The Full Picture

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Satellite-based fishery service in India

Shailesh Nayak, T. Srinivaskumar and M. Nagarajakumar;

Indian National Centre for Ocean Information Services

ndia has a 7,500 kilometre coastline and an exclusive

economic zone (EEZ) of about two million square kilometres.

About seven million people living along the coast are depen-

dent on fishing for their livelihood. The locating and catching of

fish has, however, become increasingly challenging as fish stocks

dwindle and move further offshore. This increases the search

time, cost and effort involved. A reliable and timely forecast on

the potential zones of fish aggregation would benefit the fishing

community by reducing search time and the effort involved in

locating fishing grounds.

The adaptation of fish to the surrounding marine environment is

influenced by various physical, chemical and biological factors,

including seawater temperature, salinity and dissolved oxygen. Fish

are known to react to changes in their surrounding environment by

migrating to areas where more favourable conditions exist. Availability

of food is another important factor which effects the occurrence,

abundance and migration of fish. The monitoring of these parameters

in space and time by in situ measurement is time-consuming and

expensive. In fact, a real-time picture of any one of, or a combination

of these parameters is almost impossible using in situ methods.

With the advent of ocean remote sensing during the eighties, fishery

forecast research derived much needed impetus. The link between

satellite-derived sea surface temperature (SST) and chlorophyll with

fish aggregation was established. In India, the efforts of oceanogra-

phers, remote sensing specialists and fishery scientists resulted in a

unique service called the Potential Fishing Zone (PFZ) advisory.

The PFZ forecast is issued three times a week by INCOIS, except

during the fishing ban period and on cloudy days. The validity of

such forecasts is three days. This is the only short-termmarine fishery

forecast available in the country for the benefit of small mecha-

nised/motorised sector fishermen (about 100,000 vessels). The PFZ

advisory has matured into an operational application of satellite

remote sensing, which provides timely and reliable advisories to fish-

ermen. The effort is part of the Common Minimum Programme

(CMP), lead by the Government of India.

Generation of PFZ advisories

The ability to forecast catch or seasonal abundance of stocks in differ-

ent areas requires a understanding of various aspects of the physical,

biological and chemical processes of the sea. This includes basic

knowledge of currents, upwelling areas, eddies, gyres and thermal

fronts over space and time. Remote sensing observations especially on

SST and levels of chlorophyll provide a significant part of the infor-

mation needed to assess potential fishery zones. Satellite technology

provides the additional advantage of utilizing synoptic coverage over

a short interval of one to two days.

Thermal and chlorophyll features exert a huge influ-

ence over the upper ocean biological process. The

coincidence of frontal zone positions of chlorophyll and

SST gradient represent the coupling of biological and

physical processes, and hence, fish aggregation.

SST and chlorophyll over the Indian EEZ have been

retrieved from near-synchronous thermal infrared chan-

nels of NOAA-AVHRR (10.3 to 11.3 micrometres and

11.5 to 12.5 micrometres) and optical bands in IRS-P4

OCM data, respectively, to identify PFZ along the Indian

coastline. The McSST approach suggested by Mcclain

is used to compute SST. The chlorophyll retrieval process

involves two steps: the atmospheric correction of the

visible channels of the ocean colour monitor (OCM) to

obtain normalized water radiance, and the application

of bio-optical algorithms for the retrieval of water para-

meters.

The atmospheric correction of OCEANSAT–I OCM

imagery was carried out using the approach suggested

by Mohan.

An empirical algorithm (OC2) was used

to retrieve the cholophyll.

4,5

Both the SST and chloro-

phyll images were corrected geometrically using a set

of ground control points located both on the image and

on the Naval Hydrographic Office bathymetric map. A

composite image showing SST-contours on a

colour–coded chlorophyll image was also generated.

Features such as upwelling, eddies, fronts and mean-

ders were selected.

1,6

In contrast to demersal

resources

this technique works best for pelagic and

column zones.

The identified features were delineated as PFZ lines

and overlaid on the base maps. The entire Indian coast-

line is divided into 12 sectors in order to generate the

PFZ maps: Gujarat, Maharastra; Karnataka and Goa;

Kerala; South Tamilnadu; North Tamilnadu; South

Andhra Pradesh; North Andhra Pradesh; Orissa and West

Bengal; Lakshadweep Islands; Andaman and Nicobar

Islands. The PFZ maps contain information about the

major landing centres, bathymetry latitude and longi-

tude in addition to the identified fishing zones.

The PFZ maps are translated into PFZ text, which gives

information about the latitude, longitude and depth of

the shelf at specific locations as well as angle, direction

and distance from the landing centres/light houses. These

integrated PFZ advisories are prepared in English, Hindi

and other local languages (Gujarati, Marathi, Kannada,

Malayalam, Tamil, Telugu, Oriya and Bengali). Local

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