QUANTIFICATION OF THE SINUSOIDAL TRAJECTORY IN TISSUE MASS AND CONDITION INDICES OF A BIVALVE (EGERIA RADIATA) IN THE CROSS RIVER, NIGERIA* LAWRENCE ETIM, THOMAS BREY and WOLF ARNTZ

Variation in monthly values of dry tissue mass (DTM) and two different types of condition indices of the bivalve Egeria radiata (Lamarck, 1804) was studied for 32 consecutive months. To quantify the seasonal trajectory of variation these data were fitted to oneand two-compartment sinusoidal models; and also to a quadratic, cubic and quartic models. The cyclic events in the variation of the T)TM and both indices of condition were adequately described by the one-compartment sinusoidal model whose parameters are biologically interpretable. The fit of the data is progressively improved as one moves from the quadratic through the cubic to the quartic model whose fit was best. Although biological meaning could be found for the one-compartment sinusoidal model, the biological meaning of the coefficients of the parabolic-based models are obscure. Multiple regression analysis shows that of all the four environmental parameters (temperature, salinity, pH and phytoplankton) tested, temperature has the greatest effect on DTM and one of the condition indices, while pH has the greatest impact on the other condition index. This implies that the two indices are not only intrinsically different, but that they respond to different sets of factors. Interand intra-specific comparison of the many recommended 'standard' indices of condition found in the literature, especially with regard to their innate sensitivity to seasonal variation in internal (e.g. state of gravidity) and external (physicochemical parameters of water) factors is necessary.