ESTIMATION OF GROWTH, MORTALITY, AND EXPLOITATION STATUS OF NURSE TETRA (Brycinus nurse) AND TRUE BIG SCALE TETRA (Brycinus macrolepidotus) (FAMILY: ALESTIDAE) FROM THE NEW CALABAR RIVER, NIGERIA

This study aimed to determine the growth patterns, mortality, and exploitation status of two species of Alestidae in the New Calabar River, Nigeria. For this purpose, fish samples were collected monthly from three landing sites from the local fishermen using gill nets (mesh sizes: 15-25mm), beach seine (mesh sizes: 2.3-10mm), and cast nets (mesh sizes: 15 -25mm). The length-weight relationship revealed exponent “b” value for Brycinus nurse was 3.54 and 3.21 for Brycinus macrolepidotus while the condition factors were 1.08 and 1.02 for Brycinus nurse and Brycinus macrolepidotus respectively. The growth parameters of Brycinus nurse asymptotic length (L  ) and growth coefficient (K) were 24.46 cm and 0.52 yr -1, respectively, while those for Brycinus macrolepidotus L  was 28.88 cm and K was 0.22 yr . The reproductive load (L 50 /L  ) ratio was found to be 0.59 and 0.61 for B. nurse and B. macrolepidotus, respectively. Exploitation rate (E) for B. nurse was 0.26 and 0.11 for B. macrolepidotus while length-at-first capture (L c ) was 14.49 cm for B. nurse and 17.64 cm for B. macrolepidotus. The natural mortality was greater than the fishing mortality for both species and Logistic regression of the probability of capture routine values recorded for B. nurse were higher than that of B. macrolepidotus. Maximum exploitation rate (E max ) was less than 0.5 for both B. nurse (0.41) and B. macrolepidotus (0.42). These values were close to the maximum allowable limit; therefore, the species may be unsustainable when fishery intensifies in the future. To ensure sustainable exploitation of the two Alestid species in the area, fishing effort should be regulated.


INTRODUCTION
Alestidae has been described as the most speciose of all African characiform families (Paugy & Schaefer, 2007;Arroyave & Stiassny, 2011). About 118 valid species from family Alestidae greatly vary in body and fin sizes, shapes, and occupied ecological niches (Froese & Pauly, 2016). According to Froese & Pauly (2017), about 21 species spread across eight genera (Alestes, Alestopetersius, Anorldichthys, Brycinus, Hydrocinus, Bryconaethiops, Micralestes, and Rhabdalestes) have been found in Nigerian waters. Off the eight genera, the genus Brycinus is the most prevalent in Nigerian freshwaters with ten species. The genus Brycinus is characterized by presence of rudimentary adipose eyelids, and by having two rows of pluricuspid teeth on the upper jaw (Paugy, 2003). The exploitation history of Characidae from Nigerian waters, according to the NBS (2015), revealed that the annual catch of the characid species from 2010 to 2015 ranged between 14, 784 to 23,124 metric tons (mt), where 23,124 mt was the highest landing occurred in 2014 then decreased to 18,356 mt in 2015.

Brycinus macrolepidotus (Valenciennes, 1850) and
Brycinus nurse (Rüppell, 1832) are native to freshwater systems in Africa thriving well in both lacustrine and riverine conditions (Boulenger, 2002), particularly in the Niger Delta, Nigeria. Brycinus macrolepidotus is easily identified by red marks on pectoral, ventral, and anal fins, the tip of adipose fins is reddish, the tail fin of B. nurse is bright red, other fins are tinged with red, and there is black patch on the tail peduncle (Adesulu & Sydenham, 2007). These species are said to be of commercial importance due to the fact that they are widely consumed locally and have food value (Reed et al., 1967;Saliu & Fagade, 2004). Like other commercially important fish species in Nigerian inland waters, this fish family is currently subjected to intense fishing pressure due to lack of proper management policy. Since unregulated exploitation is still on there are urgent needs to monitor and assess the status of the stocks that are being fished.
Fish stock change depends on recruitment, natural mortality, individual growth, and harvesting. The growth parameters and the mortality rate are important tools to assess the exploitation level of the pelagic species (Wang & Liu, 2006). Length and weight data of fish can be used for the estimation of the length and age structures growth and mortality rates of the fish (Kohler et al., 1995) and the condition factor that may help determine whether somatic growth is isometric or allometric (Ricker, 1975). Relationship between length and weight is required for setting up yield equation (Beverton & Hold 1957;Ricker 1968).
The biological information on fish is of paramount importance for the development of effective strategies for their management and conservation. Information on population parameters, i.e., growth, reproduction, recruitment, as well as mortality of fish, is vital to the implementation of sustainable management strategies for their better conservation (Hossain et al., 2009). The present study was thus undertaken to estimate the key biological information on two most dominant Alestid species caught in the New Calabar River, Nigeria.

MATERIALS AND METHODS
The Niger Delta is crossed by many distributaries. Off these distributaries is the New Calabar River in the Niger River Delta, Nigeria. It is a partially mixed estuary river lies between latitude 4º252 N and longitude 7º162 E (Olopade et al., 2018). It runs through the most densely populated areas in the hinterland and empties into the Atlantic Ocean at the southern tip of Bonny in the south. The river has high water volume during rainy season due to high runoff. The main rainy season is from April to September with the annual rainfall between 2000 and 3000 mm (Abowei & Hart, 2009). Rainfall is generally negligible in December and February.
Three non-overlapping landing sites were selected along the New Calabar River, namely Choba, Aluu, and Ogbogoro ( Figure 1) to provide a representative overview of the fisheries in the river. The fish samples were also collected monthly from three landing sites from the local fishermen using gill nets (mesh sizes: 15-25 mm), beach seine (mesh sizes: 2.3-10 mm), and cast nets (mesh sizes: 15 -25 mm). The fish species were identified using fish identification keys (Paugy, 2003;Adesulu & Sydenham, 2007). Total length, standard length, and weight of the fish species were measured in centimeters and grams, respectively.

Data Analysis
The length-weight relationship is expressed by the equation W = aL b, where W = Body weight (g), and L = Total length (cm), Ricker (1973) as follows: The value of the growth exponent was used to calculate the condition factor using the formula: (2) Where; K = condition factor 113-122 W = total body weight (g) L = Total length (cm) and b = growth exponent The length-frequency data for each species were collected monthly from the different sampling sites and subsequently grouped into class intervals for analysis. The data was analyzed using FiSAT II (FAO-ICLARM Stock Assessment Tools) as explained in detail by (Gayanilo et al., 2005).
The von Bertalanffy growth parameters (Pauly, 1980), L  , and annual growth coefficient K were computed by ELEFAN I (Electronic Length Frequency Analysis) (Beverton & Holt, 1966). The total mortality rate (Z) was estimated by length-converted catch curve (Pauly, 1984). The natural mortality rate (M) was also calculated by using Pauly's empirical formula (Pauly, 1980). The fishing mortality rate (F) was calculated by the difference between (Z) and (M) or The rate of exploitation (E) was calculated by the quotient between fishing and total mortality (Pauly, 1984): Relative yield per recruit (Y'/R) was estimated using the model of Beverton & Holt (Beverton & Holt, 1966) as modified by Pauly & Soriano (1986) and incorporated in the FiSAT software.
Resource status was evaluated by comparing estimates of the fishing mortality rate with a target (F opt ) and limit (F limit ) biological reference points (BRP) which were defined as F opt = 0.5M and F limit = 2/3M (Patterson, 1992).
Probability of capture against mid-length a resultant curve was used to compute the length at first capture (Lc50). The length at first maturity (Lm50) was estimated as Lm50 = (2 * L  )/ 3 (Hoggarth et al., 2006)

RESULTS AND DISCUSSION Results
A total of 196 individuals Alestid species belong to two species, namely B. macrolepidotus and B. nurse, were analyzed during this study. Brycinus nurse occurred in considerable number (64.3%) more than B. macrolepidotus with 35.7% (Table 1).  Fig. 2 and 3). Also the correlation value (r) for the two species were estimated as 0.8868 and 0.8853, respectively. The mean value for condition factor (K) recorded for B. macrolepidotus was 1.02± 0.03, and B. nurse had the value of 1.08± 0.02 (Table 3).    The growth parameters estimated for B. nurse using the length frequency data in ELEFAN I program revealed the best fit for L  = 24.46 cm and k = 0.52 per year while for B. macrolepidotus, the best fit were for L  =28.88 cm and k = 0.22 per year (Table 4). The total mortality (Z) of B. nurse estimated by the length converted catch curve was 1.88 for B. nurse while it was 0.77 for B. macrolepidotus. The natural mortality (M / year) as per Pauly's empirical formula was found to be 1.05 for B. nurse while it was 0.69 for B. macrolepidotus. The estimated fishing mortality (Z-M=F) stood at 0.83 and 0.77 while the exploitation ratio (E) was found to be 0.26 and 0.11 for B. nurse and B. macrolepidotus, respectively (Table 4).
The logistic regression of the probability of capture routine values recorded for B. nurse was lower than that of B. macrolepidotus (Table 5, Fig 4 and 5). The estimated L 50 was 14.49 cm for B. nurse and 17.64 cm for B. macrolepidotus. The L 25 was calculated as 12.88 cm and 15.13 cm while L 75 was found to be 16.10 cm and 20.16 cm for both species, respectively. In this study, the reproductive load (L 50 /L  ) ratio was found to be 0.59 and 0.61 for B. nurse and B. macrolepidotus, respectively.

Discussion
The calculated 'b' values of the LWRs, which were 3.2103 for B. macrolepidotus and 3.5405 for B. nurse, indicate positive allometric growth. The values fall within the acceptable range of 2.5 and 3.5, which is typical for tropical fish stocks (Carlander, 1969;Froese, 2006). Gopakumar et al. (1991) opined that higher b values imply a relatively productive environment. Positive allometric growth implies that the fish grows faster in weight than in length. A similar observation was also reported in B. nurse (3.0737) from the White Volta River in Ghana (Abobi & Ekau, 2013). Echi & Ezenwaji (2016) reported contrasting findings for B. macrolepidotus from the Anambra River, Nigeria with the males and females showed negative allometric growth (2.309 and 2.734, respectively); in the same vein, the other Alestid species reported in the study, such as Alestes baremoze, B. leuciscus, and Hydrocynus vitttatus, showed negative allometric growth for their respective male and female. The slight difference in the exponent 'b' values recorded could be associated with several factors, such as temperature, salinity, adequate food, seasonal changes, environmental conditions, sampling procedure, and sexual dimorphism. The condition factor "K" of a fish can be defined as a measurement of the general health condition of the fish as calculated by the ratio of body weight to body length. This factor implies that whatever is able to affect the weight of fish will definitely affect the K values. Adesulu & Sydenham (2007) reported that condition factor in fish is mainly affected by the amount of food in stomach and stage of egg development that will affect the weight of the body and then the weight of fish is a function of condition factor in fish. The "K" values of the two Alestid species in this study indicate that the population is in moderate growth condition as "K" values are slightly greater than 1. This factor implies that the fishes thrive well in the habitat (Komolafe & Arawomo, 2011). The condition factor for B. longipinnis was reported to range 1.94-2.80 in Ikomi & Sikoki (2003).
The asymptotic length (L  ) is the largest theoretical mean length that a fish could attain in its natural habitat, assuming the fish grows throughout its life (Abobi & Ekau, 2013); while the growth curvature (K) is the rate at which it grows towards this final size (Etim et al.,1999). In the present study, B. nurse had an asymptotic length (L  ) of 24.46 cm, growth curvature (k) of 0.52 per year, and t 0 (per year) of 0.04 while B. macrolepidotus had L  of 28.88 cm, k of 0.22 per year, and t 0 (per year) of 0.07 (Table 4). These results indicate that both species are fast-growing.
The total mortality (Z), natural mortality (M / year), fishing mortality (F), and exploitation rate (E) of B. nurse were found to be 1.88, 1.05, 0.83 and 0.26, respectively; while for B. macrolepidotus it was 0.77, 0.69, 0.08 and 0.11, respectively. Values of Z, M, and F were higher for B. nurse than for B. macrolepidotus; however, since natural mortality (M) exceeded fishing mortality (F) for both species, the stock is not overexploited. The exploitation rate (E) of B. nurse was 0.26. Based on the assumption that a stock is optimally exploited when F=M or E=0.5 (Gulland, 1971). The results of the present study indicate that the current exploitation rate is close to the maximum level of 0.5. The low value of E in the two species in this finding suggests that levels of exploitation may be increased in order to reduce waste of stock through natural mortality; however, caution must be taken to avoid what can be called a skewed exploitation situation (Chukwu & Deekae, 2010). Ahmad et al. (2018) reported lower values of Z (1.31), M (1.0), F (0.31), and E (0.24) for B. nurse from the Challawa Gorge dam, Kano. Higher values of the fore mentioned parameters were observed for B. nurse by Kwarfo-Ind.Fish.Res.J. Vol. 25 No. 2 December 2019: Apegyah et al. (2009 and Uneke & Nwani (2013), respectively; Z = 2.54 and 3.34; M=1.3 and 1.80; F=1.24 and 1.54; and E=0.49 and 0.46. However, the relatively lower values of E in these studies indicate that the stocks were also not over-exploited.
The result of this study showed the estimated length at first capture (L c ) of both Brycinus species to be 14.49 and 17.64 cm for B. nurse and B. macrolepidotus, respectively. The L c value recorded in this study falls within the range of the total length 7.30 to 23.00 cm for B. nurse and 11.50-27.00 cm for B. macrolepidotus. A lower value of L c (7.58) was recorded for B. nurse by Ahmad et al. (2018). Uneke and Nwani (2013) found the L c for B. nurse in Cross River to be 8.83 while Kwarfo-Apegyah (2008) found the L c for B. nurse in Bontanga Reservoir (Northern Region of Ghana) to be 10.94.
The predicted E max of the Selective Ogive procedure for B. nurse and B. macrolepidotus (0.41 and 0.42, respectively) were higher than their respective current exploitation rates E (0.28 and 0.29) showing that both Brycinus species were not over-exploited. Uneke & Nwani (2013) and Kwarfo-Apegyah et al. (2009) reported the E of B. nurse to be 0.46 and 0.49, respectively; both values are higher than the value of E estimated for the two Alestid species in this study. Values of E max , E 50 , and E for B.nurse in the Challawa Gorge dam, Kano, were reported by Ahmad et al. (2018) as 0.65, 0.2, and 0.24, respectively.

CONCLUSIONS
The present study provides some important baseline information on the growth and population of two commercially important fish species from the family Alestidae that will help in the development of efficient management strategies. The length-at-first capture (L c ) of B. macrolepidotus estimated in this study showed that this species was caught at very small sizes concerning the use of small mesh size nets, such as 2.3 mm and 15 mm. Natural mortality was greater than the fishing mortality for both B. nurse and B. macrolepidotus while exploitation rate (E) was less than 0.5 for both species. Even though the difference between biological reference point and fishing mortality is small, it would be better for the well-being of fishery to introduce some fishery regulation measures.