2014-10 - final report copper cage cahora bassa

Advance Africa Management Services of 27

FINAL REPORT

Comparative fish production trials in copper and polymer net cages in Cahora Bassa, Mozambique

October 2014

Prepared for The Copper Development Association Africa, Copalcor (Pty) Ltd. and Mozambezi Fisheries and Aquaculture by Advance Africa Management Services cc. Authors: T. Hecht and S. Daniel Reviewed by F. Formanek


Abstract

This study examined the comparative efficacy of copper alloy cages for farming of Nile tilapia in Lake Cahora Bassa. Two non-‐replicated trials with small and large mesh rigid HDPE, soft polyethylene, nylon and copper net material were undertaken in 2013 and 2014. The experiments were undertaken under faming conditions, using 5 x 5 x 6 m cages with an effective volume of 125m3. In all trials the performance of the fish with respect to weight gain, specific growth rate and condition was significantly better in the copper cages than in any of the polymer cages. On average, yield in the copper cages was 22.4% higher than in the corresponding polymer cages. The lower specific growth rate of the fish in the polymer cages was a consequence aperture occlusion resulting from biofouling by filamentous algae. Aperture occlusion reduces the rate of water exchange resulting in lower dissolved oxygen and pH levels in the polymer cages relative to the copper cages. In some instances these differences were statistically significant. Within 4 weeks of feeding the fish in the cages aperture occlusion in the polymer cages could reach levels of up to 90%, while in the copper cages occlusion levels did not exceed 10%. It was concluded that the use of copper cages for fish production in a sub tropical fresh water lake in comparison to polymer net pens, has the following advantages; the low levels of aperture occlusion, relative to polymer materials, improves water exchange and provides better conditions for fish growth. The improved conditions manifests in higher fish growth rate, better condition and higher yields. The alloy material precludes the use of predator nets and maintenance and labour requirements are reduced.


Copper was needed to protect the fish against the thieving behaviour of these guys

Getting it all together and the guys being artistic with COPALCOR alloy

Nearly done and then out into the lake.


Introduction

The aim of this study was to assess the efficacy of using copper alloy mesh cages in a subtropical lake in southern Africa. Specifically the study was designed to test the hypothesis that fish growth, and hence biomass increase, in copper alloy cages would be better than in polymer net cages. The basis of the hypothesis is that the copper mesh would not be subject to bio-‐fouling, resulting in better water flow through the net pen and hence higher dissolved oxygen levels in the water column within the alloy cages, which would manifest in superior fish growth. The growth experiments were carried out at Mozambezi, a Tilapia farm in the Chicoa basin of Lake Cahora Bassa, using Nile Tilapia (Oreochromis niloticus).

Lake Cahora Bassa is on the middle Zambezi River in Tete Province of Mozambique (15o 29’S – 26o 00’S x 30o 25’E – 32o 44’E). The lake was created in 1974 by impounding the Zambezi River in the Cahora Bassa gorge. The lake is 246 km long with a mean width of 10 km and an estimated shoreline of 1,775 km. The surface elevation of the lake is 314m ASL and it covers a surface area of 2,665 km

2and at full supply level holds 55.8km3 of water. The lake is the second largest man-‐made lake along the Zambezi River, after Lake Kariba, and is the fourth largest reservoir in Africa (Vostradovsky 1984).

The lake is climatically affected by three seasons: (1) the hot rainy season from November to April; (2) the cool and dry season from May to August and (3) the hot and dry season between September and November. The lake is stratified from September to April. Air temperatures range from a minimum of 14o C in July / August 39oC in October with the mean annual temperature between 26o C and 27o C (Vostradovsky 1984).

The farming of Nile Tilapia in Cahora Bassa is a recent initiative and was pioneered by Mr. Kurt Heyns, the owner of Mozambezi Aquaculture and Fisheries. There are no other aquaculture operators on the lake. On the other hand, Tilapia cage culture on Lake Kariba (upstream from Cahora Bassa on the Zambezi river) is a well established industry (AfDB 2011) and production currently exceeds 10 000 tonnes per annum. Several other Tilapia farms on Lake Kariba are in various stages of development. Once all farms are in operation it is anticipated that total tilapia production in Lake Kariba will exceed 40 000 tonnes of fish per annum. Cahora Bassa undoubtedly has the same production potential as Lake Kariba. It is further worth mentioning that the projected production volumes would by no means satisfy the fish deficit in the region, which is currently estimated at around 240,000 tonnes per annum. Zambia alone has a current estimated shortfall of fish in excess of 70,000 tonnes per annum.

In its mission to increase the market for copper products the International Copper Association has, since around 1970, been promoting and supporting the use of copper alloys in aquaculture. The principal advantage of using copper alloys is that the release of cupric ions prevents the settlement of invertebrate organisms on the material and hence is less prone to biofouling (Dwyer and Stillman 2009). Copper alloys are not immune to microfouling but colonization of macrofouling organisms is much restricted (Michel et al. 2011). Biofouling impedes the flow of clean, oxygenated water to the fish being cultured and provides a growth environment for parasites and pathogens that can infect fish. The removal, cleaning, and disposal of biofouled nets requires care to avoid adverse impacts.


Typical polymer nets can become biofouled within weeks. Fish farmers must therefore change polymer nets frequently, clean the nets in situ, or use antifouling coatings to maintain water flow (Dwyer and Stillman 2009). If any of these mitigating measures is not applied then aperture occlusion (Figure 1a) can create very unfavourable conditions for the fish with catastrophic consequences. Hecht et al. (2012) clearly showed the resistance to biofouling of various copper alloys in comparison to Nylon and HDPE netting in Saldanha Bay, Pemba in Mozambique and in the Seychelles. They further concluded that cage farming in Saldanha Bay would be greatly facilitated by copper alloy netting.

There are some interesting benefits when copper alloy is used to avoid fouling compared to antifouling coatings. The main one being that it does not need recoating periodically and foregoes the time and effort of removal, preparation, reapplication, and disposal. The alloy is also fully recyclable (Michelet al. 2011). Other advantages (Dwyer and Stillman 2009, ICA 2010) of using copper alloy in fish cage culture include;

• Improved water flow through cages, • Improved dissolved oxygen levels, reduced parasite load, reduced infections, lower FCR, • Reduces net fouling that serves as intermediate habitat for parasites and disease organisms

resulting in healthier fish. • Higher yield as a consequence of lower mortality (no stressful net changes; no stress from

predators) • The material is strong and predators cannot cause damage thereby reducing fish losses due

to predation and rate of escape of fish from cages. • Lower maintenance: no net changes; no net cleaning • Avoid need for predator net; avoid antibiotics • Reduced environmental impact: can be made from recycled materials; can be recycled after

use; no nets to dispose of. • Potential for consumer market positioning as more environmentally appropriate fish

production.

Despite the advantages the adoption of copper alloy netting in the aquaculture industry has been slow. However, since the development of chain link woven brass the use of copper net pen cages has been gaining momentum. The chain link woven material is flexible and highly suitable for round and square cages (Figure 1b). Currently, chain link woven brass nets are used in cages on commercial and experimental farms in Chile, China, Hawaii, USA, Tasmania, Korea, Japan and Scotland for various species including seabass, turbot, yellowtail, cobia, trout and salmon, amongst others. Several different alloys have been developed and are in use today.


Figure 1. (a) Total aperture occlusion by seaweed of a net cage in Algoa Bay, South Africa (Photograph: Gert LeRoux. (b) Circular fish cage with copper chain link woven mesh, Chile (Photograph: Langley Gace).

Moreover, unlike copper based dispersive antifouling agents very little copper is released into the environment from the copper alloys used in aquaculture. After immersion in seawater, a protective oxide layer naturally forms on the metal that inhibits corrosion, giving copper alloy mesh materials a working life of between 5 and 10 years, depending on its chemical composition. At the end of its working lifetime, the material will have lost only a fraction of its initial mass, and the remaining metal can be completely recycled to produce new net material (ICA 2010).

While copper alloy nets have been used in the marine environment since the mid 1970s there is no record of using them in freshwater. Bio-‐fouling by filamentous algae in spring and summer is a problem for cage culture in mesotrophic and eutrophic impoundments in the sub-‐topics and the tropics (pers. observations). Cahora Bassa is no exception. Nutrient loading from the 70 odd rivers that feed the lake is seasonal, occurring from November through to April, while aerial loading and nutrient inflow from Lake Kariba is of a perennial nature. Fouling in Lake Cahora Bassa is most severe in the first 2 to 3 meters of the water column, where after it is less severe and the intensity of fouling is greater in spring and summer than in autumn and winter (K.Heyns, pers. comm. 2013). The fact that there is no information on the efficacy of copper alloy nets in freshwater aquaculture and the high degree of biofouling in Cahora Bassa provided the motive for this work.

Material and methods All the juvenile fish for the experiments were provided by Mozambezi. Spawning and monosex fingerling production, using methyltestosterone, takes place in well managed ponds and once the fish reach 5-‐6 g they are transferred to nursery cages in the lake. The fish are reared for a period of 6-‐8 months and are then harvested at around 450-‐550g, which is the preferred size on the local market. Two experiments were carried out, in which performance parameters of fish in copper alloy cages were compared to polymer net cages (Table 1).


Two types of small mesh polymer nets were used, viz. soft polyethylene netting and a rigid HDPE oyster mesh material with a mesh size of 12mm in the square. Large mesh material consisted of nylon, polyethylene and HDPE oyster netting with a mesh size of 18 mm in the square. The small and large mesh copper alloy nets had mesh sizes of 9 x 15mm and 15 x 20mm, respectively (Figure 2). The copper and the HDPE oyster mesh material were rigid and this made it very difficult to harvest fish in comparison to the polyethylene and nylon cages. Copalcor (Pty) Ltd., the manufacturer of the copper alloy, is currently exploring chain link woven material, which makes harvesting as easy as in soft polymer net pens. Table 1. The number of cages, the net material, mesh size, and stocking density. Trial Cages Mesh size

(mm2) Start density (Fish/cage)

1A 1x Polyethylene 144 12 800 1x HDPE 144 12 800 1x Copper 135 12 800 1B 1x HDPE 324 16 630 1x Copper 300 16 630 2A 1x HDPE 144 4 035 1x Copper 135 4 035 2B 1x Nylon 324 4 500 1x Copper 300 4 500 All the cages were 5 x 5 x 6m deep with an effective volume of 125m3 (Figure 3). The copper alloy nets were fitted with a 0.75m skirt made of 8 mm anchovy netting between the top of the cage and the water surface.

Figure 2. Two mesh sizes of rigid copper alloy net material. The woven material was manufactured by Copalcor (Pty) Ltd. in Johannesburg.


Figure 3. The 5 x 5 x 6 m cages used for the copper alloy trials at Mozambezi in Lake Cahora Bassa. The experimental protocols were as follows: pH, temperature in degrees Celsius and dissolved oxygen (DO) in mg/L were measured daily at 09.00. Every second month a sample of 100-‐200 fish were caught from the cages with a cast net, weighed to the nearest g on a digital balance and measured to the nearest mm (total length) on a measuring board, where after they were returned to the cages. Specific growth rate was calculated using the equation, SGR (% body wt. Gain /day) = (Logn Final fishwt.(g) — Logn Initial fishwt.(g)) x 100

Time interval (days) On average, the fish were fed at 3% body weight per day. The daily ration was adjusted weekly (based on calculated fish biomass). The quantity of feed fed per day was recorded. When there was a shortage of feed then the daily ration in each cage was reduced by the same percent. The final biomass in each cage was calculated by multiplying the number of remaining fish in the cage (initial number minus mortalities) by the mean final weight of the fish. Final density was calculated by dividing the final biomass by 125 m3, which was the effective volume of all cages. Biofouling was expressed on a scale of 1 to 4, where 1 = 0 – 9% aperture occlusion, 2 = 10 – 49%, 3 = 50 – 74% and 4 = 75 -‐ 100% aperture occlusion. The condition factor (K) of the fish was calculated using the equation K = 100(W/TLb), where K=Fultons condition factor (Ricker 1975), W = weight (g), TL = total length (mm) and b = exponent of the length weight relationship. The Condition Factor K allows for a quantitative comparison of the condition of fish within a population or between populations. The length weight relationship of the fish was described by the equation W (g) = 0.00006 TL (mm)3.2634 (Figure 4).


Figure 4. The relationship between length (mm) and weight (g) for Oreochromis niloticus at Mozambezi, Mozambique (n=1215).

The cost of the copper alloy material did not allow for replication. For this reason we ran two independent trials. In all instances the experiments in the small and the large mesh cages could not be started simultaneously because of a lack of either small or larger fish. Statistical analysis All statistical analyses were performed using StatSoft Statistica 10 software. Data were tested for normality or equality of variance using Lévene’s test. A One Way Analysis of variance (ANOVA) was used to test for differences between data and if significant differences (p < 0.05) were observed, Tukey’s HSD post-‐hoc test was used to show where the differences occurred (Zar, 2009). Second or third order polynomial equations were fitted to the data to illustrate trends in growth and or biomass gain. Results

Trial 1A (small mesh)

The small mesh cages were stocked with fish on 15 May 2013 and the experiment was planned to continue for 6 months, until 15 November. Due to a misunderstanding an unknown quantum of fish was harvested from the cages on 7 October 2013. This meant that the mortality and biomass data with which to adjust the daily ration from then onwards could no longer be applied. The collection of these data was therefore stopped and the analysis for the performance parameters was curtailed to the period 15 May to 17 September 2013. The remaining fish in the copper and polyethylene cages were fed to satiation on a daily basis and their growth was monitored up to 15 November 2013. It should be noted that there was a significant difference between the initial weight of the fish in the three cages, with the heaviest fish (11.2g) in the rigid oyster mesh cage and the smallest fish in the copper cage (7.7g). The results of the trial are summarised in Table 2. Figures 5 and 6 show the growth of the fish in weight and the increase in biomass during the experiment. The fish length data show that the fish in the copper cage caught up with the fish in the HDPE cage and at the end of the experiment there was no significant difference in length between the fish in the copper cage and the HDPE cage and between the HDPE and polyethylene cage, but the fish in both the HDPE and the


copper cage were significantly larger than the fish in the polyethylene cage. However, in terms of weight gain, the fish in the copper cage had gained significantly more weight than those in the HDPE and polyethylene cages. The condition as well as the specific growth rate of the fish was also significantly better in the copper cage than in the two other cages.

The mortality rate of the fish in the three cages was low and ranged from 3.23 to 3.75%. The FCR of the fish in all three cages was excellent and ranged between 1.22 in the copper cage to 1.47 in the polyethylene cage.

Table 2. Experimental results for Trial 1A. Trial 1A Trial 1A extended

Begin and end date 15 May to 17 Sep 2013 to 15 Nov Production parameter

Poly SM

HDPE SM

Copper SM

Poly SM

Copper SM

Initial number of fish 12 800 12 800 12 800 Initial length (mm) 80 85 81 Final length (mm) 226 233 235 261 281 Length increase (mm) 146b 148ab 154a 182 200 Initial weight (g) 9.13c 11.15b 7.65a Final weight (g) 241 270 289 391a 502b Weight gain (g) 232b 259b 281a 381a 495b Specific growth rate (g/day) 2.58b 2.51b 2.88a 1Biomass gain (Kg) 2 869 3 292 3 465 Difference in final biomass gain compared to copper cage

-‐595 -‐254

Mortality (%) 3.25 3.75 3.23 2FCR 1.47 1.3 1.22 3Condition factor 0.89b 0.91b 0.95a 2.59a 3.24b Initial density (kg/m3) 0.94 0.94 0.94 Final density (kg/m3) 25 27 29 Different superscripts indicate statistical differences at P<0.05. 1Biomass gain = Final biomass -‐ Initial biomass 2FCR = Dry food fed / Biomass gain 3Condition factor = 100*(Final mean weight/ Final mean length b), where b is the exponent of the length weight relationship.

The significant difference in weight gain translated into the greater biomass in the copper cage at the end of the experiment. The results also show very clearly that the early advantage in weight of the fish in the HDPE cage was overcome within a period of 2 months. After 4 months the final biomass in the copper cage exceeded the biomass in the HDPE and polyethylene cages by 254 and 595 kg, respectively. This is highly significant from a farming perspective.


Figure 5. Growth of Nile Tilapia in small mesh copper, hard HDPE oyster mesh and soft polyethylene cages at Mozambezi, Lake Cahora Bassa from 15 May to 17 September 2013.

Figure 6. Biomass increase of Nile Tilapia in small mesh copper, rigid HDPE oyster mesh and soft polyethylene cages at Mozambezi, Lake Cahora Bassa from 15 May to 17 September 2013.

The growth of the fish in the polyethylene and copper cages for the 6 month period from mid May to mid November is shown in Table 2 and illustrated in Figure 7. By mid November the fish in the copper cage were, on average 111g heavier than those in the polyethylene cage.


Figure 7. Growth of Nile Tilapia in small mesh copper, HDPE oyster mesh and soft polyethylene net cages at Mozambezi, Lake Cahora Bassa from 15 May to 17 November 2013.

The reason for the slower growth rate and smaller final weight of fish in the polyethylene cage was most likely caused by aperture occlusion of the net, resulting in a reduction of water flow through the cage and hence lower oxygen levels. After 4 months in the water from mid May to mid September the apertures were already almost completely clogged (Figure 8) and this would have prevented adequate water exchange in the upper 2.5 metres.

Figure 8. Percent aperture occlusion in small mesh polyethylene (Stage 4) and copper cages (Stage 1) after 4 weeks (Trial 1A).

The ambient environmental conditions in the lake and in the cages during Trial 1A are illustrated in Figures 8, 9 and 10. There were no significant differences in the pH levels within the various cages and between any of the cages and the lake. However it was of interest to note that the pH level in the lake was always, except for a single occasion, higher than in the cages. This is caused by respiratory CO2 excretion by the fish which results in a decrease in pH. Dissolved oxygen was always highest in the lake but not significantly different to the DO levels in the cages (P=0.55). Water


temperature declined from 24.7oC in May to 22oC in July and then started rising again at the onset of spring in September. The lowest temperature of 22oC is the norm for the lake in July and August.

Figure 8. pH levels in the cages and the lake during the period 15 May to 10 September 2013.

Figure 9. Dissolved oxygen levels in the cages and the lake during the period 15 May to 10 September 2013.

Figure 10. Water temperature in the cages and the lake during the period 15 May to 10 September 2013.


Trial 1B (large mesh)

Trial 1B, using the larger mesh copper and rigid HDPE nets (see Table 1), started on 9 July. As for Trial 1A an unknown quantity of fish was mistakenly harvested from each cage in early October. The same remedial procedure was followed as in Trial 1A and length and weight data were obtained in mid October and at the end of the planned 6 month experimental period on 17 November 2014. The results of the trial are summarised in Table 3. Figure 10 shows the growth rate of the fish in the copper and HDPE cages from mid May to mid November and Figure 11 shows the increase in fish biomass in the cages from mid May to mid September.

Table 3. Experimental results for Trial 1B. Trial 1B Trial 1B extended

Begin and end date 9 Jul to 15 Sep 2013

to 17 Nov 2013

Production parameter

HDPE LM

Copper LM

HDPE LM Copper LM

Initial number of fish 16 630 16 630 -‐ -‐ Initial length (mm) 154 153 -‐ -‐ Final length (mm) 226 238 267 282 Length increase (mm) 72a 85b 113c 129d Initial weight (g) 82 82 -‐ -‐ Final weight (g) 250 302 427 516 Weight gain 168a 220b 345c 435d Specific growth rate (g/day) -‐ -‐ 1.23a 1.40b 1Biomass gain (Kg) 2 745 3 619 -‐ -‐ Difference in final biomass gain (kg) compared to copper cage

-‐874

-‐ -‐

Mortality (%) 1.12 0.78 -‐ -‐ 2FCR 1.11 1.08 -‐ -‐ 3Final Condition factor 0.93 a 0.94 a 2.77 b 3.32 b Initial density (kg/m3) 12.1 12.1 -‐ -‐ Final density (kg/m3) 33 40 -‐ -‐ Different superscripts indicate statistical differences at P<0.05. 1Biomass gain = Final biomass -‐ Initial biomass 2FCR = Dry food fed / Biomass gain 3Condition factor = 100*(Final mean weight/ Final mean length b), where b is the exponent of the length weight relationship.

The data show that the increase in length of the fish in the copper cage was significantly greater than for those in the HDPE cage by 15 September and at the end of the experiment. The same pattern was evident for the gain in weight. The growth of the fish during the period 9 July through to 17 November is shown in Figure 9. The higher length and weight gains are a reflection of the significantly higher specific growth rate of the fish in the copper cage. There was no significant difference in the condition of the fish in the two cages in September as well as in November. Mortality in the HDPE cage (1.12%) was slightly higher than in the copper cage (0.78%). The FCRs in both cages was nearly the same.


The 50g difference in the mean final weight of the fish on 15 September, which is reflected by the significantly higher SGR, resulted in 874 kg’s more biomass in the copper cage after the 2.5 month growth period from 9 July to 17 September 2013 (Figure 10).

Figure 9. Growth of Nile Tilapia in large mesh copper and HDPE oyster mesh cages at Mozambezi, Lake Cahora Bassa from 15 May to 17 November 2013.

Figure 10. Biomass increase of Nile Tilapia in copper and HDPE oyster mesh large mesh cages at Mozambezi, Lake Cahora Bassa from 14 July to 15 September 2013.

The environmental conditions in the cages and the lake (Figures 11, 12 and 13) provided corroborating evidence for the improved growth, FCR and condition of the fish in the copper cage in comparison to those in the HDPE cage. While there was no significant difference in DO levels between the lake and the copper cage, these values were significantly higher than DO levels in the HDPE cage (p<0.004). The significantly lower DO levels in the HDPE cage suggests a much lower water exchange rate. This supposition is supported by the high level of biofouling on the rigid HDPE material shown in Figure 14 in comparison to the copper mesh. The lower water exchange rate is also the reason for the significantly lower pH in the HDPE cage in comparison to the copper cage and the lake, between which the difference was not significantly different. The temperature during this trial was just above 22oC.


Figure 11. pH levels in the cages and the lake during the period 9 July to 15 September 2013.

Figure 12. Dissolved oxygen in the cages and the lake during the period 9 July to 15 September 2013.


Figure 13. Water temperature in the cages and the lake during the period 9 July to 15 September 2013.

Figure 14. Biofouling on strips of large mesh rigid HDPE oyster (left), copper (centre) and polyethylene (right) netting suspended in the water column from 9 July to 16 September 2013. Aperture occlusion on the HDPE and the polyethylene material was Stage 4, while occlusion on the copper material was Stage 1.


Trial 2A (small mesh)

Trial 2A began on 15 April and was terminated on 17 August 2014. Table 1 shows the specifications of the two cages. The results of the trial are summarised in Table 4. Figure 15 shows the growth rate of the fish in the copper and HDPE cages from mid April to mid August and Figure 16 shows the increase in fish biomass in the cages for the same time period.

The data show that the increase in length and weight gain in the copper cage was both significantly higher than in the polyethylene cage. The higher length and weight gains are reflected by the significantly higher specific growth rate of the fish in the copper cage. There was no significant difference in the condition of the fish in the two cages at the end of the experiment. Mortality was negligible, at an average of around 2.5% over the 4 month experimental period. The FCR in both cages was almost the same.

Table 4. Experimental results for Trial 2A. Trial 2a

Begin and end date 15 Apr to 17 Aug 2014 Production parameter HDPE SM Copper SM Initial number of fish 4 035 4 035 Initial length (mm) 115 115 Final length (mm) 200 215 Length increase (mm) 85a 100b Initial weight (g) 34 34 Final weight (g) 157 201 Weight gain (g) 123a 167b Specific growth rate (g/day) 1.89a 2.35b 1Biomass gain (kg) 481 655 Difference in final biomass gain (kg) compared to copper cage

-‐174

Mortality (%) 2.31 2.66 2FCR 1.45 1.43 3Condition factor 0.89a 0.9a Initial density (kg/m3) 1.1 1.1 Final density (kg/m3) 5 6 Different superscripts indicate statistical differences at P<0.05. 1Biomass gain = Final biomass -‐ Initial biomass 2FCR = Dry food fed / Biomass gain 3Condition factor = 100*(Final mean weight/ Final mean length b), where b is the exponent of the length weight relationship.


Figure 15. Growth of Nile Tilapia in small mesh copper and HDPE cages at Mozambezi, Lake Cahora Bassa from 15 April to 17 August 2014.

Figure 16. Biomass increase of Nile Tilapia in small mesh copper and HDPE cages at Mozambezi, Lake Cahora Bassa from 15 April to 17 August 2014.

The dissolved oxygen levels and the pH in the cages and the lake showed similar patterns (Figures 17 and 18) as observed in Trials 1A and 1B. Both pH and DO were highest in the lake and lowest in the polymer mesh cages. This persistent pattern adds weight to the argument that the lower pH and DO levels are a consequence of restricted water flow through the cages caused by aperture occlusion as a consequence of the growth of filamentous algae on the polymer nets. The temperature (Figure 19) during the experiment declined from 26.4oC in April to around 22oC in August.


Figure 17. pH levels in the cages and the lake during the period April to August 2014.

Figure 18. Dissolved oxygen levels in the cages and the lake during the period April to August 2014.

Figure 19. Temperature in the cages and the lake during the period April to August 2014.


Trial 2B( large mesh)

Trial 2B began on 15 February and was terminated on 15 August 2014 (6 months). The specifications of the cages are provided in Table 1. The results of the trial are summarised in Table 5. Figure 20 shows the growth rate of the fish in the copper and nylon cages from mid April to mid August and Figure 21 shows the increase in fish biomass in the cages for the same time period.

The data show that the increase in length and weight gain in the copper cage was significantly higher than in the nylon cage. The higher length and weight gains are a reflection of the significantly higher specific growth rate of the fish in the copper cage. There was no significant difference in the condition of the fish in the two cages at the end of the experiment. Mortality was slightly higher than during the other trials at around 4% over the 6 month experimental period. The FCR in both cages was almost identical at 1:2.8.

Table 5. Experimental results for Trial 2B. Trial 2B Begin and end date 15 Feb to 16 Aug Production parameter HDPE SM Copper SM Initial number of fish 4 500 4 500 Initial length (mm) 152 152 Final length (mm) 246 255 Length increase (mm) 94a 103b Initial weight (g) 61 61 Final weight (g) 323 365 Weight gain (g) 262a 304b Specific growth rate (g/day) 1.15a 1.25b 1Biomass gain (kg) 1124 1297 Difference in final biomass gain (kg) compared to copper cage

-‐173

Mortality (%) 3.82 4.27 2FCR 2.82 2.8 3Condition factor 0.92a 0.92a Initial density (kg/m3) 2.2 2.2 Final density (kg/m3) 11 13 Different superscripts indicate statistical differences at P<0.05. 1Biomass gain = Final biomass -‐ Initial biomass 2FCR = Dry food fed / Biomass gain 3Condition factor = 100*(Final mean weight/ Final mean length b), where b is the exponent of the length weight relationship.


Figure 20. Growth of Nile Tilapia in large mesh copper and nylon cages at Mozambezi, Lake Cahora Bassa from 15 February to 16 August 2014.

Figure 21. Biomass increase of Nile Tilapia in large mesh copper and nylon cages at Mozambezi, Lake Cahora Bassa from 15 February to 16 August 2014.

Figures 22 to 24 show ph, DO and temperature date for the duration of the experiment. During the experiment the temperature decreased from a summer maximum of around 28.7oC to the average winter temperature of 22oC. Once again, the pH and DO levels were highest in the lake followed by the copper and then the nylon cage. The DO level in the nylon cage was significantly lower than in the lake and the copper cage (P>0.038) and there was no significant difference in the DO levels in the lake and the water in the copper cage. This confirms the supposition made earlier that there would have been a greater water exchange through the copper cage than through the nylon cage, which manifests in the better performance of the fish in the copper cages.


Figure 22. Dissolved oxygen levels in the cages and the lake during the period February to August 2014.

Figure 23. pH levels in the cages and the lake during the period February to August 2014.

Figure 24. Temperature in the cages and the lake during the period February to August 2014.


Discussion and conclusion Farm conditions sometimes do not allow for experimental rigidity and hence some measure of flexibility was required. Feed shortage was compensated for by reducing the rate at which the fish were fed. For this reason it is not possible to put much weight on the FCR data. It was also not possible to start the trial in the small and large mesh cages at the same time. Starting dates were dependent on the availability of suitable sized fish for either the small or the large mesh nets. Moreover, as mentioned earlier it was not possible to run replicates because of the high cost of the copper alloy material. Never the less, and fully recognising the statistical pitfalls of drawing conclusions on non-‐replicated trials, the fish in the copper cages in all instances outperformed the fish in all the other cages by significant margins. In all the trials the SGR of the fish in the copper cages was significantly greater than in any of the polymer cages. Depending on initial size and temperature the SGR in the copper cages exceeded the specific growth rate in the polymer cages by an average of 12.6% per day (range 8 – 19.6% per day). From a farming perspective this translates into a much higher final biomass of harvestable fish. On average, yield in the copper cages exceeded that in the polymer cages by 22.4 % (range 8 – 36% or between 173 and 874 kg’s). The only experiment that ran during a period of increasing temperatures from July (22oC) through to November (28oC) was Trial 1B. By using the mid-‐November final weights of the fish in Trial 1B and assuming that the mortality rates would have remained the same as those recorded in the first half of the trail, it was possible to calculate the theoretical yield (biomass gain) in each cage. The difference in yield between the copper and the HDPE cage in Trial 1B would have been 1.4 tonnes. Expressed as a percent this means that the copper cages outperformed the polymer cages by 29.4%. This number falls well within the range of greater percent yields in the copper cages (see above). However, depending on the rate of fouling all polymer nets had to be cleaned with a broom at regular intervals. This was not necessary for the copper alloy material. In general, as soon as the young fish (5 g onwards) are put into cages in the lake they grow much better than in the nursery ponds. It is mainly for this reason that small mesh polymer net pens are used. However, depending on the rate of fouling the small mesh as well as the larger mesh net pens had to be cleaned with a broom at regular intervals. This practise is possible at this stage of the farms development. As the farm expands it would be very difficult to keep pace with having to provide optimal growing conditions for the fish by cleaning or replacing the nets. As in Lake Kariba it would not be possible to farm fish in cages on a large scale in Cahora Bassa without the use of anti-‐predator nets (Figure 25). Predators here, as in Kariba, include birds particularly cormorants, tiger fish (Hyrdocynus vittatus) (Figure 26) and Nile crocodiles. Predator nets would also be subject to bio-‐fouling and would also have to be removed and cleaned at certain intervals. On the other hand, copper alloy netting precludes the need for predator nets.


Figure 25 (left) and 26 (right). Crocodile attack on Tilapia net pen at Mozambezi, Cahora Bassa and Norm with Tiger fish (Hydrocynus vittatus).

The benefits of using copper alloy mesh cages in a sub tropical fresh water lake and the advantages over polymer nets can be summed up as follows:

• Bio-‐fouling is negligible, resulting in improved water exchange and better conditions for fish growth.

• Higher growth rate of fish • Fish are in a better condition (greater weight for a given length) • Higher yields • Protection against predators • Lower maintenance requirements (no need to clean or replace nets) • Reduced labour requirement

References

AfDB Group. 2011. Lake Harvest Aquaculture Expansion Project. African Development Bank Group. 13p.

Dwyer, R. L. and Stillman, H. 2009. Environmental Performance of Copper Alloy Mesh in Marine Fish Farming: The Case for Using Solid Copper Alloy Mesh. International Copper Association. 18pp.

Hecht, T., Daniel, S. and Formanek, F. 2012. A comparative assessment of bio-‐fouling on copper alloy chain link mesh, nylon and polyethylene netting: A contribution to the development of cage aquaculture in southern Africa and Western Indian Ocean Region. Advance Africa Report to CDA Africa. 52p.

ICA, H. 2010. Copper alloys for marine aquaculture. International Copper Association. 2pp.


Michel, J.H., Mitchels, H.T. and Powel, C.A. 2011. An Assessment of the Biofouling Resistance and Copper Release Rate of 90-‐10 Copper-‐Nickel Alloy. Paper 11352, NACE International Corrosion Conference and Expo. 2011.

Ricker, W.E. 1975. Computation and interpretation of biological statistics of fish populations. Bull. Fish. Res. Bd Can. 191: 1-‐382.

Vostradovsky, J. (1984). Fishery investigation on Cahora Bassa Reservoir (March 1983 – May 1984). A report prepared for the research and development of inland fisheries project. FAO, Rome; FAO/GCP/066/SWE; Field Document 11.

Zar, J.H. 2009. Biostatitical Analysis 5th Ed. Pearson. 960p.

Acknowledgements

The study was funded by Copalcor (Pty Ltd) and Mozambezi Fisheries and Aquaculture, through the Copper Development Association Africa. On a personal basis we should like to thank the owner of Mozambesi, Mr Kurt Heyns and his very able team on the shores of Lake Cahora Bassa for their assistance and hospitality to bring it all together, Messrs Gordon Grant and Derick Coetzee, Managing Director and Sales & Marketing Director of Copalcor (Pty) Ltd., respectively for their wisdom in seeing the potential of their alloy, Rudolf Kruger, the Quality Assurance Manager at Copalcor for his scientific and technical assistance and finally Evert Swanepoel, the Director of the Copper Development Association Africa for his encouragement and support.


Kurt Heyns (owner of Mozambezi), Margie Paterson (Hatchery / Nursery Manager) and the cage farming team.

2014-10 - final report copper cage cahora bassa

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