry

bycatch in this area (Song 2010, Song et al. 2010, Zhang et al. 2010, Song 2011, Song 2013).

Several studies have been conducted in the past on the ecological role of common minke

whales, including on their feeding habits (Haug et al. 1996, Konishi et al. 2009), prey

preferences (Lindstrøm and Haug 2001, Murase et al. 2007) and functional response (Smout and

Lindstrøm 2007). These studies showed that common minke whales are top predators that feed

on commercially important fishes, especially small pelagic fishes such as Japanese anchovy, in

addition to small crustaceans such as euphausiids and thus play an important top-down

ecological role in the marine ecosystem. In particular, some studies also reported that common

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minke whales showed prey selection for Japanese anchovy in the western North Pacific (Murase

et al. 2007).

The roles of marine mammals in the marine ecosystem were investigated by using a variety of

ecosystem models, including the Ecopath model, in many regions of the world (Mori and

Butterworth 2006, Morissette et al. 2006, Lindstrøm et al. 2009). All these studies indicated that

ecosystem models are important for the purpose of the investigation of the ecological roles of

marine mammals. In particular, a mass balance ecosystem model, the Ecopath model, is

normally used to investigate the ecosystem structure and function (Walters et al. 1997,

Christensen et al. 2000).

Recent fisheries management studies that include cetaceans are based on ecological

interactions among various taxa in the ecosystem (Walters et al. 1997, Christensen et al. 2000).

Thus, it is important to consider an ecosystem approach to fisheries in order to sustainably

maintain fisheries resources in addition to cetaceans, including common minke whales. The

ecological role of common minke whales in the post-moratorium East Sea ecosystem is poorly

understood due to a paucity of investigations (Zhang and Yoon 2003, Zhang et al. 2007).

The objective of our study was to examine the structure of the southwestern East Sea

ecosystem and the role that common minke whales have played during the post-commercial

whaling moratorium (post-CWM) period (1986-2007).

Material and methods

Data collection

The study area used for our ecosystem modeling was in the southwestern East Sea (Sea of

Japan), an area of approximately 193,000 km2, which includes Korea-Japan provisional fishing

zones (Fig. 1). The East Sea is located east of the Korean peninsula, between Korea and Japan.

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The average water depth and the total area of the East Sea are 1,700 m and 1,000,000 km2,

respectively. There is approximately a total of 960 km of coastline in the East Sea. This area is

one of the most productive areas in Korean waters compared with other areas (Kim and Kang

1998).

Species abundance and distribution information for this area were investigated with a survey of

the relevant literature (NFRDI 1994, Park and Choi 1997, NFRDI 1999, NFRDI 2000). A time

series of the catch data from 1986 to 2007 was obtained and analyzed from databases within the

Ministry for Food, Agriculture, Forestry and Fisheries (MIFAFF) in Korea (MIFAFF 1987-

2008). The catch data of each taxonomic group was calculated from the sum of the average catch

of each species within each group, including the bycatch of common minke whales during this

period (Kim et al. 2004, Kim 2008).

The grouping of species in this area was conducted by using the self-organizing mapping

(SOM) method, which is a neural network pattern recognition technique (Lek and Guegan 1999).

An artificial neural network (ANN), which is a black box approach that has a great capacity for

predictive modeling, is a non-linear mapping structure based on the function of the human brain.

All characters describing an unknown situation should be presented to the trained ANN, and the

prediction is then given in a self-organizing map. Based on the method of Lek and Guegan

(1999), a total of nine variables were used to classify species, including mobility (weak, medium,

strong), body size (small, medium, large, very large), bone type (soft bone, hard bone, mollusk,

carapace), habitat depth (epipelagic, pelagic, semi-demersal, demersal), body shape (spindle, flat,

streamlined), habitat type (surface, bottom, sand/mud, rock, sand/rock, sandy bottom, nektonic),

feeding type (filter feeding, beak/tooth), longevity (0-5, 5-10, 10-15, over 15 years) and food

type (herbivorous, omnivorous, carnivorous).

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Data analysis

The structure of the southwestern East Sea ecosystem and the role of common minke whales in

this ecosystem during the post-CWM period (1986-2007) were examined using an Ecopath

model (Christensen et al. 2000) (Fig. 2). This period was selected to investigate the potential

effect of the CWM in 1986 on this ecosystem. The Ecopath model is based on two equations.

The first, a mass-balance equation, described how the production rate of each group (i) is divided

into components. This is fulfilled using the following equation,

Pi = Yi + Bi·M2i + Ei + BAi + Pi·(1-EEi) (1)

where Pi is the total production rate of group i, Yi is the total catch rate of group i, Bi is the

biomass of group i, M2i is the total predation rate of group i, Ei is the net migration rate of group

i, BAi is the biomass accumulation rate of group i, Pi·(1-EEi) is the other mortality rate of group i

and EEi is the ecotrophic efficiency, i.e., the fraction of the production that is utilized within the

system for predation or export, of group i. The second equation balances the input and output

energy of all groups in the model by accounting for energy flows.

Consumption = Production + Respiration + Unassimilated food (2)

The input parameters of the catch (C), biomass (B), production to biomass (P/B) ratio and

consumption to biomass (Q/B) ratio were estimated for each taxonomic group classified by

SOM. The biomass (B) of common minke whales was calculated from the abundance and

average body weight (Trites and Pauly 1998, Park et al. 2009). In particular, a shipboard sighting

survey using the line transect method was conducted to estimate the abundance of common

minke whales in this area, and the abundance was estimated using the DISTANCE program

(Buckland et al., 1993). The biomass (B) of other groups was calculated from the B=C/F

relationship (C=catch, F=instantaneous coefficient of fishing mortality which is associated with

fishing activity). The biomass (B) of zooplankton and phytoplankton was calculated using

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previously published information (Kang et al. 2000). Since the production to biomass (P/B) ratio

is equal to the biomass-averaged total mortality, it was calculated from the instantaneous

coefficient of total mortality (Z), which consists of fishing mortality and natural mortality

(NFRDI 1994, NFRDI 2000). Due to a lack of data for the Q/B ratio in this area, it was assumed

to be the same as that of the Bering Sea ecosystem, because the temperature and species

composition of the Bering Sea ecosystem are relatively similar to those of the southwestern East

Sea ecosystem compared with other areas (Trites et al. 1999). The diet composition of common

minke whales was calculated from direct observation of the forestomach contents of common

minke whales incidentally bycaught during fishing operations in Korean waters from 2000 to

2008 in our other study. The diet composition of other taxonomic groups was calculated from

several scientific papers and reports on prey species of each taxonomic group (NFRDI 1994,

NFRDI 1999, NFRDI 2000).

Results

Ecosystem structure

Nine variables (i.e. mobility, body size, bone type, habitat depth, body shape, habitat type,

feeding type, longevity and food type) were used to classify species in the southwestern East Sea

ecosystem. As a result, they were classified into 14 taxonomic groups by a SOM method, except

for zooplankton, phytoplankton and detritus. In particular, a total of four species, specifically

Japanese anchovy (Engraulis japonicus), Pacific saury (Cololabis saira), walleye pollock

(Theragra chalcogramma) and common squid (Todarodes pacificus), were subjectively

classified as single taxonomic groups because of their importance as prey species for common

minke whales (Table 1).

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The input parameters of the catch (C), biomass (B), production to biomass (P/B) ratio and

consumption to biomass (Q/B) ratio for each taxonomic group are shown in Table 2. The catch

and biomass of common squid was the highest in this ecosystem during the post-CWM period

(catch=0.302 t/km2, biomass=1.031 t/km2). The diet composition for each species group is

shown in Table 3.

Although the southwestern East Sea ecosystem consists of primary producers, primary

consumers, secondary consumers and terminal consumers, most taxonomic groups were

classified as secondary consumers (Fig. 3). Common minke whales were classified as terminal

consumers along with apex predators. The trophic level of common minke whales in this

ecosystem was estimated to be 3.34 during this period (Table 4), and the mean trophic level of

this ecosystem was estimated to be 2.91.

Ecosystem function

The relative contribution of common squid to the total energy flow at trophic level III, which

includes common minke whales, was the highest (72.0%) (Fig. 4). However, the relative

contribution of common minke whales at this trophic level was low (1.3%) compared to other

taxonomic groups.

Discussion

The results from our model suggested that the structure of the southwestern East Sea

ecosystem during the post-CWM period was similar to that reported by previous studies (Zhang

and Yoon 2003, Zhang et al. 2007), including the result that most taxonomic groups were also

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classified as secondary consumers. However, our results concerning the ecological role of

common minke whales in this ecosystem are not directly comparable with these previous studies

because in those cases common minke whales were not analyzed as an independent taxonomic

group.

Our calculated trophic level value (3.34) for common minke whales was similar to the value

(3.40) for common minke whales in other geographic areas (Pauly et al. 1998). Other studies

involving the stomach contents of common minke whales in our study area show that the

proportion of macrozooplankton, including krill, was slightly higher than those in published

reports of other areas (Kasamatsu and Hata 1985, Kasamatsu and Tanaka 1992, Tamura et al.

1998, Tamura and Fujise 2002). Because of the higher proportion of macrozooplankton in

stomach contents of minke whales in our study, the trophic level of common minke whales in

this area is slightly lower than those of published reports of other areas (Pauly et al. 1998).

Generally, baleen whales tend to feed on mainly small crustaceans like krill, while toothed

whales tend to feed on several kinds of fishes, crustaceans and cephalopods, although there is

some variation in the composition of prey species according to the region (Horwood 1990). Our

result show that the trophic level of common minke whales in this area (3.34) was lower than

that of the finless porpoise (Neophocaena phocaenoides) in the Yellow Sea (4.21) (Park et al.

2002). Due to the common minke whale’s baleen feeding habit, the trophic level of common

minke whales in this area is probably lower than those of other predatory fishes as well as of the

finless porpoise.

Self-organizing mapping is a valuable method for grouping and visualizing species according

to their ecological characteristics (Lek and Guegan 1999). Zhang et al. (2007) also reported the

utility of this method for grouping species, and they used seven variables, specifically the

mobility, body size, bone type, habitat depth, body shape, habitat and feeding type, to classify

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species. Our study used nine variables, including longevity and food type, to classify species

because we aimed to generate a grouping that better represents the trophic structure in our study

area.

Even though an Ecopath model can be used to study the ecosystem structure and function, the

accuracy of the modeling of an ecosystem mainly depends on the input parameters including the

biomass (B), production to biomass (P/B) ratio and consumption to biomass (Q/B) ratio.

Consequently, more accurate data need to be obtained regarding the input parameters which

generally require extensive time and effort. A limitation of our model was that the data on the top

predators (e.g., sharks and whales), including the biomass and prey composition, was sparse.

Previous studies on the structure of the southwestern East Sea ecosystem also had this limitation

(Zhang and Yoon 2003, Zhang et al. 2007). Therefore, future work using similar Ecopath models

may require more information on the top predators in order to have more realistic applications

toward the management and conservation of common minke whales in the North Pacific.

Acknowledgements

We wish to acknowledge the many volunteers who reported bycatch events of common minke

whales in Korean waters. We thank Mr. Hyeok Chan Kwon and Dr. Jong-Hee Lee (Pukyong

National University, Republic of Korea) for their assistance with the Ecopath and SOM analysis.

This research was supported by a grant from the Cetacean Research Institute, National Fisheries

Research and Development Institute in Republic of Korea.

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Table 1. Classification of groups by species in the southwestern East Sea ecosystem

Biota Group name

Sharks and whales Apex predator

Common minke whale

Small sharks

Birds Seabirds

Fishes Japanese anchovy

Pacific saury

Walleye pollock

Small pelagic fish

Large pelagic fish

Semi-demersal fish

Benthic demersal fish

Cephalopods

Common squid

Cephalopods

Benthos Benthic feeders

Epifauna

Infauna

Gastropods

Algae Benthic algae

Plankton Zooplankton

Phytoplankton

Other Detritus

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Table 2. Basic input parameters for the post-CWM model

Habitat area

(fraction)

Biomass

(t/km2)

Production/

Biomass

(/year)

Consumption/

Biomass

(/year)

Catch

(t/km2)

Apex predator 1.000 0.003 0.280 11.156 -

Common minke whale 1.000 0.031 0.280 8.331 0.002

Small sharks 1.000 0.000 0.280 11.156 0.000

Seabirds 1.000 0.006 0.800 60.000 -

Japanese anchovy 1.000 0.036 0.630 3.650 0.029

Pacific saury 1.000 0.021 1.250 3.650 0.017

Walleye pollock 1.000 0.074 0.600 5.487 0.034

Common squid 1.000 1.031 3.200 10.667 0.302

Small pelagic fish 1.000 0.089 1.610 3.650 0.073

Large pelagic fish 1.000 0.025 1.000 3.145 0.010

Semi-demersal fish 1.000 0.049 0.934 2.226 0.031

Benthic demersal fish 1.000 0.042 0.910 2.611 0.027

Cephalopods 1.000 0.044 3.200 10.667 0.013

Benthic feeders 1.000 0.298 0.934 7.690 0.103

Epifauna 0.050 0.109 1.051 5.777 0.041

Infauna 0.050 0.041 0.863 12.000 0.018

Gastropods 0.050 0.167 1.156 5.777 0.065

Benthic algae 0.050 0.569 10.000 - 0.014

Zooplankton 1.000 12.740 5.545 22.000 -

Phytoplankton 1.000 13.062 718.754 - -

Detritus 1.000 - - - -

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Table 3. The estimated proportions of prey eaten by predators in the post-CWM model

Prey\Predator 1 2 3 4 5 6 7 8 9

1 Apex predator

2 Common minke whale 0.015

3 Small sharks 0.005

4 Seabirds

5 Japanese anchovy 0.062 0.079 0.100 0.001 0.0002

6 Pacific saury 0.019 0.030 0.001 0.0001

7 Walleye pollock 0.239 0.383 0.010 0.001

8 Common squid 0.356 0.120 0.100 0.051 0.082

9 Small pelagic fish 0.010 0.053 0.027 0.100 0.100 0.100 0.015 0.001 0.020

10 Large pelagic fish 0.005 0.008

11 Semi-demersal fish 0.009 0.014 0.007

12 Benthic demersal fish 0.019 0.017 0.001

13 Cephalopods 0.037 0.100

14 Benthic feeders 0.224 0.100 0.100 0.100 0.012

15 Epifauna 0.017 0.033 0.016 0.001

16 Infauna 0.010 0.020 0.010 0.0001

17 Gastropods 0.074 0.147 0.001

18 Zooplankton 0.868 0.700 0.800 0.800 0.839 0.9317 0.8519

19 Benthic algae 0.003 0.008

20 Phytoplankton 0.017

21 Detritus 0.019

SUM 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000

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Table 3. Continued

Prey\Predator 10 11 12 13 14 15 16 17 18

1 Apex predator

2 Common minke whale

3 Small sharks

4 Seabirds

5 Japanese anchovy 0.061 0.005 0.009 0.005

6 Pacific saury 0.018 0.001 0.002 0.001

7 Walleye pollock 0.030 0.034 0.020

8 Common squid 0.219 0.063 0.110 0.008 0.003

9 Small pelagic fish 0.300 0.101 0.050 0.020

10 Large pelagic fish 0.023 0.004 0.003 0.002

11 Semi-demersal fish 0.030 0.028 0.015

12 Benthic demersal fish 0.015 0.007 0.005

13 Cephalopods 0.083 0.004 0.060

14 Benthic feeders 0.097 0.100 0.253 0.322 0.082

15 Epifauna 0.016 0.028 0.034 0.064

16 Infauna 0.013 0.023 0.028 0.053

17 Gastropods 0.043 0.075 0.092 0.172

18 Zooplankton 0.282 0.494 0.250 0.598 0.698 0.171 0.130

19 Benthic algae 0.001 0.289 0.338

20 Phytoplankton 0.003 0.159 0.333 0.331 0.762

21 Detritus 0.001 0.128 0.092 0.667 0.331 0.108

SUM 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000

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Table 4. Estimated trophic levels in the southwestern East Sea ecosystem for the post-CWM

model

Group name Trophic level

Apex predator 4.17

Common minke whale 3.34

Small sharks 3.95

Seabirds 3.09

Japanese anchovy 3.24

Pacific saury 3.24

Walleye pollock 3.13

Common squid 3.07

Small pelagic fish 3.08

Large pelagic fish 3.76

Semi-demersal fish 3.47

Benthic demersal fish 3.46

Cephalopods 3.48

Benthic feeders 3.18

Epifauna 2.50

Infauna 2.00

Gastropods 2.00

Zooplankton 2.02

Benthic algae 1.00

Phytoplankton 1.00

Mean trophic level 2.91

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Fig. 1. Study area for ecosystem modeling in the southwestern East Sea (Sea of Japan).

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Fig. 2. Flowchart of the steps and processes for conducting ecosystem modeling.

Fig. 3. Estimated trophic levels and relative biomass of species in the southwestern East Sea

ecosystem for the post-CWM model.

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Fig. 4. Relative contribution (%) of species to the total energy flow (throughput) at trophic level

III in the southwestern East Sea ecosystem.

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