Mud banks and Fisheries

^#Mahesh. V, ^#Harshavardhan D. Joshi, ^#Milind B. Katare, Vijay S. Hapase ^@Harshal P. Wankhade, ^*Snehal S. Lokhande, ^*Amrapali M. Gajbhaye, ^*Nileshwari M. Warhekar,*Priyanka S. Naik

#College of Fisheries, Mangalore, Karnataka Veterinary Animal and Fisheries Sciences University, Bidar, Karnataka, India 575 002.

College of Fisheries,Veraval,Junagadh Agriculture University,Junagadh,Gujarat,India

^*BBKKV, College of Fisheries, Ratanagiri, Maharashtra, India

Corresponding Author's e-mail: mahesh.fishco@gmail.com

Introduction

      The calm, turbid regions in the coastal waters of Kerala are called the mud banks. The well known mud banks are formed along the coast of Cochin and Alleppy. The mud banks appear during the south west monsoon season. The mud gets churned up and this mud is kept in suspension making the water highly turbid. The muddy waters are free from surface disturbance their being no wave even during peak monsoon season. Portion of the beach corresponding to mud banks are free from wave action. While regions of the adjacent beach experiences severe erosion. The mud banks acts as a barrier and save the beach being eroded. These mud banks are unique in nature, which are not reported from anywhere in India. It is periodic phenomenon taking place between Cannanore and Quilon.

      First mention of mud banks recorded in 1755 by caption cope and described in his book entitled “A new history of East Indies” later attempts were made to study the Chemistry, Physics, and Geology of these mud banks. To substantiate the calmness of the mud bank, there are two schools of thoughts.

1.     According to W. King (1881) attribute to the presence of oil along with mud which causes damping of the waves.

2.     Keen and Russle attribute the calming effect to the mud itself- ie mud in suspension increases viscosity of water and causes damping of waves. This is also reflected as liquid mud.

      Generally, the mud colour is dark green and contains some foraminifera remains. The fineness of mud gives it an oily feeling consisting of ¾ th clay and ¼ th of silt. The rich Phosphate the rich content of the mud was observed by Jayaraman and Sheshappa. 

Theories regarding the formation of mud banks.   

The proposed theories are

1. Underground discharge of mud from mud banks 

2. Wave action on bottom mud 

3. Upwelling and mud bank formation

      This theory was put forwarded by Ramasastry and Myrland in 1959. According to this, mud bank formation is associated with upwelling and divergence near the bottom between 20 to 30 meter depth along the coast line. This produces vertical acceleration resulting in lifting of fine bottom mud.

4.floculation and deflocculation

        Suspended particles in sea water either be deflocculated depending on whether the salinity is high or low. In lower salinity conditions particles remain in suspension. While in higher salinity condition they get flocculated. A flocculated suspension can be deflocculated by lowering the salinity. This deflocculation takes place when the salinity falls below 2.5 ppt  and flocculation above 20 ppt. between these two salinity range, the flocculated mud gets deflocculated and kept in suspension, thus become the responsible factor for formation of mud banks. 

Fish and Fishery

      The S.W.Monsoon period is generally an off season for fisher folk on the west coast of India, who still use non-mechanised fishing crafts. In this period of general idling, the calm areas created by the mud banks natural, attract fishermen in large numbers, from far and wide. These calm areas, varying in extent from about 10km² to 25km2, provide safe harbourage to the country crafts. Launching and landing of canoes are very easy in the mud bank areas. Most of the canoes from the fishing villages of Cochin-Quilon and Cochin-Ponnani congregate respectively at the mud banks of Alleppy and Nittika, and these places then become centres of intense fishing activity. 

Craft and gear

      Dugout and rigged canoes (Vallom or Vanchi in Malayalam) were the main crafts used in the fishing operations. Catamarans of Tamilnadu with hook and lines were also seen. The canoes were mainly of two sizes, a larger one of 9.5 m, manned by 15 persons, and a smaller one of 6 m, manned by 9 persons. Canoes fitted without board motors, which had been introduced in 1980, were also in operation. The main gears of operation were drag nets (Thangu vala) and gill nets (Mathi-chala vala) Cast nets were also used effectively near the shore in the mud bank area. 

Thangu vala

        This is a rectangular net made of Cotton or Nylon and is about 50-60 m in length and 15-20 m in width, with a narrow end measuring about 6-9m. the mesh size is about 20mm. on sighting a shoal, one person jumps in to the water holding one end of the net and remains stationary, while the Conoe moves paying out the net to encircle the shoal. The Canoe on reaching the person, collects the other end also from him, and the net is hauled. Hauling the ends, a bag like belly in the centre is formed, where the fishes are collected and removed.  
 

Mathi-chala vala

      This is a gill net made of Cotton or Nylon pieces. Each piece, measuring about 2.5 to 3 m long and 4 to 6 m broad, with a mesh of 9 to 25 mm one or two conoes are operated to lay the net and these Canoes carry 7 to 8 pieces of net is rapidly paid out in a semi circular fashion, the fishermen making loud noise. Thus driving the shoals toward the net they are gilled on the nets and removed. 

Fish

      The monsoon fishery was composed of 50 species of fish and 6 species of prawns. Fishes of the families Carcharinidae, Clupeidae, Dussumieridae, Dorosomidae, Engraulidae, Tachysuridae, Ambassidae, Theraponidae, Chirocentridae, Sillaginidae Sciaenidae, Siganidae, Trichiuridae, Scombromoridae, Stromatidae, Cyanoglossidae and Drepanidae were encountered in the landings. Of the prawns, Pennaeus indicus, P monodon, P semisulcatus, Metapenaeus dobsoni, M. monoceros and M affinis represented in the catch. While the monsoon fishery of the Alleppey mud bank region was dominated by M dobosoni, Stolephorus sp, Sardinella sp and Leiognathus sp. The mager bulk of the catch of the northen sector was contributed by Sardinella sp, P indicus and M dobsoni.

      The monsoon fishery in Kerala has a legal protection from the State Government, providing exclusive operational rights during monsoon to Canoes and Catamarans, especially so in the vicinity of the mud bank region. Fishing by mechanised vessels is strictly prohibited at the mud bank and nearly areas. However, indigenous crafts fitted with outboard motors, introduced in 1980, are allowed to operate in the mud bank regions.  

Reference 

CMFRI bulletin No. 31, Mudbanks of Kerala Coast 

CMFRI bulletin No. 44, National Symposium on Research and Development in Marine Fisheries