Some Biological Characteristics of Chrysichthys nigrodigitatus from Luubara Creek, Niger Delta, Nigeria

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Introduction
The fishes that make use estuarine environment are economically important group and Chrysichthys nigrogigitatus belong to this group (Froese and Pauly, 2020;Okyere and Boahemaa-Kobil, 2020).C. nigrodigitatus species is among other species that have been reclassified to the family Claroteidae (Paugy et al., 2003;Olaosebikan and Raji, 2004;Yem, 2015) and highly used as important fish food species in Nigeria and Africa (Esenowo et al., 2017).
C. nigrodigitatus, commonly known as ″silver catfish″, can actually grows to attain 3kg weight and even above, which can be found both in fresh and brackish waters.Silver catfish is tasty and possess a tough good quality flesh.Silver catfish is vital and highly valued finfish, common in inland waters of Nigeria (Akinsanya et al., 2007;Saliu, 2008;Olarinmoye et al., 2009;Okyere and Boahemaa-Kobil, 2020).C. nigrodigitatus is known to be characteristically hardy and can accommodate diverse ecological conditions and these qualities project it culturation in some countries (Pangni et al., 2008a;Pangni et al., 2008b;Okyere and Boahemaa-Kobil, 2020), and the fish is well consumed for its nutritional value.The species is endemic in Nigeria (Yem, 2015) and also distributed in most basins from Senegal to Angola and other African countries (Paugy et al., 2003;Okyere and Boahemaa-Kobil, 2020),

Suggested Citation
occurring over mud and fine sand bottoms in shallow waters of lakes, rivers, estuaries and swamps (Okyere and Boahemaa-Kobil, 2020).Given the importance of this Silver catfish to the fisheries in Nigeria, it is important to understand some biological characteristics of this endemic and commercial species so as to employ appropriate management routine and strategies for the stocks.

The Study Area
The study was carried out in Luubara creek in Khana Local Government Area of Rivers State of the Federal Republic of Nigeria for a period of one year from September, 2018 to August, 2019 (Figure1).The creek is found between longitudes 7 0 15 // E -7 0 32 // E and latitudes 4°321-4°37 // N in the eastern part of the Niger Delta (Deekae, 2009;Deekae, et al., 2010aDeekae, et al., , 2010b;;Gbarakoro et al., 2014).The Luubara creek has a dry and wet seasons climatic rotation.This phenomenon shows that the area is in the humid tropical zone (Gbarakoro et al., 2014).A bogged down detailed of the study area are given by (Deekae et al., 2020;Nkuene, 2020;Gbarakoro et al., 2014).

Collection of Fish Samples
Three stations (Wiiyaakara, Bane and Luubara-Fig 1) and sampled were collected from each station twice a month from the fisher men catches from normal operations using multiple and single longline (Plate 1).The fish species were identified to species level from monographic description, checklist and keys from authors (Holden and Reed 1972;Idodo-Umeh, 2003).Total length (cm) of Chrysichthys nigrodigitatus was taken by measuring the fish from the tip of the mouth to the end of the caudal fin using measuring board and the weights (g) was taken by placing the C.

Statistical Analysis
Data collected were analyzed through the use of FAO-ICLARM Stock Assessment Tools (FISAT11) 2007 software:

Growth Parameters
The annual growth parameters (L∞, K and t0) of The growth performance index (Ø) of C. nigrodigitatus were calculated as: Ø= Log10K + 2Log10 L∞ (Pauly and Munro, 1984) as incorporated into the scan of k-value routine.
The state of exploitation of the fish species were calculated from the ratio of F/Z.
The relative yield-per-recruitment (Y/R) and relative biomass-per-recruitment of C. nigrodigitatus was determined by the knife-edge recruitment approach which is identified by Beverton and Holt (1959) as yield per-recruit model and incorporated into the recruitment routine in FISAT (Pauly. 1983;Gayanilo, et al., 1995).

Results and Discussion
The results of the estimated growth parameters L∞, K, to and Ø of C. nigrodigitatus based on the FISAT software.The result gives L∞ as 76.87, K as 0.740, to, 0.27 and Ø as 2.37. Figure 2 shows the K scan of C. nigrogrodigitatus, Figure 3 is the length-converted graph of C. nigrodigitatus and Figure 4 is the relative yield-per-relative biomass of C. nigrodigitatus.

C.nigrodigitatus from Luubara Creek
The result of a year's total mortality (Z), natural mortality (M) and fishing mortality (F) of the species using the length-converted curve protocol were.3.23,1.18 and 2.05 for Z, M and F respectively while the exploitation rate (E) stands as 0.635.The ratio of length at first capture to asymptotic length of the foregoing species (Lc/ L∞) is 0.15, meaning that the species is caught at a small size or that the fish were caught at 15% of growth.
The relative yield per recruit and relative biomass of C. nigrodigitatus of the creek was examined using the Beverton-Holt ogive in Fig. 4. The result indicated that € produced the value of Emax, E10 and E50 as 0.426, 0.357 and 0.157, respectively.

Discussion
The K value for C. nigrodigitatus estimated at 0.74 was ˃ 0.5, which is typical for rapid growth pattern (Sparre & Venema, 1998).This implies that the higher the growth coefficient the faster the silver catfish approaches the asymptotic length.The Asymptotic length in this study is greater than those found for the same species by Kone et al. (2022), Kongbeh et al. (2015), but closer to Ajagbe et al. (2021).The implication is that Fish with a smaller asymptotic length may initiate reproduction earlier, potentially allowing them to support a larger biomass in comparison to individual with larger length (Ahti et al., 2022).Moreover, the value of Ø at 2.37 indicates that C. nigrodigitatus species from Luubara creek have a consistent fast growth.The growth performance index, Ø exhibit close values within species and closely related taxa.This is similar to the result of Kone et al (2022) andkongbeh et al., (2015) for C. nigrodigitatus in diferent locations in Africa.For the close species, Chrysichthys auratus, growth performance index was 2.55 (Abdellatif et al., 2022).Generally, the asymptotic length can be influenced by differences in habitats such as available food supply and differences in environmental temperature while K is only affected by temperature ((Beverton & Holt, 1959;Latuconsina et al., 2020), accounting for the differences in the value obtained in this study compared with others from other locations.
The instantaneous rate of total morality, natural mortality and fishing mortality recorded in this study are higher than the result reported by Francis and Erondu (2010) and Sikoki (2013) about the natural and fishing mortalities of the aforementioned species.In this study, the instantaneous fishing mortality of C. nigrodigitatus is higher than natural mortality.This means that the death of C. nigrodigitatus recorded during the period was mostly caused by human exploitation, which is inconsistent with report of Francis (2003), Francis and Erondu (2010) and Sikoki (2013) on C. nigrodigitatus from Andoni river system.The death of C. nigrodigitatus caused by human is as a result of the consumers' preference which made the fishers to develop specific gears for harvesting it (Uneke and Nwani, 2014).Therefire, with the exploitation rate (E = F/Z) estimated to be 0.635, C. nigrodigitatus fishery of Luubara suffers from over exploitation.This is based on the assumption that a stock is optimally exploited when the fishing mortality (F) equals natural mortality (M) or E = F/Z = 0.5 (Gulland, 1971).If fish mortality in an aquatic ecosystem occurs naturally, the stock is not at risk.However, when the mortality is a result of human exploitation, it poses a threat to the fish stock and may lead to overfishing.Olopade et al. (2019) advised that exploitation ratio should be maintained at 0.4.The mortality values obtained in this study is scientifically valid and can be used for management purposes since the M/K value is within the range of 1.0 and 2.5.
The Lc was estimated at 0.15cm depicting that it was harvest at smaller size before attaining full potential and the gloomy situation could be regarded as growth overfishing (Olopade et al., 2019).Udoidiong et al ( 2016) encountered similar scenario on the species in the Lower Cross River.Thus, from a management standpoint, the situation is unfavorable for the resource's health and there is a need for a management strategy that enables the removal of such sizes from the equipment used in resource exploitation to manage and protect C. nigrodigitatus fishery of the Creek.

Conclusion
The study on some biological characteristics of C. nigrodigitatus is important for two reasons.First, there is scanty data available for use in the management of artisanal or subsistence fishery and stock depletion is pronounced daily for multispecies and specific species fisheries.For the sustainability of the fish in the creek, this study become sacrosanct.C. nigrodigitatus is a commonly consume fish species and viable for commercial business.The growth coefficient and growth performance index show that the species grows fast and regularly while the result of the mortalities shows that the abovementioned species died mainly via fishing.This now call for the need to putting up appropriate management measures to watch over the lurking incident of overfishing.This will require Figure 1.Map of Luubara creek showing study stations Source: Deekae, 2009

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Figure 3. Length Converted Curve of C.nigrodigitatus from Luubara Creek