Climate Exchange

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] 73

griculture

Variability of oceanographic parameters and

fisheries production

The oceanographic parameters of Indonesian waters

were affected by monsoon wind, ENSO, and IODM.

During southeast monsoon between May and August,

the wind blows from the southeast towards the north-

west along the southern coast of Java-Bali-Nusa

Tenggara (eastern Indian Ocean or EIO), causing

water masses along the coast to be pushed away from

the coast, and producing seasonal upwelling within the

regions. The upwelling process brings deep-water mass

with nutrient-rich cold water to the surface, and that

affects thermocline layer shallowing and increases the

fertility of waters. Time series data from the SeaWiFS

in the Bali Strait upwelling region clearly indicated the

increase in chl-a concentration due to the upwelling

process, which begins in May and peaks in September

in this area.

Bigeye tuna (Thunnus obesus) was mostly caught in

the layer of 10-15° C isotherm, because this layer is suit-

able for the species.

Time series data for the catch rate

(hook rate or HR) of bigeye tuna over 15 years showed

that during the upwelling period, the bigeye tuna HR

generally increased.

Generally, the long line hook used to catch bigeye

tuna varied in depth from 100-250 metres. At the time

of upwelling, the 10-15° C isotherm was about 50

metres shallower than normal, resulting in a growing

number of hooks reaching the bigeye tuna fishing

layer – hence the increased hook rate. At the time of

upwelling, phytoplankton abundance also increased,

resulting in an abundance of small pelagic fish as a food

source for bigeye tuna.

The climate change impacts of ENSO and IODM are

seemingly anomalous from SST and chl-a concentra-

tions in 1994, 1997-1998 and 2006-2007. During ENSO

and IODM positive periods, more intense upwelling

occurs, the thermocline layer becomes shallower and

chl-a concentrations increase significantly in the EIO.

SST data from January 1982 to December 2009 and chl-a concentra-

tion from January1998 to December 2011 were obtained from the

National Oceanic and Atmospheric Administration and the National

Aeronautics and Space Administration websites.

In general, SST values in Indonesian waters varied from 25° C

to 31° C. Relatively high SST values were encountered during the

May-July period (summer), while the November-January period

(winter) showed relatively low values. The SST pattern was heavily

influenced by seasonal wind conditions such as the monsoon

winds.

During the northwest monsoon (November-February),

relatively cold winds and high moisture from the South China Sea

will lead to high rainfall and lower SST in the Indonesian region.

In contrast, during the southeast monsoon (May-August), the flow

of wind from mainland Australia carries little moisture and leads

to a relatively high SST with little rainfall (dry season).

The El

Niño Southern Oscillation (ENSO) and Indian Ocean Dipole Mode

(IODM) also influenced the inter-annual variability of SST in

Indonesian waters.

Some specific waters such as the Java Sea and Makassar Strait

had slightly different SST patterns. Relatively high SST values were

commonly found in October-December and March-May, while

relatively low values were seen in July-September and in January-

February. The Bali Strait and Arafura seas had different SST patterns

from other locations, with a maximum SST in November-March

and minimum SST in May-October. These patterns demonstrated

that the SST fluctuations in the Bali Strait, Arafura Sea, Java Sea and

Makassar Strait were influenced by monsoon winds and by the ocean

circulation in this area.

In general, the SST trend at various locations in Indonesian waters

over the last two decades has tended to increase. Over the same

period, it has tended to decrease in the Indian Ocean south of Java,

the Bali Strait and Arafura Sea. This may be due to an intensified

upwelling phenomenon in these regions.

In contrast to SST, the pattern of chl-a concentration tended to

decrease in general, except in the Indian Ocean south of Java, the

Bali Strait and Arafura Seas where intensified upwelling was evident.

Climate warming has reduced vertical mixing since the water

column is stabilized by thermal stratification, decreasing nutrients

the in upper layer.

Source: IOTC, 2012

http://www.iotc.org

Bigeye tuna production rates for Indonesia, Taiwan, and Japan in EIO, 1973-2010