Assessing the exploitation status of main fisheries resources in Ghana’s reservoirs based on reconstructed catches and a length-based bootstrapping stock assessment method

Figures & data

Figure 1. Map showing the locations of Tono, Bontanga, and Golinga reservoirs in Ghana.

Table 1. Morphometric characteristics of Tono, Bontanga, and Golinga reservoirs.

Parameter	Tono	Bontanga	Golinga
Surface area (km^2)	18.6	6.7	0.62
Reservoir length (m)	3471	1900	690
Mean depth (m)	6.6	5.9	2.7
Maximum depth (m)	13.32	9.70	4.95
Catchment area (km^2)	650	165	165
Volume (m^3)	93 × 10^6	25 × 10^6	1.23 × 10^6
Water level variation (m)	5.46	5.38	2.68

Table 2. Summary statistics and length‐weight relationships of the target fish species from Bontanga, Tono, and Golinga reservoirs of northern Ghana.

Reservoir	Family	Species	N	TL (cm) range	Lmean (cm)	Lc (cm)	BW (g) range	a (Cl95%a)	b (Cl95%b)	r^2
Bontanga	Cichlidae	Coptodon zillii (Gervais 1848)	2393	5.6–24.0	9.2	7.2	1.1–274.3	0.0236 (0.0224–0.0248)	2.89 (2.86–2.91)	0.9598
	Cichlidae	Oreochromis niloticus (Linnaeus 1758)	2678	6.1–37.5	12.3	10.7	2.1–1190.3	0.0226 (0.0213–0.0240)	2.93 (2.91–2.95)	0.9567
	Cichlidae	Sarotherondon galilaeus (Linnaeus 1758)	2677	6.0–21.9	11.4	10.9	4.6–196.8	0.0210 (0.0199–0.0222)	2.97 (2.95–2.99)	0.9605
Tono	Cichlidae	Coptodon zillii (Gervais 1848)	2584	7.0–19.6	11.2	9.0	4.0–126.9	0.0155 (0.0145–0.0165)	3.04 (3.02–3.07)	0.9513
	Cichlidae	Oreochromis niloticus (Linnaeus 1758)	2638	9.2–24.7	15.2	13.4	13.5–243.9	0.0248 (0.0232–0.0265)	2.88 (2.86–2.91)	0.9537
	Cichlidae	Sarotherondon galilaeus (Linnaeus 1758)	2700	9.9–22.4	15.3	12.7	18.4–169.9	0.0275 (0.0255–0.0297)	2.84 (2.82–2.87)	0.9388
Golinga	Cichlidae	Coptodon zillii (Gervais 1848)	261	6.5–23.3	10.5	8.6	5.3–219.6	0.0141 (0.0121–0.0165)	3.10 (3.03–3.16)	0.9710
	Cichlidae	Oreochromis niloticus (Linnaeus 1758)	1157	6.4–26.7	13.5	12.1	5.1–381.8	0.0219 (0.0203–0.0236)	2.95 (2.92–2.98)	0.9718
	Cichlidae	Sarotherondon galilaeus (Linnaeus 1758)	971	5.8–21.2	12.4	11	1.3–196.5	0.0194 (0.0180–0.0210)	3.00 (2.97–3.03)	0.9745

Note. N, sample size; Lmean, mean length of catch; L_c, length at first capture; TL, total length (cm, min–max); BW, body weight (g, min–max); a, intercept; b, slope; CI, confidence interval; r², determination coefficient.

Table 3. Catch (in tonnes) of Coptodon zillii, Oreochromis niloticus, and Sarotherondon galilaeus recorded through field survey at Tono, Bontanga, and Golinga reservoirs from July 2016 to June 2017.

Species	Tono reservoir	Bontanga reservoir	Golinga reservoir	Total
Coptodon zillii	12.36	9.72	0.14	22.22
Oreochromis niloticus	52.01	19.75	4.28	76.04
Sarotherondon galilaeus	103.06	48.9	3.13	155.09
Total	167.43	78.37	7.55	253.35

Figure 2. Size distribution of O. niloticus, S. galilaeus, and C. zillii landings observed from July 2016 to June 2017 at Bontanga, Tono, and Golinga reservoirs of northern Ghana. The vertical lines represent the length-based reference values: length at first maturity (dashed line), optimum length Lopt (dotted line), and starting length of mega-spawners (“dot-dashed” line).

Figure 3. Scatter histogram of bootstrapped ELFFAN with genetic algorithm optimisation for the target species from 3 reservoirs using TropFishR. The points represent the individual combinations of L_∞ and K estimates, while the contours represent the density of the combinations. The histograms represent the marginal distributions of the L_∞ and K estimates, respectively.

Table 4. Parameter estimates (mode of marginal distribution, Mod) of the seasonally oscillating von Bertalanffy growth function of the target fish species from reservoirs assessed with the bootstrapped electronic length frequency analysis with genetic algorithm function of TropFishR. Estimates based on the pooled length-frequency data of Tono, Bontanga, and Golinga reservoirs collected from July 2016 to June 2017. Lower and upper denote 95% confidence interval of the estimates.

Species	Parameter	Mod	Lower	Upper
C. zillii	L∞ (cm)	20.60	15.62	22.34
	K (/yr)	0.71	0.33	1.22
	tanchor	0.41	0.19	0.87
	C	0.76	0.21	0.94
	ts	0.64	0.24	0.79
	Φ	2.48	1.91	2.79
O. niloticus	L∞ (cm)	25.92	19.80	28.23
	K (/yr)	0.85	0.21	1.05
	tanchor	0.52	0.17	0.87
	C	0.52	0.16	0.92
	ts	0.74	0.11	0.94
	Φ	2.76	1.91	2.92
S. galilaeus	L∞ (cm)	17.79	17.16	19.10
	K (/yr)	0.79	0.52	1.15
	tanchor	0.61	0.21	0.79
	C	0.70	0.30	0.90
	ts	0.41	0.23	0.70
	Φ	2.40	2.19	2.62

Figure 4. Length-frequency histograms with the growth curves (dashed lines) obtained through the bootstrapped ELEFAN with GA analysis superimposed for Coptodon zillii, Oreochromis niloticus, and Sarotherodon galilaeus. The bars represent the restructured length frequency data, where black bars indicate positive peaks and white bars represent negative peaks. The method tries to maximize the number of positive peaks hit. The faint blue and red colors emphasize positive and negative peaks, respectively.

Figure 5. Bootstrapped linearized length-converted catch curve based on 1-yr catch data with selectivity correction. (a) C. zillii-Bontanga, (b) C. zillii-Tono, (c) C. zillii-Golinga, (d) O. niloticus-Bontanga, (e) O. niloticus-Tono, (f) O. niloticus-Golinga, (g) S. galilaeus-Bontanga, and (h) S. galilaeus-Golinga. The circles represent the points of the catch curve, which was applied to each resampled data set. The line represents the regression line corresponding to the maximum density estimates of the 500 estimated catch curves. The blue shaded area represents the 95% confidence interval for the maximum density regression line.

Table 5. Estimated mortality values (Z, M, and F), exploitation rate (E), and biological reference points of fishing mortality (F_MSY, F_max, F_0.1, F_0.5) for the target species of Tono, Bontanga, and Golinga reservoirs. Mod: mode of the marginal distribution. Low (lower) and Upp (upper) denote 95% confidence interval of the estimates. Estimates are based on the pooled growth parameters with moving average of 5, hence the M estimate of a species is a common value for all reservoirs.

Species	Parameter (/yr)	Bontanga			Tono			Golinga
		Mod	Low	Upp	Mod	Low	Upp	Mod	Low	Upp
C. zillii	Z	2.16	1.06	3.53	1.94	1.69	5.84	2.37	1.30	4.06
	M	1.17	0.66	1.80	1.17	0.66	1.80	1.17	0.66	1.80
	FMSY	1.02	0.57	1.57	1.02	0.57	1.57	1.02	0.57	1.57
	F	0.94	0.35	1.84	1.29	0.61	4.48	1.17	0.40	2.46
	E	0.43	0.33	0.52	0.66	0.36	0.77	0.49	0.31	0.61
	F0.1	0.93	0.76	1.85	1.17	0.56	1.79	1.15	0.63	1.81
	Fmax	6.97	1.91	7.00	2.50	1.22	7.00	2.19	1.73	7.00
	F0.5	0.57	0.49	1.42	0.85	0.42	1.52	0.83	0.42	1.33
O. niloticus	Z	1.88	1.04	4.28	1.74	0.97	4.48	3.54	1.05	5.18
	M	1.25	0.43	1.48	1.25	0.43	1.48	1.25	0.43	1.48
	FMSY	1.09	0.37	1.29	1.09	0.37	1.29	1.09	0.37	1.29
	F	1.15	0.51	3.02	0.96	0.49	3.08	0.81	0.61	3.73
	E	0.61	0.49	0.71	0.55	0.50	0.69	0.23	0.58	0.72
	F0.1	1.02	0.82	2.78	1.89	1.05	3.54	1.79	0.66	2.17
	Fmax	4.16	1.80	7.00	6.97	3.59	7.00	6.99	1.89	7.00
	F0.5	1.50	0.49	1.76	1.67	0.95	3.19	1.21	0.43	1.65
S. galilaeus	Z	2.64	1.48	3.64				2.03	1.82	3.36
	M	1.32	1.00	1.77				1.32	1.00	1.77
	FMSY	1.15	0.87	1.54				1.15	0.87	1.54
	F	1.34	0.46	2.05				0.91	0.70	1.66
	E	0.51	0.31	0.56				0.45	0.38	0.49
	F0.1	1.54	0.68	5.06				1.72	1.48	4.40
	Fmax	4.92	1.65	7.00					7.00	7.00
	F0.5	1.26	0.51	2.07				1.68	1.61	2.83

Table 7. Proportions of mature fish (Pmat), optimum-sized fish (Popt), larger than optimum size fish (Pmega), and Pobj (= Pmat + Popt + Pmega) for each species, and from the 3 reservoirs based on the indicators proposed by Froese (2004) and the formulas described in the Methods section. Stock condition interpretation is based on a decision tree proposed by Cope and Punt (2009), aimed to assess whether spawning biomass (SB) is above (>) or below (<) a reference point (RP) of 0.4 unfished biomass. The last column indicates the estimated probability of SB being lower than 0.4 of unfished biomass based on the same authors. For further details refer to Supplemental Table S4.

Species	Reservoir	Pmat	Popt	Pmega	Pobj	Stock condition interpretation	Probability
C. zillii	Bontanga	0.66	0.25	0.66	1.57	SB < RP	100%
	Tono	0.98	0.02	0.98	1.98	SB ≥ RP	0%
	Golinga	0.95	0.03	0.95	1.94	SB ≥ RP	0%
O. niloticus	Bontanga	0.55	0.40	0.34	1.29	SB < RP	100%
	Tono	0.98	0.10	0.90	1.97	SB ≥ RP	0%
	Golinga	0.80	0.37	0.56	1.73	SB < RP	100%
S. galilaeus	Bontanga	0.58	0.42	0.36	1.37	SB < RP	100%
	Tono	1.00	0.01	0.98	1.98	SB ≥ RP	0%
	Golinga	0.81	0.24	0.65	1.70	SB < RP	100%

Table 6. Biomass (in tonnes) of Coptodon zillii, Oreochromis niloticus, and Sarotherondon galilaeus estimated using Jones’s cohort analysis.

Species	Tono reservoir		Bontanga reservoir		Golinga reservoir
	Biomass (t)	Productivity (t/km^2)	Biomass (t)	Productivity (t/km^2)	Biomass (t)	Productivity (t/km^2)
C. zillii	16.24	0.87	20.61	3.08	0.28	0.45
O. niloticus	112.17	6.03	41.50	6.19	8.09	13.05
S. galilaeus	173.63	9.33	71.44	10.66	10.23	16.52
Total	302.04	16.23	133.55	19.93	18.60	30.02

Table 8. Growth parameters (L_∞ = Asymptotic length and K = Growth coefficient), mortality estimates (Z = Total mortality, M = Natural mortality, and F = Fishing mortality), and exploitation rates (E) from different fishing areas in West Africa, including the estimates of the present study.

Species	L∞ (cm)	K (yr)	Φ	Z (yr)	M (yr)	F (yr)	E (yr)	Country	Locality	Reference
C. zillii	27.83	0.46		3.16	1.14	2.02	0.64	Nigeria	Cross River basin	(Uneke and Nwani 2014)
	34	0.20	2.36					Burkina Faso	Sourou Reservoir	(Baijot and Moreau 1997)
	21.9	0.6	2.46					Niger	River Niger	(Mérona 1983)
	21.53	0.6	0.44	1.99	1.41	0.58	0.29	Ghana	Bontanga Reservoir	(Ofori-Danson et al. 2008)
	20.6	0.71	2.48	2.02	1.17	0.85	0.42	Ghana	Bontanga Reservoir	This study
	20.6	0.71	2.48	1.85	1.17	1.23	0.66	Ghana	Tono Reservoir	This study
	20.6	0.71	2.48	2.16	1.17	0.97	0.45	Ghana	Golinga Reservoir	This study
O. niloticus	26.2	0.7	2.68					Burkina Faso	Kokologho Reservoir	(Baijot and Moreau 1997)
	23.7	0.29	2.21					Burkina Faso	Boulmigou Reservoir	(Baijot and Moreau 1997)
	21.3	0.52	2.37					Burkina Faso	Petit Balé Reservoir	(Baijot and Moreau 1997)
	15.8	0.75	2.27					Burkina Faso	Sourou Reservoir	(Baijot and Moreau 1997)
	23.63	0.58	2.55	1.9	1.35	0.55	0.29	Ghana	Bontanga Reservoir	(Ofori-Danson et al. 2008)
	25.9	0.85	2.76	1.84	1.25	1.12	0.61	Ghana	Bontanga Reservoir	This study
	25.9	0.85	2.76	2.1	1.25	1.29	0.61	Ghana	Tono Reservoir	This study
	25.9	0.85	2.76	3.19	1.25	0.57	0.18	Ghana	Golinga Reservoir	This study
S. galilaeus	14.7	1.11	2.38					Burkina Faso	Loumbila Reservoir	(Baijot and Moreau 1997)
	17.4	0.45	2.13					Burkina Faso	Ramitenga Reservoir	(Baijot and Moreau 1997)
	26.2	0.73	2.70	1.76	1.51	0.27	0.15	Benin	Lake Doukon	(Lederoun et al. 2016)
	23.6	0.87	2.68	2.21	1.74	0.47	0.21	Benin	Lake Togbadji	(Lederoun et al. 2016)
	36.8	0.26	2.55	1.36	0.7	0.66	0.48	Ghana	Bontanga Reservoir	(Ofori-Danson et al. 2008)
	17.8	0.79	2.4	2.66	1.32	1.36	0.51	Ghana	Bontanga Reservoir	This study
	17.8	0.79	2.4		1.32			Ghana	Tono Reservoir	This study
	17.8	0.79	2.4	2.05	1.32	0.93	0.45	Ghana	Golinga Reservoir	This study

Appendix 1. Parameter estimates (mode of marginal distribution, Mod) of the seasonally oscillating von Bertalanffy growth function based on moving averages (MA) of 3 and 7 of the target fish species from reservoirs assessed with the bootstrapped electronic length frequency analysis with genetic algorithm function of TropFishR. Estimates based on the pooled length-frequency data of Tono, Bontanga, and Golinga reservoirs collected from July 2016 to June 2017. Lower and Upper denote 95% confidence interval of the estimates.

Species	Parameter	MA = 3			MA = 7
		Mod	Lower	Upper	Mod	Lower	Upper
C. zillii	L∞ (cm)	20.56	18.19	22.41	20.56	15.55	22.27
	K (/yr)	0.73	0.30	1.08	0.69	0.51	1.55
	tanchor	0.27	0.19	0.83	0.36	0.18	0.89
	C	0.68	0.21	0.94	0.81	0.35	0.97
	ts	0.36	0.16	0.83	0.65	0.35	0.78
	Φ	2.49	1.99	2.73	2.47	2.09	2.89
O. niloticus	L∞ (cm)	26.03	24.26	28.08	26.06	19.51	28.15
	K (/yr)	0.26	0.22	0.69	0.83	0.44	1.53
	tanchor	0.60	0.16	0.84	0.45	0.18	0.78
	C	0.60	0.15	0.89	0.50	0.22	0.95
	ts	0.53	0.18	0.86	0.81	0.12	0.93
	Φ	2.24	2.11	2.73	2.75	2.23	3.08
S. galilaeus	L∞ (cm)	22.91	17.24	24.29	17.60	17.15	18.00
	K (/yr)	0.32	0.30	1.31	0.82	0.63	1.10
	tanchor	0.57	0.14	0.87	0.63	0.21	0.80
	C	0.70	0.17	0.90	0.68	0.33	0.90
	ts	0.41	0.11	0.89	0.39	0.23	0.63
	Φ	2.23	1.95	2.89	2.41	2.27	2.55

Appendix 2a. Estimated mortality values (Z, M, and F), exploitation rate (E), and biological reference points of fishing mortality (F_max, F_0.1, F_0.5) for the target species of Tono, Bontanga, and Golinga reservoirs based on the pooled growth parameters with moving average setting of 3. Mod, mode of the marginal distribution; Low (lower) and Upp (upper) denote 95% confidence interval of the estimates. The M estimate of a species is a common value for all reservoirs.

Species	Parameter (/yr)	Bontanga			Tono			Golinga
		Mod	Low	Upp	Mod	Low	Upp	Mod	Low	Upp
C. zillii	Z	1.13	1.03	3.26	1.91	1.68	5.47	1.42	1.27	3.63
	M	0.73	0.61	1.65	0.73	0.61	1.65	0.73	0.61	1.65
	F	0.40	0.32	1.64	1.15	0.89	4.31	0.67	0.56	2.17
	E	0.35	0.31	0.51	0.61	0.53	0.79	0.47	0.44	0.60
	F0.1	0.95	0.75	1.56	0.67	0.52	1.62	1.25	0.66	1.54
	Fmax	6.95	1.82	7.00	4.78	1.16	7.00	2.27	1.68	7.00
	F0.5	0.59	0.48	1.03	0.48	0.39	1.25	0.88	0.43	1.24
O. niloticus	Z	1.13	1.03	2.82	1.15	1.03	2.69	1.19	1.01	3.52
	M	0.52	0.45	1.08	0.52	0.45	1.08	0.52	0.45	1.08
	F	0.62	0.50	1.78	0.67	0.50	1.65	0.68	0.53	2.40
	E	0.55	0.49	0.63	0.58	0.48	0.61	0.57	0.53	0.68
	F0.1	1.02	0.84	1.99	1.66	1.13	3.06	0.81	0.68	1.61
	Fmax	4.34	1.73	7.00	6.96	3.81	7.00	7.00	1.98	7.00
	F0.5	0.58	0.50	1.23	1.65	0.99	2.64	0.52	0.45	1.16
S. galilaeus	Z	2.49	1.75	6.38	2.99	2.35	3.19	2.10	1.82	3.90
	M	0.81	0.59	1.85	0.81	0.59	1.85	0.81	0.59	1.85
	F	1.58	0.85	5.01	1.40	0.87	1.46	1.27	0.89	2.68
	E	0.63	0.48	0.78	0.47	0.37	0.46	0.61	0.49	0.69
	F0.1	0.99	0.65	2.52	2.59	2.03	2.85	0.85	0.68	4.46
	Fmax	2.13	1.51	7.00	6.72	6.02	7.00	7.00	3.50	7.00
	F0.5	0.67	0.46	1.82	2.85	1.81	3.05	0.65	0.52	2.32

Appendix 2b. Estimated mortality values (Z, M, and F), exploitation rate (E), and biological reference points of fishing mortality (F_max, F_0.1, F_0.5) for the target species of Tono, Bontanga, and Golinga reservoirs based on the pooled growth parameters with moving average setting of 7. Mod, mode of the marginal distribution; Low (lower) and Upp (upper) denote 95% confidence interval of the estimates. The M estimate of a species is a common value for all reservoirs.

Species	Parameter (/yr)	Bontanga			Tono			Golinga
		Mod	Low	Upp	Mod	Lower	Upp	Mod	Low	Upp
C. zillii	Z	2.14	1.60	5.02	3.36	1.87	15.80	2.26	1.61	6.62
	M	1.68	0.92	2.24	1.68	0.92	2.24	1.68	0.92	2.24
	F	0.91	0.44	3.12	1.91	0.62	13.92	1.14	0.37	4.73
	E	0.43	0.27	0.62	0.57	0.33	0.88	0.50	0.23	0.71
	F0.1	1.53	0.96	2.35	1.14	0.90	2.65	1.10	0.88	5.50
	Fmax	7.00	1.69	7.00	2.57	1.81	7.00	1.96	1.55	7.00
	F0.5	1.00	0.61	1.99	0.91	0.67	2.41	0.79	0.65	1.34
O. niloticus	Z	3.34	1.70	4.99	3.07	1.36	4.01	3.49	2.05	5.53
	M	1.25	0.81	2.08	1.25	0.81	2.08	1.25	0.81	2.08
	F	2.04	0.70	3.40	1.85	0.26	2.49	2.21	1.05	3.95
	E	0.61	0.41	0.68	0.60	0.19	0.62	0.63	0.51	0.71
	F0.1	2.28	1.38	3.28	2.49	1.30	3.96	1.82	1.19	2.92
	Fmax	4.16	2.80	7.00	6.96	3.30	7.00	3.72	2.57	7.00
	F0.5	1.44	0.94	2.29	2.15	1.04	4.76	1.21	0.79	2.68
S. galilaeus	Z	2.70	1.87	3.38				2.87	2.86	2.89
	M	1.32	1.13	1.72	1.32	1.13	1.72	1.32	1.13	1.72
	F	1.29	0.48	1.72				1.19	1.15	1.24
	E	0.48	0.25	0.51				0.42	0.40	0.43
	F0.1	2.24	1.52	3.44				2.25	2.18	2.31
	Fmax	5.30	4.98	7.00					7.00	7.00
	F0.5	2.07	1.24	2.13				2.35	2.33	2.37

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