Shark Fishing in Ghana: What We Ought to Know

Abstract

The main objective of the study was to assess the abundance and distribution of sharks in Ghana’s coastal area. Samples were collected daily from the three sampling sites from April – to December 2021. The data obtained from this study were assessed for species abundance and composition, ecological indices; species diversity, and exploitation status using PAST and Microsoft Excel. A total of eight species were recorded with Prionace glauca (68%) as the dominant species and Carcharhinus leucas (1%) as the least dominant species. The mean species richness index (SRI) of 0.9 indicated infinite diversity of the species. The mean species evenness index (SEI) of 0.5 showed an evenly distributed species. The mean species dominance index (SDI) of 0.4 implied that the habitat was not dominated by only one or two particular species. The mean Shannon Weiner index (SWI) was less than 3, which is suggestive of pollution and habitat degradation. The exploitation status indicated that the stock of the shark species is in healthy condition. The month with the higher index was October which is possibly the aftermath of the close season and the major upwelling season. Extending the period of the closed season and reducing fishing effort are recommended.

1. Introduction

Sharks are a small, evolutionarily conservative group, comprising approximately more than 1200 species that have functioned successfully in diverse ecosystems for 400 million years [1]. Sharks perform numerous essential functions, both ecologically and economically. For instance, ecologically, shark species can act as both apex predators thereby securing the food web both directly by regulating prey dynamics through predation and indirectly by modifying prey behaviour and function as well as macro predators in line with a diverse group of reefs fish [2]. Although shark species are evolutionary successful, yet still some sharks are threatened with extinction because of human activity, climate change, pollution, and shark fin marketing as about 73 million sharks are killed every year according to an analysis of the Hong Kong shark fin trade [3]. Economically, shark fisheries sustain a substantial number of coastal communities’ livelihoods [4]. Shark population declines can have unforeseeable implications, such as the collapse of significant fisheries. Many shark species have experienced significant population reductions as a result of the harmful effects of both target and non-target shark fishing [5]. Because sharks are especially sensitive to overfishing, there is evidence suggesting that some populations of large sharks have declined regionally by 90% or more [6]. Also, according to global reports, the shark population has declined by 70–80 percent [3]. Worldwide, shark populations are in grave peril. Despite questions over the sustainability of shark fisheries globally, information on world shark catches is often inadequate and regionally incomplete [7]. Consequently, few information is known on and landings of elasmobranchs, and species-level data are almost non-existent until recently, little attention has been afforded to the management of elasmobranch resources either by fisheries scientists and managers or by conservationists [8]. As a result of this, the status and trend of sharks globally are not appealing, and thus difficult to obtain the exact trend of the shark population globally. Apparently, over the last 60 years, shark catches by industrial, artisanal, and sport fisheries have increased around the world and sharks are now among the most threatened marine animals [9].

In Ghana, the shark fishery first developed around 1974 [10]. Shark fishing in West Africa has been undertaken as a commercial activity since the beginning of the 19th century, developed as a result of the growing demand for shark oil for lighting purposes [11]. Many fishers and traders generated between 80 and 100% of their income from shark fisheries [3]. In Ghana, much attention has been shifted to Shark fishing since their demand in the world is higher and hence valuable. One major threat to shark fisheries in Ghana is the unregulated of species mainly because they are caught as bycatch, and the meat is mostly used as bait for higher commercial species such as tuna, anchovies, and mackerels. Since the late 1950s, shark landings have been increasingly erratic in Ghana, peaking at 11,478 tons, in the last decade, the total reported shark catches fluctuated considerably. The catch peaked at 10,000 tons in 2013 and dropped to 8152 tons in 2015. In Ghana, shark fishing activities are not regulated mainly because the species are caught as bycatch, and the meat is mostly used as bait for higher commercial species such as tuna, anchovies, and mackerels, as today, shark products can reach expensive prices in Ghanaian markets [3, 12]. In Ghana, studies on sharks’ distribution, abundance and species composition include the study by on ‘Fishing for survival in the Western region of Ghana, [12] studies on ‘Detection of illegal, unreported, and unregulated (IUU) fishing in sharks using barcoding in the Greater Region of Ghana. Another study was carried out by [3] on’ Species composition, seasonality, and biological characteristics of Western Ghana’s elasmobranch fishery. These studies however did not assess the maturity composition and the exploitation status of shark species. This study aimed to assess the diversity of shark species, estimate the diversity indices of shark species, determine the maturity composition of shark species, and determine the exploitation status of shark species on the coast of Ghana which could be used in aiding the sustainable management of fishery resources.

2. Materials and methods

2.1 Study area

The study was conducted in three coastal communities in the Western Region namely Axim and, Dixcove and Greater Accra Region namely Tema in Ghana (Figure 1). These three communities are the hotspots of shark fisheries in Ghana. Dixcove is located in Ghana’s Western Region (N 04.79368°, W 01.94612°), Axim is located in the Nzema East district (N 04.8665° N, N 04.2409° W) and the Tema fishing community is situated at the Tema Newtown within the Tema Metropolitan Assembly.

Figure 1.

Map showing the study sites.

2.2 Data collection

Samples were collected daily from three (3) sampling sites, i.e., from April to June and from August to December 2021. Majority of the fishers in these study areas use drift gill nets (DGN) in fishing for shark’s species. However, in July there was no sampling due to the one (1) month banned on fishing activities in Ghana. On the field, the fish catch was identified to species level according to [13]. The length of fish samples was measured using a tape measure. Mostly the colour and the local names (mostly asked by the local fishermen) of sharks are noted down.

2.3 Species relative abundance

Relative species abundance is how rare or common a species is relative to other species in a defined location. This was expressed in percentage, using the expression below [14]:

$$\frac{\text{Number of individuals of species}}{\text{Total number of individuals}}\ast 100.$$E1

2.4 Diversity indices

2.4.1 Shannon–Weiner index

Shannon Weiner’s diversity index according to [15] considers both the number of species and the distribution of individuals among species. This index was calculated using the formula [16]:

$$H=-\sum \left[\left(\frac{m}{N}\right)\times ln\left(\frac{\mathit{ni}}{N}\right)\right]$$E2

where ni is the number of individuals in species i and N is the total number of individuals in the community.

2.4.2 Species richness

This is the number of different species represented in an ecological community. Margalef index (d) was to measure the species richness by using the formula [17];

$$d=\frac{\left(S-1\right)}{lnN}$$E3

Where S is the number of different species represented in the sample and N is the total number of individual organisms in your sample.

2.4.3 Species dominance

Simpson diversity index is the measure of diversity that takes into account the number of species present as well as the relative abundance of each species. This index was estimated using the formula [18]:

$$D=\sum n/N^2$$E4

The value of D ranges between 0 and 1. With this index, 1 represents infinite diversity and 0, no diversity.

2.4.4 Species evenness

This index refers to how close in number each species in an environment is. Pielou’s evenness index was used to calculate the evenness of the fish species in the sample. This was estimated using the formula [19]:

$$J=\frac{H^I}{H^I\mathit{max}}$$E5

where H′ is the number derived from the Shannon diversity index and H′_max is the maximum possible value of H′ (if every species was equally likely).

2.5 Exploitation rates

A rapid evaluation of the exploitation status of the most frequently landed species in the artisanal fishery was performed from a simple length-frequency framework developed by [20] for data-deficient fisheries and provides a first approximation of population parameters in these fisheries. The length-frequency framework uses empirical relationships between the asymptotic length (L_∞, cm), the mean length at first maturity (L_m, cm), and the length corresponding to the mean age in years at maximum possible yield per recruit, known as the optimum length (L_opt, cm). The following empirical relationships from [20] were used to estimate L_∞, L_m, and L_opt:

Asymptotic length (L_∞) was estimated from the maximum observed length (L_max) using the equation [20]:

$$log\mathrm{L}\infty =0.044+0.9841log{\mathrm{L}}_{\max }.$$E6

Length at first maturity (L_m) was estimated from L_∞, as follows [20]:

$$log\mathrm{Lm}=0.8979log\mathrm{L}\infty –0.0782$$E7

where the standard error (SE) provides a measure of variability around the regression coefficient.

Length at maximum possible yield per recruit (L_opt) was estimated from L_m for unsexed fish, as follows [20]:

$$log\text{Lopt}=1.053log\mathrm{Lm}–0.0565.$$E8

The derived growth parameters (L_∞, L_m, and L_opt) were then indicated on the length-frequency distributions of the species to evaluate the exploitation status and sustainability of sharks caught in the artisanal fishery.

2.6 Data analysis

Descriptive statistics such as the mean, median and range were estimated using the length frequency distribution, and species diversity indices data. Frequency statistics were applied in showing the number of species obtained in each sampling area with other species. The statistical packages used for the study were Microsoft Excel and Palaeontological statistics software (PAST) Version 4. The Microsoft Excel Tool was used in estimating the descriptive statistic of the recorded length data of the species which involved the mean, median and range. The species diversity indices were done using the PAST V4.0 software.

3. Results

3.1 Species composition

Overall, eight (8) sharks’ species were recorded during the study period (April 2021 to Dec 2021) as shown in Figure 2. They were Carcharhinus leucas, Carcharias taurus, Isurus oxyrinchus, Prionace glauca, Sphyrna lewini, Carcharhinus brevipinna, Alopias supercilliosus, and Rhizoprionodon acutus. The highest number of shark species were recorded in August and October (i.e. 7 species) and the least number of shark species were recorded in April, June and December (i.e. 4 species). The month of July was a close season so therefore there were no sharks landing (Table 1).

Figure 2.

Species abundance and composition from the study.

Species	April	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Carcharhinus leucas	√	√	—	—	√	—	√	—	—
Carcharias taurus	√	√	—	—	—	—	√	√	—
Isurus oxyrinchus	—	√	√	—	√	√	√	√	—
Prionace glauca	√	√	√	—	√	√	√	√	√
Sphyrna lewini	√	√	√	—	√	√	√	√	√
Carcharhinus brevipinna	—	—	√	—	√	√	√	—	√
Alopias supercilliosus	—	—	—	—	√	√	—	—	—
Rhizoprionodon acutus	—	—	—	—	√	√	√	√	√
Total	4	5	4	—	7	6	7	5	4

Table 1.

Temporal composition of shark species from the study.

NB: ‘√’ means Present ‘-’ means absent.

3.2 Species abundance

Figure 2 shows the overall relative abundance of species recorded from shark species landed in all the three selected communities from April 2021 to December 2021. From the Eight (8) species of sharks were observed with Prionace glauca as the most dominant species (68%) followed by Rhizoprionodon acutus (14%) as the second dominant and both Carcharhinus leucas (1%) and Alopias supercilliosus (2%) as the fewer dominate species.

3.3 Maturity composition

The composition of adults and juveniles’ shark’s species are showed in Figure 3 where juveniles formed the lowest composition of all the species recorded except for Carcharias taurus. Adult individuals dominated the shark species with A. supercilious recording only adult individuals.

Figure 3.

Composition of adult and juvenile shark species landed during the study period.

Overall, 92% of sharks landed were adults whilst only 8% were in their juvenile stage (Figure 4).

Figure 4.

Maturity composition of shark species landed during the study period.

Analysis of the maturity composition of the sharks landed in the studies area shows that August was the month of the most landed shark period with the highest number of adults and juveniles followed by October. A few adults’ sharks were landed in April while juvenile sharks are the least landed in November (Figure 5).

Figure 5.

Abundance of juveniles and adult shark species obtained during the sampling period.

3.4 Diversity indices

Figure 6 shows the diversity indices for the species obtained during the sampling period. The Species diversity index (D) ranged from 0.3 to 0.7 with a mean of 0.4. The minimum ‘D’ was recorded in June and December (2021) while the highest ‘D’ was recorded in October (2021). The Shannon-Wiener index (H) ranged from 0.5 to 1.4 with a mean of 0.9. The minimum ‘H’ was in June (2021) while the highest ‘H’ was observed in October (2021). The Species Evenness index (J) ranged from 0.4 to 0.7 with a mean of 0.5. The minimum ‘J’ was recorded in May, June, August, September, November, and December (2021) while the highest ‘J’ was recorded in April (2021). The Species Richness index (d) ranged from 0.6 to 1.1 with a mean of 0.9. The minimum ‘d’ was recorded in June (2021) while the highest ‘d’ was recorded in May and October (2021). The diversity indices recorded during the study did not show significant difference over sampling period (ONEWAY ANOVA, df = 31, f-value = 1.55, p-value = 0.199).

Figure 6.

Diversity indices during the sampling period.

3.5 Exploitation status of dominant shark species

Figure 7 shows the exploitation status of dominant shark species in terms of length-based measurement. The dominant species from the study was Prionace glauca which recorded the highest L_max, L_opt, L_inf and L_m while Carcharhinus leucas recorded the lowest L_max, L_opt, L_inf and L_m (Figure 7).

Figure 7.

Parameters for exploitation status of species from the study.

4. Discussion

4.1 Species abundance

From the study conducted, eight (8) shark species were identified at the three sampling locations in the Western region (Shama and Axim) and Greater Accra region (Tema).

The number of species identified in the current study was in variance with other studies. Previous studies by [3, 12, 21] were similar to that reported in other works. Based on the aforementioned studies findings from the current study were similar to that reported in other works. The reasons for the variation in findings in comparison to other studies could be a result of the following factors, environmental factors, time of sampling, sampling duration, depth and type of fishing gear, biological activities of fish species, geographical location, the possibility of tear of fishing gears and the intensity of fishing activities [22]. Concerning the environmental factors, [23] stated that human impact, however, has become a driving force in shaping the spatiotemporal patterns of species abundance and distribution through direct and indirect effects of fishing exploitation, climate change. These impacts have promoted the immigration and expansion of thermophilic taxa, habitat destruction, and pollution which have affected the populations of cartilaginous fish. Multiple species that are now extremely rare or no longer present in the different study areas were a result of prolonged and intense fishing exploitation in the region. Similarly, [24] mentioned that marine fish abundance and distributions are dependent upon a variety of biotic and abiotic factors and may change temporally and/or spatially. According to [25], changes in species assemblages are partly reliant on changes in environmental conditions. For instance, the tide has been suggested as an important factor in influencing the seasonal abundance of fish in the Gulf of Mexico [25]. It is worthy to note that the distribution of sharks responds to seasonal changes, light levels, food availability, predator avoidance, various water quality parameters, and reproductive purposes [24].

Constantly varying the population of the fish assemblages at the shore zone as a result of the changes in the geology of the shore zone is another difference in species distribution variation. Differences in sampling technique, length, and mesh size of gears used are known to affect the abundance of species encountered [26]. Nunoo et al. [25] reported that the duration of sampling influences the abundance of species caught. For instance, [21] over a period of eleven (11) months recorded twenty-three (23) species of sharks from both the artisanal fisheries and trawlers compared to the current studies, which buttress the ascertain that sampling period affects the abundance of species.

In terms of numerical abundance of species the shark (Prionace glauca) was the most dominant species within the studied area. Similarly [12] stated that out of the seven (7) species identified, P. glauca accounted for the majority of the catch which conforms to the findings from the current study. Depending on the fisheries, areas, and seasons, the blue shark catches can be very significant in the overall catch and some specific cases can account for more than 50% of the total fish catch and around 85–90% of the total elasmobranch catch [27]. The high abundance of blue shark from the current study suggests that the environmental conditions of the marine waters of Ghana are conducive to its sustenance. However, to ensure the sustainability of this dominant shark species in the marine waters of Ghana from future collapse, there is a need for an assessment of its status in the marine waters of Ghana to be conducted. Findings from such studies will help in ensuring proper management measures are drafted and implemented for the sustenance of this species.

4.2 Maturity composition

In Ghana, information on the maturity stages of shark species is lacking, therefore, findings from this study will serve as a baseline for the management of shark fishing in Ghana. According to [28], for many species the percentages of matured specimens in the catch were inversely related to the maximum size. The maturity stage of the sharks recorded from the current study shows that 92% of sharks landed were adults whilst only 8% were in their juvenile stage. The findings were in agreement with a study done by [3] who revealed that 63–76% of the shark species were in the matured stage. However, the finding from the present study was in variance with studies by [21] who reported that about 90% of the species landed were within the juvenile stage. Also, [29] documented that the majority of the shark species captured were juveniles. The reasons for the observed variation in the composition of matured and juvenile shark species landed by artisanal fishermen could be attributed to factors such as seasons, fishing gears and study area. For instance, [29] stated that the highest catches are mostly observed during a reproductive aggregation period from April to June on the Campeche coast, with the adults of both sexes being the most commonly landed specimens.

Consequently, harvesting more adults than juveniles could reduce the recruitment potential of shark species into the stock leading to severe biological and economic repercussions on both the ecosystem and dependent fishing households. This is because elasmobranchs typically have a relatively slow life history due to their large body size, late maturity, slow growth, and low fecundity, which results in low population growth rates. These traits make them exceptionally vulnerable to overfishing and typically result in decreased chances of recovery from population decline and can also lead to stock collapse. The results obtained from the current study could also show that fishers apply destructive fishing methods just to catch more of the adults in order to meet their maximum profits for the fishery, signifying the over-reliance on shark for survival with limited options for alternative livelihoods. Hence, there is a need for the development of proper management measures geared toward a sustainable shark population in the marine waters of Ghana.

The monthly variation for the maturity composition could be alluded to a plethora of factors such as fishermen targeting the matured specimens for economic benefits and the aftermath effect of the upwelling period. Targeting mostly matured individuals by fishermen could reduce the number of species to be recruited into the shark fishery, leading to possible collapse in the future, especially in the absence of proper management measure.

4.3 Diversity indices

Estimation of diversity indices in fishery studies can be useful as changes can be detected in the structure of commercially exploited populations. The Shannon Weiner Index (H) for dominance species range obtained from the current study ranged from 0.5 to 1.4 with a mean of 0.9. The mean of the H for dominance species reported by [24, 30] was 3.81 and 1.1 respectively. The values obtained from the current study were lower than the above-mentioned researchers’ values. According to [16], the H value above 3 indicates that the structure of the habitat is stable and balanced and the values below 3 indicate that there are pollution and degradation of habitat structure. From the study, the H was less than 3.0 which indicates possible pollution with the existence of some level of habitat degradation. This could be characterised by the higher number of fishing vessels and thus causing a higher level of fishing and overexploitation.

Species Eveness Index is an important component of diversity indices and expresses the uniform distribution of individuals among different species [31]. The ‘J’ ranged from 0.4 to 0.7 and with a mean of 0.5 from the current studies. Comparably, the ‘J’ recorded by [32] in the Mexico waters for Atlantic Sharpnose Sharks was 0.54 as the mean and ranged from 0.34 to 0.69, and that for Bull Sharks was 0.52 as the and ranged from 0.20 to 0.83. The values obtained from the current study was in variance to values reported by the aforementioned researchers. ‘J’ values close to one (1) indicate an even distribution of a species within an ecosystem, while values closer to zero (0) indicate a site preference [32]. From the current studies, the ‘J’ was closer to zero (0) indicating site preference for the species. The blue shark was the dominant species (68%) in the study area, which is an indication of site preference for this species.

The Species diversity index (D) refers to its relative importance in its habitat which determines the degree of influence of the species on the ecosystem [22]. The ‘D’ obtained from the current study ranged from 0.3 to 0.7. and with a mean of 0.4. The mean of the ‘D’ reported by [33] was 0.04 which was lower than the mean of the ‘D’ obtained from the current study. Habitats with more interference, tend to have high ‘D’ (> 0.6), consisting of only one or a few species, and relatively large populations [22]. In this survey, the mean of the ‘D’ from the current study was 0.4 indicating a relatively stable habitat with less interference hence the composition of dominant species is relatively balanced and not dominated by only one (1) or two (2) particular species. According to [34], Species richness is a diversity of order 0 (which means it is completely insensitive to species abundances); the higher the value, the greater the diversity. The mean of ‘d’ from the current study (0.9) was closer to one (1) representing an infinite diversity.

The reason for the monthly variation among the various diversity composition could be both the major and minor upwelling season and the aftermath of the close season that was in July and August. Also, migrating pattern of fishes, the difference in sampling period, type of fishing gear could serve as potential factors [22, 35]. Amponsah et al. [22] noted that the variation in species diversity resides in the level of nutrients influx from the coastal lagoon linking each of these marine environments and can be attributed to the linkage of the marine environment and coastal lagoon systems which ensures the exchanges between fresh and marine waters. Tavares and Arocha [30] observed that the highest diversity levels are likely related to the oceanographic factors associated with high marine productivity and the most important factors are the seasonal upwelling.

4.4 Exploitation status

Research on the exploitation status of sharks has not been done or published in Ghana. Therefore, this current study is the first research on the exploitation status of sharks in Ghana waters. The exploitation status from the current status was based on three indicators to deal with overfishing proposed by [36]. Indicator one (1) was described as letting them spawn and it was measured as a percentage of mature specimens in the catch. The target would be to let all fish spawn at least once before they are caught to rebuild and maintain healthy spawning stocks. Comparing this analysis to the current study from Table 2, the majority of the species were matured except for Carcharhinus leucas. However, this analysis seems to favour the current study indicating a healthy spawning stock. Indicator two (2) is the ‘Let them grow!’ and is measured as the percentage of fish caught at the optimum length. This is where the Optimum length is typically a bit larger than the length at first maturity. From the study, the optimum length (L_opt) estimated for all the species was larger than the length at first, maturity (L_m) which satisfies indicator 2. Indicator three (3) is described as allowing the mega spawners to live!’ and is measured as the percentage of old, large fish in the catch.

From the current study, the percentage of matured specimen caught within the study period was 92%, and the juveniles as only 8%, which is highly greater than the threshold of 20% of the catch which indicates a healthy size structure. According to [36] if L_m is less than L_opt less than L_inf (L_m < L_opt<L_inf), it conveys that, all fish are given a chance to reach the size of maximum growth rate (and reproduce before being caught, so growth and recruitment overfishing are theoretically impossible and impact on expected life-time fecundity per recruit is reduced. The results from the current study fit perfectly into the framework as shown in Table 2 which buttresses the fact that the stock of these species is in healthy condition. In effect, this condition improves the ecosystem resilience and stability which serves as preconditions for reliable ecosystem services.

Species	Mega spawners (%)	Percentage of; juveniles (%)	Matured Specimen (%)
Isurus oxyrinchus	78.0	16.7	83.3
Carcharhinus leucas	66.6	61.5	38.5
Carcharias taurus	23.0	22.2	77.8
Sphyrna lewini	61.9	37.5	62.5
Carcharhinus brevipinna	42.5	37.7	62.3
Rhizoprionodon acutus	28.0	37.6	62.4
Alopias supercilliosus	100	0.0	100
Prionace glauca	38.4	29.7	70.3

Table 2.

Percentage of mega spawners, juveniles, and matured specimen.

5. Conclusion

The study aimed to assess the abundance and distribution of sharks in Ghana’s coastal area. Overall, eight (8) species of sharks were obtained with Prionace glauca (68%), as the most dominant species with both Carcharhinus leucas (1%) and Alopias supercilious (1%) as the fewer dominate species. The percentage of the maturity composition of adult to juvenile sharks is 92:8% with August happens to be the month of the most landed shark period with the highest number of adults and juveniles landed possibly as a result of the close season in July, followed by October. The diversity indices showed the marine environment is moderately polluted, the species structure is evenly distributed with infinite diversity and not dominated by only one (1) species. The exploitation status of the species based on length measurement indicates that the stock of the shark species is in healthy condition.

Acknowledgments

The authors express sincere gratitude to the fishermen and the personnel from Axim, Dixcove, and Tema fisheries commission for their assistance provided during the measurement and identification of the samples.

Conflicts of interest

No conflicts of interest.