Abstract
BackgroundThe black-chinned tilapia, Sarotherodon melanotheron, is the most abundant fish species in the Nakwa (an open lagoon) and Brenu (a closed lagoon) in the Central Region of Ghana. Aspects of the life history characteristics and the ecology of the fish populations in both lagoons were studied to assess the bio-ecological status of this important resource.MethodsFish samples were obtained from fishermen that fish on the Nakwa and Brenu lagoons using cast, drag and gill nets. The age of the fish was assessed from otoliths analysis and its growth modelled following the von Bertalanffy growth function. Morphometric characteristics of the fish populations were analysed using power regression and ANOVA for parameters comparisons, and Student’s t test to determine whether species grew isometrically. The percentage occurrence method was used to analyse the stomach contents of the fish.ResultsA total of 382 fish samples from both lagoons were measured, comprising 209 from Nakwa lagoon and 176 from Brenu lagoon. The size and weight of fish samples ranged between 3.9–11.5 cm total length and 1.0–27.3 g for Nakwa Lagoon and 5.6–12.8 cm total length and 3.2–29.8 g for the Brenu Lagoon. The estimated von Bertalanffy growth parameters were L∞ = 12.04 cm and K = 2.76/year for the Nakwa Lagoon samples and L∞ = 13.44 cm and K = 3.27/year for Brenu Lagoon samples. Daily otolith incremental rate ranged from 0.01–0.03 mm per day to 0.01–0.02 mm per day for Nakwa and Brenu lagoons, respectively. Stomach content analysis of the fish samples revealed that the species are planktivorous and the range of food varied between the lagoons. Green algae were the most prevalent food item in the stomachs of the fish samples from Nakwa with the frequency of 69% whilst diatoms (80.5%) were most prevalent phytoplanktonic food item for the fish in Brenu lagoon.ConclusionsThe estimates of asymptotic length for the species in both lagoons are close to known values of the species length at first sexual maturity and points to intensive fishing pressure. As a consequence, a comprehensive sample-based survey is required in both lagoons to derive estimates of management reference points. The results of the stomach content analysis are beneficial to the construction of diet matrix for ecosystem models of the two systems.
Highlights
Ghana’s coastline is endowed with many lagoons that support the livelihood of adjacent local communities (Dankwa et al 2004; Davies-Vollum et al 2019)
The standard lengths of the fish samples were in a range of 3.0–8.6 cm and 4.2–9.9 cm for Nakwa and Brenu lagoons, respectively (Additional file 1: Figure S1)
The results show that fish from Brenu lagoon were generally bigger compared with the population from Nakwa lagoon
Summary
Ghana’s coastline is endowed with many lagoons that support the livelihood of adjacent local communities (Dankwa et al 2004; Davies-Vollum et al 2019). Tilapias constitute an important food source, providing protein and play a very important role in the economy of coastal inhabitants, especially during the off season for marine fishing (Blay and Asabere-Ameyaw 1993) Due to their potential to adapt and thrive in varying environmental conditions, they have either invaded or have been introduced into several water bodies in various regions around the world (Lazard 1990). The species, though not developed for aquaculture, is an important resource in the commercial and subsistence fisheries of many West African lagoons (Ekau and Blay 2000) It constitutes a high proportion (50–95%) of total catch in the lagoons along the coast of Ghana (Blay and Asabere-Ameyaw 1993; Koranteng et al 2000; Panfili et al 2004; Dankwa et al 2016). Aspects of the life history characteristics and the ecology of the fish populations in both lagoons were studied to assess the bio-ecological status of this important resource
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