Offem Benedict Obeten, Akegbejo-Samsons Yemi, Omoniyi Isaac Tunde
Department of Fisheries, Faculty of Agriculture and Forestry, Obubra Campus, Cross River State, Nigeria.
Rev Biol Trop. 2008 Dec;56(4):1785-99. doi: 10.15517/rbt.v56i4.5759.
The silver catfish, Chrysichthys nigrodigitatus (Lacépède: 1803) is a highly valued food-fish included among the dominant commercial catches exploited in major rivers of Africa. To provide useful biological data for management, samples were collected monthly between January (2005) and December (2007) in three zones: I: Upper Cross River (grassland), II: Middle Cross River (mixed forest and grassland), and III: Lower Cross River (rainforest)] along 200 km length of the Cross River, Nigeria. Data from 1 248 specimens were processed using: allometric coefficient (b), gonado-somatic index, Fulton's condition factor and diversity indices. Male dominance was observed in all populations; C. nigrodigitatus reached maturity at 11.5 cm (male) and 16.7cm (female) total length. Gonado-somatic index was higher for females with a peak in the early rains. The breeding period was between April and August with mean fecundity ranging between 4522 +/- 1924 eggs and 20321 +/- 5543 eggs. This was directly related to total length and weight by the regression models: F = 2365.88 + 560.22 log L and F = 5025.81 +56.34 log W respectively. Exponential equations for length-weight relationships were Wt = -1.997 Lt(2.206) (Zone I), W = -2.831 Lt(3.040) (Zone II) and Wt = -2.245 Lt(2.995) (Zone III). The mean allometric coefficient (b) showed significant departure from cubic value (negative allometry) for Zone I while Zones II and III showed no difference, indicating isometry. Dominant items in the diet were fish and fish parts constituting 21.6% with Food Richness and Gut Repletion Index of 18 and 100% respectively, in all zones. Diet Breadth of 0.802 (Zone 1), 0.922 (Zone II) and 0.910 (Zone III) indicate a high trophic flexibility that enables switching from one diet to another according to availability. Mean condition factor for males varied from 0.718 +/- 0.117 minimum in Zone I to 0.996 +/- 0.062 maximum in Zone III. Forest inland wetlands (Zone II and III) of Cross River provided better condition for C. nigrodigitatus.
银鲶鱼，黑指丝尾鳠（Lacépède：1803年）是一种极具价值的食用鱼，是非洲主要河流中主要商业捕捞的鱼类之一。为了提供有助于管理的生物学数据，于2005年1月至2007年12月期间，每月在尼日利亚克罗斯河沿岸200公里长的三个区域采集样本：I：克罗斯河上游（草原），II：克罗斯河中游（混交林和草原），III：克罗斯河下游（雨林）。对1248个样本的数据进行了如下处理：异速生长系数（b）、性腺-体指数、富尔顿体况系数和多样性指数。在所有种群中均观察到雄性占优势；黑指丝尾鳠雄性全长11.5厘米、雌性全长16.7厘米时达到性成熟。雌性的性腺-体指数较高，在早雨期达到峰值。繁殖期在4月至8月之间，平均繁殖力在4522±1924枚卵至20321±5543枚卵之间。通过回归模型，繁殖力分别与全长和体重直接相关：F = 2365.88 + 560.22 log L和F = 5025.81 +56.34 log W。长度-重量关系的指数方程分别为：Wt = -1.997 Lt(2.206)（区域I），W = -2.831 Lt(3.040)（区域II）和Wt = -2.245 Lt(2.995)（区域III）。区域I的平均异速生长系数（b）显著偏离立方值（负异速生长），而区域II和III没有差异，表明为等速生长。所有区域的食物中，占主导地位的食物是鱼类及鱼的部分，占21.6%，食物丰富度和肠道饱满指数分别为18和100%。区域I的食物宽度为0.802，区域II为0.922，区域III为0.910，这表明其营养灵活性高，能够根据可获得性从一种食物转换为另一种食物。雄性的平均体况系数从区域I的最小值0.718±0.117到区域III的最大值0.996±0.062不等。克罗斯河的森林内陆湿地（区域II和III）为黑指丝尾鳠提供了更好的生存条件。
