Condition Index, Meat Yield and Population Structure of the Marine Gastropod, Thais coronata, off Cross River Estuary, Nigeria

Abstract

Samples of the (rock shell) marine gastropod Thais coronata were obtained at monthly intervals from the Cross River estuary, Nigeria over a five month sampling period (February-June). The condition index, CI% (dry tissue weight expressed as a percentage of shell weight) and population structure showed distinct monthly variation. The values of the condition index was maximum (in May) during the beginning of the rainy season and minimum during the dry months (March). The population structure showed that generally T. coronata in the study area is composed principally of 33.5cm rock shells with distinct peaks or modal mean lengths stable at 33.5cm for February through April with triple peaks in March and April. The monthly variations in meat yield (MY%) and condition index indicate a buildup of somatic and gonodal materials during the months of April and May, followed by rapid decline attributed to the shedding of reproductive material through spawning. A standard specimen of 4cm yields CI = 7.1% and MY = 16.3%. Hence, the population spawns during the rainy season. The pattern of variation of water content seems a reverse of that of dry weight, with a fall in water content accompanied by an increase in dry weight. This might imply that the species absorbs more water after spawning. Results indicate harvest and purchase of T. coronata are best in the month of May in the study area and the waste shells have a high potential in income generation as source of calcium for animal feed production.

Key words: seasonal variations, shape, spawning and shore animals.