MUD BANKS OF(F) KERALA: FISHWORKER’S DELIGHT

Prof. Thrivikramji.K.P., University of Kerala, Kariavattom Campus 695 581

thrivikramji@gmail.com

Introduction

Mud banks (MB) are very special to certain segments of the coastal waters of Kerala and were first reported by Bristo, the founder of the Cochin port. This phenomenon, not known elsewhere in the world is of interest not only to the fishing industry and Marine biologists, but also to physical and chemical oceanographers as well as geoscientists. The immediate attraction to the fishworkers is the very profitable fisheries opportunities due to very special fish species attached to the MB or fishing ground the mud banks are synonymous with. Further, such a phenomenon is also not noticed reported from elsewhere in the country. In what follows, one would come across a useful summary or rather a review of the various aspects the MB.

Fisheries potential

The huge fisheries potential that exist in the mud banks attract hundreds of migratory fish-workers with their crafts and tools, financiers, buyers, seasonal workers in peeling and processing units and others like truckers, porters, small business persons who find numerous but indirect business opportunities. Due to abundance of Prawns, sardines, mackerels and soles in the waters during the MB season, the small and big towns in the adjacent coastal land plunges into a frenzy, and Thakazhi Sivasankara Pillai very aptly used this canvas for his great love story – Chemmeen, which later on Ramu Karyat immortalized in his epic film of the same name.

Previous work

Ever since Bristo made the first mention during his preliminary studies in connection with the design and development of the Cochin Port, the MB attracted the curiosity of geologists as well as Marine Biologists. The latter always worried and wondered about its fisheries potential, and the causative biological and physical and chemical parameters. The Geoscientist, on the other hand focused on the mineralogy of the (silty-, clayey-) sediment in suspension on the body of the MB, and thereby the source/s and the mechanism that triggered, maintained, even controlled the loci. Jacob, Dora, Narayana and Nair made significant contributions on the varied geological aspects, whereas, Kurup examined the phenomenon from a Physical Oceanographers standpoint.

What are MBs?

The MB are very special and uniquely present only in the coastal waters of Kerala and that too these become active only during the SW monsoon season. Another, attraction of the MB is the excellent fisheries potential they offer the fishing industry. This phenomenon appears only in the segment between Thirkunnapuzha and Kozhikode (distance=270Km). Another important attribute is that mud banks are migratory in nature, i.e., these migrate up coast or down coast, in that they do not recur in the same spot or sector year after year.

 Nair reports at least at 27 locations in this sector, where MB had appeared at one time or other. The marine geology of the MB sector is generalized as below. A map of the sea bed geology of the shelf published by Siddique, Mallick and Srivastava (1978), all of the Geological Survey of India, shows the presence of an inner-shelf mud in the MB region, in addition to a narrow ribbon of sand lacing the sea bed from north to south – a seaward extension of the sediment in the beach face.

Kurup reported that usually low phosphate content of the mud water rises during the MB generating SW monsoon season, but salinity down by 8% of the normal. The June-July period is roughest in the Laccadive Sea with highest 10% waves. Mud content in the range of 1200 mg/lit on the surface waters of MB to 1500 mg/lit in the bottom layer exists. The mud is characteristically made of clay (45-65%; Kaolinite, 60 to 65%; montmorillonite, 15-20%; and illite 15-20%; and traces of chlorite, quartz and gypsum), followed by silt and very little sand. The temperature between surface and bottom waters in the MB differs only by 1.0- 1.5 o C.

In the late sixties, a Norwegian research ship had collected bottom sediment samples in the coastal waters of the MB region. The samples archived in the KERI were studied for characterizing the clay mineralogy, and there is a report on the presence of coarse sand grade Selenite grains (crystals) - a transparent variety of gypsum – showing a nucleus of sand particle, clay lumps or shell fragments.

I believe that these Selenite grains would have rather formed in the pore waters of tropical beach sediment due to super-saturation of its pore water with respect to gypsum. Absence of such crystals in the eroding modern beach (sediment) then must be taken as a robust evidence for the occurrence of beach complexes (sand bodies) in the offshore that formed during the low sea level stands in the Pleistocene. Selenite along with the mud is stirred up by the wave climate of the SW monsoon. Kurup too supported a wave initiated triggering mechanism for the MB.

Why Mud-banks?

This phenomenon offering the twin benefits of (a). rather very safe and harbour like clam pool of water on its shore ward side, during the other wise hostile sea conditions and (b). a stupendous fishery potential, is an important contributor to the nations GDP. Therefore, from a scientific point of view, the mud banks are “an essential phenomenon” for the marine biologist and the Marine Geologist. More over, a coastal sea sans MB for Kerala and the nation will be equally devastating. The “erosion shadow” offered to the beach by the MB is equally beneficial to the settlers in the respective backshores, as well as to the exchequer. Therefore these themes have legitimately become a field of study to examine the sustainability of the phenomenon and resource potential.

The MB phenomenon has been attributed to mundane geological phenomenon like submarine spring sapping and the sea bottom sediment stirring monsoon wave climate. However, the investigations by various groups and individuals have come to suggest the role of waves on a sea bed with large quantity of dominantly muddy sediment. The migratory nature of mud banks obviously reflects the link between availability of suitable bottom mud (sediment) and wave climate – the latter however is a trans-Laccadive sea phenomenon. The nutrient that surfaces due to upwelling caused by the wave activity attracts marine life of sorts to this region creating a transient but rich fishing ground.

What lies ahead?

Considering the huge reserves of sandy-clayey, clayey bottom sediment that had been accumulating since the Pleistocene (since 1.8 Ma) in the continental shelf of Kerala, the state shall not have to worry about the rebirth of the MB in the coastal waters and hence the income from the MB related fisheries. The chances are that the size of reservoir of mud must be on the increase, as a result of selective shoreward transport of ilmenite rich sand (now exploited by the Rutile manufacturing industry), leaving behind a lag sediment rich in mud.

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