Trophic ecology, growth pattern and wellbeing of blueback herring (Alosa aestivalis) from Iko River estuary, Southeastern Nigeria

Abstract

Background: Studying the trophic ecology, growth pattern and well-being of Alosa aestivalis are important aspects of fish biology. It is a crucial knowledge needed for its mass cultivation by fish farmers and also its implication for its management in the wild. The main objective of the study was to investigate the food and feeding habits, growth and well-being of Alosa aestivalis from Iko River estuary in Eastern Obolo Local Government Area, Nigeria. Methods: Samples of Blueback Herring (Alosa aestivalis) were collected from December, 2022 to April, 2023 from fisher catches at Iko landing point. The specimens were preserved with formalin immediately after capture in transparent container and transported to the Department of Zoology Laboratory and kept prior to analysis. Specimens were weighed with electronic weighing balance to the nearest 0.01 g and total length, standard length, and fork length of each fish sample were measured to the nearest 0.01 cm using a measuring board. Specimens were dissected and the gut contents examined, individual food items sorted, counting the number of each food item present in the gut of the species and summing up these numbers to obtain the grand number of all food items in the guts. Frequency of occurrence, numerical abundance, point method, Index of Relative Importance (IRI) and Gut Repletion Index were all computed. The length-weight relationship and condition factor of Alosa aestivalis were also determined. Results: Alosa aestivalis from Iko River estuary fed mainly on micro-plant and animal, macro-animal and non-living materials such as zooplankton, detritus, crayfish, small fish, phytoplankton and sand. The stomach content analysis revealed that Alosa aestivalis from Iko River estuary is an omnivorous species. Based on index of relative importance, Zooplankton formed the major food item with % IRI of 51.81. Detritus, second most important food item composed 35.63% IRI. Crayfish ranked third in the importance with an IRI value of 5.63% while small fishes ranked fourth with an IRI value of 3.45%. However, Phytoplankton and sand constituted a little above 3%. The species was found to be an active feeder based on the Gut Repletion Index of 85.03%. The length-weight relationship 'b' values of males, females and combined sexes were 2.253, 2.278 and 2.253 respectively. Conclusion: Based on the food items isolated from the stomach contents, the species is considered to be an omnivore. The condition factor values obtained for both male and females depict a negative allometric growth. This implies that the fish was in poor condition during the duration of study.