Mortality and Exploitation Rates of Challawa Gorge Dam Fishes, Kano State, Nigeria

Livelihoods and food security of millions of people across the world are supported by small-scale fisheries that if it is managed appropriately, it can make tremendous contributions to socio-economic development of fishing communities. In Nigeria, fish forms an important part of the diet of many households, especially in rural areas [1,2]. It makes up around 40% of Nigeria’s protein intake, with fish consumption at 13.3 kg/person/per year [3]. Total fish production per year is close to 1 million metric tons (313,231 metric tons from aquaculture and 759,828 metric tons from fisheries). The majority of this fish is consumed domestically, while around 10% is exported [4].


Introduction
Livelihoods and food security of millions of people across the world are supported by small-scale fisheries that if it is managed appropriately, it can make tremendous contributions to socio-economic development of fishing communities. In Nigeria, fish forms an important part of the diet of many households, especially in rural areas [1,2]. It makes up around 40% of Nigeria's protein intake, with fish consumption at 13.3 kg/person/per year [3]. Total fish production per year is close to 1 million metric tons (313,231 metric tons from aquaculture and 759,828 metric tons from fisheries). The majority of this fish is consumed domestically, while around 10% is exported [4].
But inland fish resources, most of the time, seems to be on a steady decrease [5,6]. This can be attributed to overexploitation of the resources, illegal fishing methods [7], minimal conservation commitments and majorly lack of review, compliance or enforcement of fishery management policies and laws [8]. It has been noticed that inland fishery resources are often over-exploited because of the fishing pressure and the fact that bigger aquatic resources -Seas, Oceans, Estuaries, Lagoons etc. -which produce the bulk of fish resources are far from the inland areas, therefore the pressure on the inland freshwaters [9,10].
Fish mortality is a parameter used to infer the loss of fishes through death. Mortality in exploited fisheries is either natural or fishing mortality. Natural mortality is all causes of death that are not as a result of anthropogenic harvest. These include; Diseases, predation, unfavourable environmental conditions (such as pollution), senescence, competition, cannibalism or any other factor(s) that might cause death in fisheries.
On the other hand, fishing (harvest) mortality is the removal, from fish stock, of fishes using any fishing methods [11]. Fish mortality may account for the shrinkage of fish population overtime. Mortality in fishes is highest at the egg/larval stages due to vulnerability to predation, starvation and unfavourable conditions [12] and decreases as the fish progresses in their life history [13].
Many fisheries are being direly over-exploited, including those in countries with well-developed fishery management institutions. Example the Canadian cod fisheries have not recovered from a collapse and closure of the fishery in 1992 [14,15]. Top predator species are continually being over-harvested leading to their noticeable decline in stock [16,17].
According to Chukwu [18] small-scale fisheries in Nigeria are continually negatively affected by pollution and obnoxious fishing practices. But as reported by Bergkamp et al. [19] in a review for the World Commission on Dams (WCD) one pertinent issue that escapes our notice is the fact that pre-impoundment population tend to reduce in biodiversity post-impoundment, and this definitely affects the fisheries yield, while Jackson et al. [20] added that after impoundment, the fish communities become distinct depending on the geography and climate of the basin.
But illegal fishing methods, especially in third world countries, are a major concern to the conservation and yield of the fisheries [7]. In a survey by Ogundiwin [21] on the fishing gears and crafts of Kainji lake, Nigeria she discovered that there is a strong relation between part-time fishing and reduced stock. The reduction in catches is supplemented by; largely farming and other vocational activities.

Mortality rates
Below in Figures 2a-2j, are the length-converted catch curves of the species. The mortality parameters: Total mortality (Z), Natural mortality (M), Fishing mortality (F) and Exploitation rate (E) were estimated from length-converted catch curves by inputting the corresponding L ∞ and K values.

Probabilities of capture
The probabilities of capture of the species were determined from the length-converted catch curves. Length-frequencies of the species were used to extrapolate the length at which they might be vulnerable to fishing gears. It is given in percentages of 25, 50 and 75. They are presented below in Figures 3a-3j.
In Table 3 below, the values of Z, Z/K, M, F, E and Probabilities of capture are presented. Clarias lazera had the highest total mortality followed by Late niloticus. From the Z/K values [31] The Population of B. nurse and P. sidori had growths that dominated their mortality. S. schall and M. senegalensis mortality lightly dominated growth. While the population of Bagrus b. macropterus, Oreochromis niloticus, Clarias lazera, Lates niloticus, Schilbe uranoscopus and Auchenoglanis occidentalis all had mortality that dominated growth greatly (Overexploitation). M. senegalensis had the highest natural mortality while S. uranoscopus had the lowest natural mortality. Lates niloticus was the most fished species in Challawa dam, whereas B. nurse was the least fished species. Lates niloticus was the most exploited fish species followed by C. lazera. B. nurse was the least exploited species followed by P. isidori. The probabilities of capture of the species show that fingerlings of all the species with the exception of M. Senegalensis were caught and retained.
The Z/K values for the species indicate the extent of mortality in relation to growth, for the given species in a year: • Z/K values of <1 indicates Growth dominates Mortality;

Materials and Methods
The study was carried out in Challawa gorge dam, Kano state, Nigeria with coordinates 8 0 06 ' 58.04 '' E 11 0 41 ' 21.95 '' N. The dam was completed in 1993 on the Challawa River. It is the 2 nd largest of the 23 dams along Hadejia-Jama' are River Basin [24]. It is 42 m high and 7.8 km in length. The dam has a full storage capacity of 904,000,000 m 3 . The direct catchment area is 3857 km 2 . Apart from irrigation and township water supply, the dam was constructed with hydropower potential of around 3 MW.

Data collection
Fish samples were collected from three of the seven landing sites of the dam from artisanal fishermen, on a monthly basis. The 3 landing sites were -TURAWA: 11 0 40 I 52.4 II N 008 0 02 I 31.9 II E, SAKARMA: 11 0 39 I 543 II N 8 0 00 I 611 II E and PEGIN MA: 11 0 39 I 28.1 II N 007 0 581 I 18.9 II E. The Total lengths and Weights of the fish samples were measured using a measuring tape and a digital weighing balance (Sarturius T630) to the nearest 0.1 cm and 0.1 g respectively. The study was conducted from March -August, 2017.
The fish were identified to species level and Olaosebikan and Bankole [3]. The sampling sites were chosen based on a reconnaissance survey and responses of the fishermen that the 3 sites have the largest landings by volume. The fishermen use non-motorized canoes and the predominant fishing gears were gill net and hook and line. The data were analysed using the FAO-ICLARM fish stock assessment software, FiSAT_II.

Mortality parameter
Total mortality rates (Z) of each of the species were estimated using Length-converted catch analysis taking 26. Where K is the VBGF growth constant and L ∞ is the asymptotic length. From the length-converted catch curve, Probabilities of Capture (L 25 , L 50 and L 75 ) were estimated using running average. These parameters indicate the length at which 25%, 50% and 75% of the fish population will be vulnerable to a fishing gear.

Exploitation parameters
Employing Beverton and Holt Y/R analysis [25,26], the relative biomass per recruit (B'/R) was estimated as B'/R =(Y'/R)/F [27]. The     • Z/K value of 1 indicates population is in equilibrium (Where mortality balances growth), • Z/K values of >1 indicate mortality domination; and • Z/K value of ≥2 indicates the fish population is highly exploited [31]. These values are relative to the population size, fecundity and the exploitation rate for a given species.
The growths of B. nurse and P. isidori dominated their mortalities. The mortalities of S. schall and M. senegalensis dominated their growths. While, Bagrus b. macropterus, O. niloticus, C. lazera, L. niloticus, S. uranoscopus and A. occidentalis were highly exploited more than their growth could replace their biomass.

Exploitation status of Challawa fishery
Using the FiSAT_II software, the exploitation status of the individual species studied from Challawa dam were estimated. The required inputs for this function are: M/K, L c /L ∞ , "a" and "b" values (from length-weight relationships). Below are the outputs (graphs) generated for each of the species in Figures 4a-4j. L c /L ∞ of less than 0.5 indicates that small-sized fishes are exploited. M/K and L c /L ∞ were used as inputs for the Beverton and Holt exploitation analysis. The Selection ogive was preferred because it was more practical to the fishery of the area under study. Because fishes caught below the L c (50) are retained and never returned to the water. When E current =0.5, that is the optimum level of exploitation. Therefore when F=M, then E=0.5.
From Table 4 below, L c /L ∞ of less than 0.5 indicates that small-sized fishes are exploited. M/K and L c /L ∞ were used as inputs for the Beverton and Holt exploitation analysis. The Selection ogive was preferred because it was more practical to the fishery of the area under study. Because fishes caught below the L c (50) are retained and never returned to the water. When E current =0.5, that is the optimum level of exploitation. Therefore when F=M, then E=0.5.

Bagrus b. macropterus: Bagrus b. macropterus population in
Challawa dam are overly exploited. The Maximum Sustainable Yield (MSY) was 0.398, but the current exploitation rate was 0.64. The Maximum Economic Yield (MEY) which is the value that will ensure maximum economic return to the fishermen was (E 10 ) 0.320. It was slightly below the MSY which shows that the Bagrus population is highly sought after by the fishermen, but its exploitation is highly unsustainable. Hence the probable reason for its rapid decline. At current exploitation rate the Y/R and B/R stand at 0.011 and 0.049, respectively, which is far lower for replenishing depleting stocks, considering the exploitation rate (Figure 4a). Where; Z=Total instantaneous mortality, M=Natural mortality, F=Fishing mortality, E=Exploitation rate L 25 =Length at which 25% of the population will be vulnerable to fishing gear. L 50 (Length at first capture)=Length at which 50% of the population will be vulnerable to fishing gear.
L 75 =Length at which 75% of the population will be vulnerable to fishing gear. Where; E c =Current Exploitation rate; E max =Exploitation rate that will produce maximum sustainable yield; E 10 =Exploitation rate at which the marginal increase in Y/R is 10% of its virgin population.   Schilbe uranoscopus: Schilbe uranoscopus population is overexploited. Its exploitation rate should be decreased by about 19% because further fishing pressure to the juvenile stock will decrease the E 10 (Maximum Economic Yield) by about 10%. The Y/R and B/R were found to be about 0.57 and 0.26. These indicate that recruitment into the fishery per outweighed fishing mortality (Figure 4i).

Auchenoglanis occidentalis:
Auchenoglanis occidentalis population is overexploited by about 17%. The exploitation rate of 0.67 shows that it was beyond the MSY=0.54 and the E opt =0.5. As with other species, fingerlings and juvenile overfishing was very high considering the length at first catch and length at first recruitment (L 50 )=11.50 cm ( Table  4). The Optimum exploitation level, E 50 =0.30 for this specie indicated high fishing pressure. The high instantaneous fishing mortality rate of 1.49 out of 2.22 suggests that it is one of the species that command considerable market value (Figure 4j).

Bagrus b. macropterus
The ideal length at first maturity for Bagrus is 17.1 cm (Fishbase. org). The Challawa species' L c =9.30 cm. This showed that there is growth and recruitment overfishing, characterised by small-sized fishes in the landings. From the study ( Table 4) the value of L c /L ∞ was 0.12, which was lower than 0.5 [31]. This affirms the presence of many smallsized Bagrus spp. in the landing, indicating that small mesh sizes were employed.
The estimated Z/K ratio (Table 3) for Bagrus this study was greater than 1, which depicts mortality domination for the targeted species. A Z/K value of ≤ 2 shows that the species are lightly exploited. From Table 4, the E current for this species was estimated at 0.64, which was above the E opt =0.5. Also, the E max (Exploitation level that maximises Y/R or B/R) was relatively lower at 0.40 (Table 4). In comparison to a similar study conducted in Lake Akata, Benue state, Ikongbeh et al. [32] reported lower total mortality of 1.43 compared to 1.56, lower fishing mortality of 0.381 compared to 1.00, lower exploitation level of 0.266 compared to 0.64. Also, Akombo et al. [33] found out that the population parameters C. auratus (Bagridae) of River Nile, Egypt were Z=1.44, M=0.60, F=0.84, E=0.58, Y/R=0.0158.

Synodontis schall
The length at first capture 10.08 cm (Table 3) was far below the length at first maturity -21.00 cm (fishbase.org). This indicates serious growth overfishing that does not spare fingerlings. The L c /L ∞ of 0.32 < 0.5, confirms the fact that smaller sized fishes dominated the catches.

Oreochromis niloticus
The estimated length at first capture (12.24 cm). The Z/K ratio (2.36) indicates that Tilapia are mortality dominated, but the E value (0.51) in Table 4, shows that the fish stock are replenished enough to counter the exploitation, and adjusted to optimal level. Abdul et al. [35] studied the Tilapia of Ogun estuary and they got; Z=3.93, M=1.46, F=2.47, E=0.63 and L r(50) =10.23 cm. The asymptotic length and growth rates were higher than this study, but the length at first maturity in this study (12.24 cm) was higher than the Ogun estuary which means Tilapia in the Estuary are recruited earlier than in Challawa dam. The L r for Lake Timsah, Egypt [36] was even lower at 8.4 cm. But these lower values might be due to differences in fishing gears and mesh sizes employed in the localities [7,26,37]. In a WorldFish Center report [38], on lake Nasser Egypt in 1972 and 1992 showed; L ∞ =50. 39

Clarias lazera
The estimated L c (13.76 cm) was found to be lower than the L r which was 30.86 cm. This indicates recruitment overfishing. The Z/K and E values of 2.67 and 0.77, respectively show that Clarias are highly exploited in this fishery. In a report on Lake Kainji [39]

Conclusion
Small-sized fishes are highly exploited in the dam for all the species with the exception of Marcusenius senegalensis probably because their adults are 12-15 cm. As with similar findings, species deemed to be of commercial value were over-exploited and species of lower demand were mostly under-exploited.

Funding
This work was supported by Abubakar Tafawa Balewa University, Bauchi state, Nigeria, which provided a meager allowance that helped to offset some of the financial burdens of this research.