The Composition in Different Size Groups and Index of Relative Importance (Iri) of Callinectes amnicola (De Rochebrune, 1883) Food from Okpoka Creek, Niger Delta, Nigeria
Abstract: The composition in different size groups, and index of relative importance of Callinectes amnicola food from Okpoka creek was studied for a period of two years (January 2006 -December 2007. Crabs less than 10mm were absent in the catch. Crab appendages, bivalve shells, bivalve tissues, gastropod shells and annelids were absent in smaller crabs (10-19.9 mm) and (20-29.9 mm). Algae cells were absent in crab (40-49.9 mm to 70-79.9 mm) size groups. Crustacean, Pisces and mollusca were common in the stomachs of the larger size group. The numerical method showed that algae cells (46.9%) were most abundant in size group (10-19.9 mm) followed by 20-29.9 mm (33.1% ) size group and 30-39.9 mm (20.0% ) size group. Crustacean (shrimp parts 32.7% and crab appendages 35.2%) was numerically higher in (70-79.9 mm) size class. The frequency of occurrence method showed that, Fish flesh accounted for 19.8% 17.6%, 15.4% 13.2%, 11.0% and 9.9% of the stomach contents in the 30-39.9 mm, 20-29.9 mm, 10-19.9 mm (50-59.9 mm, 70-79.9 mm), 40-49.9 mm, 60-69.9 mm size group respectively. Fish scales occurred most in (70-79.9 mm) size group with a value of 19.4% followed by 15.3% (50-59.9 mm) size group whereas the other size groups had between 10.4% and 14.6% in occurrence. The percentage occurrence of fish bones /spines in 10-19.9mm to 70-79.9mm size groups ranged between 8.7% and 17.4%. The highest (17.4%) was recorded in (40-49.9 mm) size group while the least (8.7%) was obtained in (10 -19.9mm) size group. Bivalve shells (29.5%) and bivalve tissue (28.6%) led in occurrence in (70-79.9mm) size group whereas 28.6% (gastropod shells) encountered in (70-79.9 mm) size group, followed in decreasing order of occurrence by size groups 60-69.9mm (25.0%), 40-49.9 mm (21.2%), 50-59.9 mm (16.3%) and 30-39.9 mm (15.4%). Shrimp parts and crab appendages occurred consistently in (50-59.9 mm to 70-79.9 mm) size groups with the highest values (33.0% and 31.1%) in (70-79.9 mm) size group while the lowest (7.4%) was for (30-39.9mm) size group. Algae cells and higher plant parts accounted for 40.6% and 28.0% respectively of the stomach contents in 10-19.9mm. The point's method that showed shrimp parts contributed 33.3% and 26.3% of the stomach content in the 70-79.9mm and 60-69.9mm size groups respectively. Similarly, crab appendages accounted for 35.5% in 70-79.9mm size group and 31.2% in 60-69.99mm size group. In the 70 -79.99mm size group, fish flesh was 38.0%, fish scales 18.9% and fish bones/spines (18.9%) respectively. The highest values bivalve shells (31.9%) and bivalve tissues (32.1%) were recorded in (50-59.9mm) and 60-69.9mm) size groups respectively. Whereas the 10-19.9mm and 20-29.9mm size groups had no bivalve shell or tissue in their stomachs. Gastropods shells values ranged from 17.7% to 23.2% by points with the highest (23.2%) recording in size groups (30-39.99 to 70-79.99 mm) while the least (17.7%) was in (70-79.9 mm) size group. Annelids scored between 4.2% and 37.5% points amongst (30-39.9 mm to 70-79.9 mm) size groups with the highest and lowest values recorded in (70-79.9 mm) and (30-39.9 mm) size groups respectively. Index of relative importance (IRI) in the food items showed importance ranged from 7.14% to 0.07% in 2006 with the highest percentage index of relative importance recorded for bivalve shells (7.14%) and the lowest (0.07%) for the annelids. On the contrary, the percentage index of relative importance values varied from 0.34% to 21.67% based on increasing order of importance in 2007. Bivalve shells had IRI value of 427.44 (21.67%) whereas Annelids had IRI valve of 6.66 (0.34%). Similarly the highest value of 21.67% and lowest (0.34%) were recorded for bivalve shells and annelids respectively
