The dynamics of an exploited population of a siluroid catfish (Schilbe intermidius Reupell 1832) in the Cross River, Nigeria

Abstract

We studied the breeding biology, growth, mortality, recruitment pattern and yield per recruit of a freshwater siluroid catfish (Schilbe intermedius) in the Cross River, Nigeria. Monthly variation in mean gonadosomatic and mean condition indices shows that the species breeds twice in a year (March and September). We used 12 consecutive months length–frequency data together with FiSAT software to study the population dynamics of this catfish which is exploited by artisanal fishermen. Fitting the seasonalized von Bertalanffy growth function to our length–frequency data gives the following growth parameters: L_∞=27.5 cm total length, K=0.29 yr⁻¹, C=0.5, WP=0.46. Using the seasonalized length converted catch curve, we estimated the instantaneous total mortality coefficient Z as 1.85 yr⁻¹. The instantaneous natural mortality coefficient M was 0.81 yr⁻¹ while the instantaneous fishing mortality coefficient F was 1.04 yr⁻¹, giving the current exploitation rate E=0.56. From the analysis of probability of capture of each length class we estimated the length at first capture L_c to be 10.9 cm. The relative yield per recruit analysis predicted the maximum exploitation rate E_max=0.54. The current exploitation rate E is more than this predicted maximum E_max. Thus, the species is on the verge of being overexploited.

Introduction

Catfish are valued food species. They are heavily exploited and widely cultivated. The current world aquaculture annual production is about 3 00 000 tons. That makes them the fourth most widely cultivated freshwater fish after carp, salmons and tilapia.

The schilbeid catfish are salient components of the ichthyofauna of many freshwater bodies. Their general ecology has been studied in many Nigerian aquatic ecosystems. For example, the elucidation of the feeding habits of Schilbe mystus in the Ogun River (Adebisi, 1981), the Tiga (Bankole, 1989) and Kainji (Olatunde, 1978b) lakes revealed that the species feeds mainly on fish with insect and larvae as secondary items in the diet. In Lake Kainji the analysis of abundance of five schilbeid species revealed that Eutropcus niloticus is the most abundant, followed by Physailia pellucida, Schilbe mystus, Schilbe uranoscopus and Siluranodon auritus in that order (Olatunde, 1978b). In the river Ase, Idodo-Umeh and Victor (1993)described S. mystus as the most important species where it makes up 85.2% by number and 94.7% by weight of all the schilbeids landed by the artisanal fishers.

The limnology of the Cross River has been studied (Löwenberg and Kunzel, 1987; Akpan, 1994) and its effect on the fisheries elucidated (Moses, 1987). There are two seasons in southern Nigeria – the dry season (November–March), and the rainy season (April–October). Rainfall has far-reaching effect on the hydrography of the river system. During the rains, an enormous amount of run-off water carrying nutrients and debris from land flows into the main river channel, resulting in the flooding of adjacent plains at the peak of the rainy season. There is then an increase in water depth from ≈4 to 12 m, a lowering of conductivity, and a fall in water temperature from ≈32°C in March to about 24°C in July (Löwenberg and Kunzel, 1987). During this period the flow is unidirectional (no longer subject to tidal reversals), and current speed much faster (increasing from 0.6 to 1.0 m/s). More detailed results of the limnological studies of the Cross River have been given by Akpan (1994).

The ecology of the schilbeid catfish in the Cross River has not been studied before. In this work, we studied the cycles of breeding activity, growth parameters, mortality rates and recruitment pattern of Schilbe intermedius in the Cross River. We also computed the exploitation rate and the relative yield per recruit. Knowledge of these population and fisheries parameters are necessary for the scientific management and proper conservation of the stock.