Population dynamics of Cynoglossus senegalensis from the coastal waters of Greater Accra, Ghana

1. Introduction

Flat fishes are highly commercial and traded in fresh, smoked, or dried form [1]. Cynoglossus senegalensis, a flat fish also known as the Senegalese left-eyed tongue sole is an economically important fish species in Ghana, ranked among the country’s fish exports [2]. Belonging to the family Cynoglossidae, it possesses a long hook on the snout overhanging the mouth, without pectoral and pelvic fins with its eyes located on the left side of the body [1, 3]. They are found in the tropical subtropical oceans, mainly in shallow waters and estuaries, distributed mainly in the Eastern Atlantic from Mauritania to Angola [1]. C. senegalensis inhabits sandy and muddy bottoms of coastal waters with a depth range of 10–110 m and feeds on mollusks, shrimps, crabs, fish, and seaweeds, particularly the green seaweed “sea lettuce” (Ulva) [3, 4]. High catches of this species occur from June to September or July to September using tengiraf nets or soluya and tengirafiibibii nets, which are left in the sea for a week and visited daily [4]. According to Kwei and Ofori-Adu [2], C. senegalensis is a typical bottom-dweller, mostly caught by bottom trawlers and beach seiners. In terms of food security, this species is highly patronized by fisherfolks as protein source, chiefly sold in the fresh form [4]. Regardless of its importance to the economic and food security of fishing households, few studies on its abundance, condition factor, and length–weight relationship and population parameters exist (e.g., [1, 5]). Given this, the objective of this study was to assess some population parameters of the species from the coast of Ghana. Such information will contribute to the sustainable management of the species in the country and the region at large, thus ensuring food security and economic wellbeing of dependent households.

2. Materials and methods

2.3. Population parameters

Based on the von Bertalanffy growth function (VBGF), growth parameters, including growth rate (K) and asymptotic length (L_∞), were calculated with the aid of the electronic length frequency analysis (ELEFAN) programming found in the TropFishR package.

The longevity (T_max) of the species was estimated based on the method:

$T_{\max }=\frac{3}{K}$

(Anato [8])

The growth performance index was determined using the formula:

$\Phi \prime =2\log L_{\infty }+\log K$

(Pauly and Munro [9])

The theoretical age at length zero (t₀) was obtained with the aid of the equation:

${\mathrm{Log}}_{10}\left(-t_0\right)=-0.3922–0.2752{\log }_{10}{L}_{\infty }–1.038{\log }_{10}K$

(Pauly and David [10])

A linearized length-converted catch curve was applied in estimating the total mortality (Z) rate [11].

The natural mortality rate (M) was calculated using the equation:

$M=4.118K^{0.73}L{\infty }^{-0.333}$

(Then et al. [12])

Figure 1

A map of the study area showing sampling areas.

Figure 2

A picture of Cynoglossus senegalensis.

Fishing mortality (F) was calculated as:

$Z-M$

(Qamar et al. [13])

The exploitation rate (E) was calculated using:

$\frac{F}{Z}$

(Georgiev and Kolarov [14])

The length of capture at 25%, 50%, and 75% was obtained from the ascending data points of the selectivity curve that was obtained using linear regression [15]. The length at first capture was taken as Lc₅₀.

Virtual population analysis (VPA) assumes that the fish are caught continuously, which has to be solved by the trial-and-error method [16]. The inputs for length-based VPA estimation include terminal exploitation rate, the length–weight relation constants (a and b), mortality parameters, and the catches. The relative yield-per-recruit (𝑌/𝑅) was calculated using the knife-edge method [17].

3. Results

Figure 3 shows a reconstructed length distribution overlaid with growth curves for C. senegalensis. The asymptotic length (L_∞), growth rate (K), and growth performance index (Φ′) were 57.2 cm, 0.40 per year, and 3.115. The longevity (T_max) and age at zero-length (t_o) were eight years and 0.37, respectively.

From Figure 4a, total mortality rate (Z) was 0.81 ± 0.23 yr⁻¹. Natural and fishing mortality rates were 0.56 and 0.26 yr⁻¹, respectively. The current exploitation rate (E) was obtained at 0.31.

From Figure 4b, the age at probability of capture (t₅₀) was 0.95 years (Figure 4). In addition, the age at 75% and 95% probability of capture was t₇₅ = 1.29 years and t₉₅ = 1.87 years, respectively. The corresponding lengths at probability of capture (Lc) were Lc₅₀ = 18.1 cm, Lc₇₅ = 23.1 cm, and Lc₉₅ = 30.1 cm.

Figure 5 shows the VPA of C. senegalensis, where the number of survivors decreased serially with increase in the length of the individuals. Natural mortality was the most contributor to the decline of individuals with the highest impact on individuals with length size 13 cm. Similarly, fishing rate was highest (F = 0.57 yr⁻¹) for individuals within the length size of 43 cm (Figure 5).

The biological reference points as shown in Figure 6 include fishing mortality rate at maximum sustainable yield (F_msy) = 1.0 yr⁻¹, fishing mortality rate at 0.5 yield (F_0.5) = 0.5 yr⁻¹, exploitation rate at MSY (E_msy) = 0.64, and exploitation rate at 0.5 (E_0.5) = 0.47.

Figure 3

Reconstructed length–frequency distribution with growth curves.

Figure 4

(a) Linearized length-converted catch curve (b) Probability capture of C. senegalensis, Greater Accra, Ghana.

Figure 5

Virtual population analysis of C. senegalensis.

Figure 6

The relative yield plot of C. senegalensis with the yellow line = E_max; red line = E₅₀.

4. Discussion

The growth constant of the fish in the present study (K = 0.4 yr⁻¹) was markedly higher than estimates from Ameworwor [5] who sampled from the coast of Central region, Ghana (K = 0.18 yr^-1). However, the maximum theoretical length (L_∞ = 57.2 cm TL) recorded for the species from the study was lower than that reported by Ameworwor [5] who documented L_∞ to be 60.38 cm. This might suggest that the species from the coast of Greater Accra, Ghana, is exposed to harsh conditions than individuals available in the central waters of Ghana. Growth performance index of the Ghana stock (φ′ = 3.115) was also higher than the populations in the coastal waters in the Central region of Ghana (φ′ = 2.82). This may suggest conducive environmental conditions, despite harsh anthropogenic conditions. The estimated growth rate was lower than 0.5 which according to Kienzle [19] implies that this species is slow growing.

The length at first capture from the study was lower than stock from the Central region of Ghana by Ameworwor [5] who reported Lc₅₀ to be 40.6 cm. This might suggest that the Greater Accra stock are exposed to higher fishing pressure than their counterpart, which is the Central region stock. Furthermore, the high fishing pressure could also suggest that this species is highly patronized by consumers in the Greater Accra region than the Central region. Nonetheless, the critical size at capture, which is the ratio of the length at first capture to the asymptotic length, was 0.31. This was markedly lower than the optimum level of 0.5, implying that the stock is vulnerable to growth overfishing, as individuals are harvested way before reaching the maximum size [20]. Comparing the Lc₅₀ the estimated length at first maturity as reported in [5] is Lm₅₀ = 40.6 cm. The estimated length at first capture was lower than the length at first maturity, signifying that the individuals are harvested before reaching the matured stage. Consequently, this situation could have severe repercussions on the recruitment potential of the stock, leading to possible collapse of the fishery if necessary measures are not put in place.

The VPA results show that individuals within the length classes of 38–43 cm are highly vulnerable to intense fishing pressure, which might be due to the high economic value of large-sized individuals. However, it is worthy to note that excessive fishing of large-sized individuals may result in the poor recruitment potential of stock as there will not be enough matured individuals to spawn and recruit juveniles into stock. Hence, there is the need to reduce the fishing pressure on the large-sized individuals.

Fishing mortality coefficient of the Greater Accra stock (F = 0.26 yr⁻¹) was higher than that of the populations in the Central region (0.14 yr⁻¹, [5]) which might explain the higher exploitation ratio (E = 0.31) of the Greater Accra population compared to the Central region stock (E = 0.26). Nonetheless, the current exploitation rate was lower than the optimum level of exploitation (E = 0.5), which implies that this species in underexploited in the coast of Ghana. This finding conforms to studies by Ameworwor [5] who also reported that the Central region stock are also underexploited. To buttress this assertion, the current exploitation rate (E) was markedly lower than the exploitation rate at the maximum sustainable yield (E_msy). This also suggests that the species in the coast of Ghana is far from being overfished. Nonetheless, there is the need to monitor the fishing pressure over time.

5. Conclusion

The study aimed at assessing the population status of C. senegalensis from the coast of Ghana. From the study, the species is a slow-growing type with a growth rate of 0.40 per year. The critical length at capture indicated the presence of more juveniles than adults, which suggests that the species could be vulnerable to growth overfishing in the future, amidst high fishing pressure. Nonetheless, the exploitation rate showed that the species is underexploited. Though the species is underexploited, it is recommended that fishing effort should be monitored periodically to avoid possible collapse of the fishery as well as improvement in the data collection.