Relationship between opercular girth, maximum girth and total length of fishes caught in gillnets in the estuarine and lower river sections of Shatt al-Arab River (Basrah Province, Iraq)

Abstract

Summary In fisheries, the two morphological parameters of opercular and maximum girth are related to the effectivity of capture methods in gilled and wedged fish, respectively. The present work investigates the relationship between opercular (Gope) and maximum girths (Gmax) to total length (Lt) for 10 fish species captured from Shatt al-Arab River, Basrah, Iraq. Data were collected October 2005 to December 2006. Cyprinids were the best represented family with six species; engraulids, silurids, heteropneustids and mugilids were each represented by one species. Gope and Gmax were found to increase linearly with total length of all species, all r2 values being greater than 0.73 and statistically significant (P < 0.01). When Gope and Gmax for all species were plotted against total length, two groups were identified (G1, G2), corresponding to general girth-length relationships: (a) G1 = −0.252 + 0.424 Lt and G2 = −0.262 + 0.600 Lt for opercular girth and, (b) G1 = 1.538 + 0.419 Lt and G2 = 1.538 + 0.696 Lt for maximum girth. These groups correspond to different body shapes of fishes: G1-round and G2-compressed. These relationships have implications when using length data and mesh size to determine size selectivity of gill nets.