Chanos Chanos) Aquaculture in Mtwapa Creek and Gazi Bay Kenya

Assessment of Some Key Success Indicators in Community Milkfish (Chanos chanos) Aquaculture in Mtwapa Creek and Gazi Bay Kenya.

Item Type Report

Authors Mirera, D.O.

Publisher Western Indian Ocean Marine Science Association

Download date 29/09/2021 05:02:21

Link to Item http://hdl.handle.net/1834/7855

ASSESSMENT OF SOME KEY SUCCESS INDICATORS IN COMMUNITY MILKFISH (Chanos chanos) AQUACULTURE IN MTWAPA CREEK AND GAZI BAY- KENYA

BY

David O. Mirera

REPORT NO: WIOMSA/MARG-I/2009 –05

Introduction Fish farming accounts for more than one-quarter of the total fish directly consumed by humans, using about 220 finfish and shellfish species (Naylor et al ., 2000). Tilapia, milkfish, catfish, carps and marine molluscs contribute 80 % of the global aquaculture output amounting to 29 million tones in 1997 (Naylor et al ., 2000). Global aquaculture output is shooting towards the 60 million metric ton level (Bueno et al , 2006), implying that Aquaculture has the greatest potential to fill the gap between supply and demand for fish products. In order to make aquaculture a long- term strategy to contribute to growth and development, aquaculture should be sustainable, which means that it should not only be technically feasible and economically viable but should also be environment-friendly and socially equitable. There are key issues and concerns; however that should be addressed to make this possible like, seasonality and inconsistency of seed supply, impacts of release of cultured seed stocks, environmental degradation and diseases among others (Platon et al, 2006).

With the current state of technology and increasing population in the world, wild fish populations look insufficient to meet the increased protein demand. Hence aquaculture has been put forward as one of the solutions to the present protein problem in the world (Pillay, 1993). In many parts of the developing world, small scale aquaculture projects provide an inexpensive source of protein-rich food for personal consumption or as cash crop (Rice, 2003). Small scale artisanal mariculture projects are a mainstay in Asia where most mariculture is carried out on small family farms using low-trophic level marine mammals.

Cultivation of low-trophic-level marine species even though low priced, could alleviate some of the impacts of farming animal species that require high levels of organic inputs, such as marine finfish which the local people can not afford due to their high prices. One possible way to achieve this is through direct replacement of high-input species with low-input species, e.g., replacing production of carnivorous finfish (such as groupers) with omnivorous species (such as milkfish and rabbitfish). Milkfish aquaculture has a long history in Nauru (Pacific Ocean) where fry were caught in the surf and transferred to brackish water Ponds Island’s interior which caused mortality of most fry but a large number survived (Spennemann, 2002). Milkfish culture can be

2 traced back about 700 years in Indonesia (Ronquillo, 1975), and at least 400 years in Philippines (Ling, 1977).

Although pellet diets are available for a range of marine finfish as well as some crustaceans, there remain important constraints to the widespread use of compounded diets: Farmer acceptance of pellet diets is low because they see these diets as much more expensive than trash fish and organic manure. Farmers often do not appreciate that the food conversion ratios of pellet diets (usually 1.2–1.8:1) is dramatically better than that of ‘trash’ fish (usually 5–10:1, but sometimes higher) (Bueno et al ., 2006). Lack of farmer experience in feeding pellets may result in a lot of wastage. Distribution channels for pellet feed are not widely available in rural areas, which limit accessibility to and increase the cost of feed. Small-scale farmers operating fish ponds may not have access to the financial resources necessary to invest in purchase of pellet diets or infrastructure such as refrigeration, finding it easier to collect organic manure and use to increase pond productivity.

Most fish farmers in Kenya are rural, non-commercial, small-scale fish farmers. Total fish production is currently estimated at around 1 000 tones (Nyandat-unpublished data). Mariculture contributes insignificantly though there are several experimental mariculture initiatives along the coast of Kenya which have not been put to significant production while the inland aquaculture comprises about 0.6 percent of the total national fish production.

The mariculture experiments are mainly dependent on the natural production systems with limited use of animal manure (Mirera, 2008-in press). Chicken manure is very expensive and on a unit weight basis approaches the price of chemical fertilizers making it difficult to use by small scale mariculture farmers along the coast. Cereal brans, kitchen waste and vegetables are commonly used by the non-commercial farmers in the inland aquaculture but have not been mainstreamed for mariculture productions. Also for successful mariculture interventions there is need to develop formulations and make pellets using simple improvised machinery that can be available for the local farmers at affordable prices.

3 Milkfish aquaculture has attracted a lot of focus as the main candidate for marine finfish farming in East Africa because its herbivore, hardy (tolerate wide environmental conditions) and more profitable to rural fish farmers than predatory species or carnivores. With the promotion of milkfish as a livelihood option in the WIO region, some progress has been made in feed formulation and composition, response to feeds in laboratory conditions, occurrence and availability, influence to water quality in the open sea inclusive growth rate in earth ponds with season (Mirera, 2007; Mwaluma, 2003; Mwangamilo and Jiddawi, 2003 and Mmochi et al ., 2002). The current research addressed stocking density in earthen ponds and influence of supplementary feeds on fish growth.

This report shows in detail results of research trials carried out in earthen ponds to assess the effect of stocking density and feed on milkfish growth rate in earthen ponds along the coast of Kenya. The research was formulated under three key specific objectives:

1. Determine the effect of stocking density on milkfish growth in earthen ponds without supplementary feeding. 2. Establish the optimal feeding level required for milkfish in earthen ponds for effective and economical milkfish growth in earthen ponds. 3. Assess the effect of milkfish production with respect to site

Materials and Methods 1. Site The research was carried out at Mtwapa creek (Majaoni youth Development culture ponds) north of Mombasa-city and Gazi bay (Makongeni’s Baraka conservation group culture ponds) south of Mombasa city. Majaoni is located on Mtwapa creek 30 km from Mombasa; 6 km inland of Shanzu town- the site has a stand of mangroves and an upper intertidal sands. The sandy flat where ponds are located falls within a zone of Avicennia marina on the landward Rhizophora mucronata on the seaward side. Makongeni’s Baraka conservation group’s culture site is located on Gazi bay, 60 km south of Mombasa - 5 km before Gazi village. The culture ponds fall in an open sandy flat that is surrounded by Avicennnia marina (Figure 1).

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Figure 1: Map showing the sites (Majaoni and Makongeni) where milkfish pond culture was done along the coast of Kenya

2. Experimental design Each site had two earthen ponds that were used for the experiment giving a total of four ponds. The ponds of size 12 m x 12 m (144 m 2) were renovated, prepared for this study using labour from the community. Each site had only one replicate per treatment with a culture period of between 60 and 80 culture days, implying that each treatment had two replicates for the whole experiment.

5 Four growth experiments were conducted using earthen ponds at two stocking density and at different feeding rates. Fish size, stocking density, feed and growth periods differed between the two experiments. In each experiment, average fish weight (to ± 0.01 g) and total length (to ± 0.01 cm) were measured at the start and end of the growth experiment. Average growth rate (AGR) was calculated as:

AGR = (w 2 – w1)/ T

Where w 1 is the weight at the start of the experiment, w 2 is the weight at end of the growth period and “T” is duration of the days for the experiment.

3. Stocking density Milkfish fingerlings were seined in the mangrove channels and pools using mosquito nets, sorted, measured for length and weight then stocked in the respective culture ponds. Ponds were stocked at two stocking densities; 2fish/m 2 and 3fish/m 2 giving a total of 288 fish per pond for the first treatment and 432 fish/pond for the second respectively. The culture period was 69 days at Majaoni and 78 days at Makongeni. The fish were not given any supplementary feed and were sampled on a bi-weekly to establish their growth rates. A minimum of 20 individuals were sampled from each pond to get a representative sample of the growth condition for the fish. To establish weight, fish were measured using a weighing balance to nearest grams (with a capacity of 500g) while standard and total length was measured with a ruler to nearest millimeter.

4. Feeding rate The best treatment for stocking density was selected for use in assessing response of the fish to feed. Based on availability and cost; wheat bran was chosen as the food to be used for this culture experiment. Food composition was declined at this point due to limited availability technology wise to the local farmers who are pioneers of milkfish farming in the coast of Kenya. The fish used in the stocking density experiment were reconditioned for two weeks in bigger ponds before being subjected to the feeding experiment. Here fish were stocked at 2fish/m 2 with a supplementary feed of wheat bran at the rate of 5% and 10% of the body weight twice a day (10am and 4pm) respectively. The culture period in each experiment was between 63 and 71

6 days respectively. Fish sampling was done bi-weekly for a minimum of 20 fish and weight measured to the nearest grams, total length/standard length nearest to millimeters.

5. Water quality Water quality parameters (Temperature, Dissolved Oxygen and Ph) were measured weekly while other parameters (salinity, nutrients, ammonia) were measured bi-weekly.

Results

a. Growth trends In both treatments milkfish increased in weight and length from that observed during stocking though at different rates. ANOVA indicated that relative growth rates differed significantly between the two sites (P< 0.05). Both stocking density and supplementary feeds affected growth rate significantly at different rates (Table 1). Stocking density at 2 fish/m 2 and no feed recorded the highest mean growth rate (1.10 g/fish/day) as opposed to all the other treatments. Also the 10% body weight feed with wheat bran provided a higher growth compared to 5% body weight feed with the same feed (Table 1).

7 Table 1: Description of milkfish ( Chanos chanos ) growth experiments at two sites along the Kenyan coast under earthen ponds culture system Parameter Site No feed- No feed- 2fish/m 2-5% 2fish/m 2-10% 3fish/m 2 2fish/m 2 BW feed BW feed Initial mean Majaoni 6.6±0.51 19.6±2.74 14.25±1.17 22.55±2.99 total length (cm) Makongeni 13.1±2.33 10.6±2.63 16.43±4.43 13.96±1.99

Final mean total Majaoni 14.25±1.17 23.14±1.63 16.16±1.06 24.91±4.43 length (cm) Makongeni 14.24±2.05 20.17±1.63 22.21±3.75 19.86±3.74

Initial mean Majaoni 3.2±0.80 91.8±23.27 33.88±6.27 106.5±34.16 weight (g) Makongeni 22.7±11.53 22.7±18.03 68.81±47.90 29.33±10.93

Final mean Majaoni 33.88±6.27 119±19.26 49.58±6.56 153.63±76.28 weight (g) Makongeni 32±9.79 111.15±21.62 89.28±45.36 98.98±33.2

Growth rate Majaoni 0.56±0.40 a 0.61±0.82 a 0.23±0.07 b 0.76±0.07 c (g/day) Makongeni 0.19±0.27 b 1.58±0.99 c 0.93±1.18 c 0.98±0.57 c Combined 0.38±0.34 1.10±0.91 xx 0.58±0.63 0.87±0.32 xx

Growth rate Majaoni 0.111±0.06 a 0.051±0.01 c 0.030±0.01 b 0.037±0.02 b (cm/day) Makongeni 0.015±0.01 b 0.123±0.01 a 0.086±0.04 c 0.088±0.04 c Combined 0.063±0.03 0.087±0.01 xx 0.058±0.02 0.063±0.03

Mean culture Majaoni 69 69 63 63 days Makongeni 78 78 71 71 Combined 74 74 67 67 a, b, c : Letters with different superscript indicate significant difference at P< 0.05 xx : indicate significantly different growth rates compared to all the rest in the same level at P <0.05

8 Due to dependency on wild capture for fingerlings, it was not possible to get fingerings of same cohort for the experiment. Due to none uniformity in size at stocking, there were wider variations in growth rates of fish during the culture period even within the same treatment. This is evident through the outliers and extremes observed in Figure 2. The figure illustrates what could be obtained from an earthen pond culture system that depends on wild fingerling supply. As growth progresses different fish sizes changes their feeding strategy to make the best use of the available resource. Though, in the absence of feed due to high stocking density over along time, the sizes classes seem not to have any option but to behave in a more similar way. Provision of complementally feed in the second experiment, especially 10% body weight, lead to reduction of extremes from the experiment but increased the number of outliers in the same system. Such observation will not be made if all the fingerlings were hatchery produced and hence same cohort or size class.

6

5

4

3

2

1 Growth rate(g/day) 0

-1

-2

Mean ±SE No feed No 3fish/msq feed No 2fish/msq feed 5% feed 2fish/msq feed 5% ±SD 10% feed 2fish/msq feed 10% Outliers Extremes Treatment Figure 2: Growth rates of milkfish cultured in earthen ponds. The results show extremes and outliers in the results indicating differences in growth of fish in a pond due to inability to get fish of same size from wild seed stock collection.

9 The actual growth rate was compared with the expected growth rate (Figure 3). The results from all the treatments indicated some close links between the expected and attained growth rates. However, there were wider variations from the expected in three treatments (no feed 3 fish/m 2, 5 % feed 2fish/m 2, and 10 % feed 2 fish/m 2). Exceptionally, the no feed, 2 fish/m 2 indicated a very close relationship between the actual and expected growth rates. The results equally indicate that some fish within the pond culture systems experienced negative growth with respect to the expected which points towards what farmers should expect in returns when making use of wild fingerlings with varied sizes for stocking.

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3

2

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0

-1 Expected NormalExpected Value

-2

-3 -2-1 0 1 2 3 4 5 6 -2-1 0 1 2 3 4 5 6

Treatment: No feed 3fish/msq Treatment: No feed 2fish/msq 4

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-1 Expected NormalExpected Value

-2

-3 -2-1 0 1 2 3 4 5 6 -2-1 0 1 2 3 4 5 6

Treatment: 5% feed 2fish/msq Treatment: 10% feed 2fish/msq

Figure 3: Comparison of expected growth rate as per computer simulation and the real attained growth rate from the study

10 PCA analysis revealed that in terms of determining growth rate and consequently milkfish production, stocking density supersedes supplementary feeds. Stocking density contributed 66.82% towards growth rate of milkfish in the earthen pond experiments while feeds contributed 33.18% of the attained growth rate (Figure 4). This gives a general indication that farmers need to be given the right stocking density for milkfish in their ponds which can in turn be supplemented with feed to fasten growth for improved production. The significantly higher contribution of stocking density to growth rate is an inside into how researchers need to address mariculture so as to achieve profitability to the farmers who always rely on production.

1.0

No feed 2fish/m 2 No feed 3fish/m 2 0.5

*5% feed 2fish/m 2 *10% feed 2fish/m 2

0.0 Feeding :Feeding 33.18%

-0.5

-1.0 Active -1.0 -0.5 0.0 0.5 1.0 Suppl. Stocking density: 66.82%

Figure 4: Principal component analysis diagram with indications on how stocking density and feed impacted fish growth rate in earthen ponds

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b. Length-weight relation To establish the relations in weight and length of fish during the study, initial readings were made during stocking, harvest and during the growth process. Figures 5, 6 and 7 shows the body- weight to total-length relation as indicated by the respective regression equations which was determined (w = - a *10 -x Lb). The equations at stocking were established to be:

W = - 1.78 *10 -2 L2.7455 (1) - graph a W = - 4.06 * 10 -2 L2.5913 (2) - graph b W = - 4.53 * 10 -2 L2.4719 (3) - graph c W = - 2.91 * 10 -2 L2.6827 (4) - graph d

Where “w” is expressed in “g” and “L” in “cm” but represented by x in the graphs.

In all the initial equations, a similar “b” value was observed indicating that all the fish had similar in their wild habitats where they were captured from.

300 300 300 300 280 280 280

260 260 260

240 240 250 240 220 220 220

200 a 200 b c 200 d 200 180 180 180

160 160 160 y = 0.0453x2.4719 140 140 150 140 weight (g) Weight(g) Weight (g) 2 2.5913 weight(g) R = 0.9284 120 120 y = 0.0178x2.7455 120 y = 0.0406x y = 0.0291x2.6827 100 2 R2 = 0.822 100 R = 0.9635 100 100 R2 = 0.8593 80 80 80 60 60 60 50 40 40 40 20 20 20 0 0 0 0 0 5 10 15 20 25 30 0 5 10 15 20 25 30 0 5 10 15 20 25 30 0 5 10 15 20 25 30

Total length (cm) Total length (cm) Total length (cm) Total length (cm) Figure 5: Initial length-weight relationship of fish cultured in earthen ponds under different treatments: a – stocking at 3fish/m 2, b- stocked at 2fish/m2, c-stocking at 2fish/m2 and feeding at 10% body weight, d-stocking at 2fish/m2 and feeding at 5% body weight.

At the end of the culture experiment the regression equations were determined to be: W = - 5.55 * 10 -1 L1.5282 (5) –graph e W = - 3.154 *10 -1 L1.9067 (6) - graph f W = - 2.6 *10 -3 L3.3798 (7) – graph g W = - 9.46 *10 -2 L2.1983 (8) – graph h

12 The variations within the obtained “b” values were wider. This was an indication that the culture fish under different treatments adopted different modes of growth; where others became wider in girth with growth while others narrower in girth with growth. In three treatments there was a weak relationship between total length and weight at harvest (Figure 6).

300 300 300 300 280 280 280 280 260 260 260 260 240 240 e 240 f g y = 0.0026x3.3798 240 220 220 220 2 h 220 R = 0.986 200 200 200 200 180 180 180 180 160 160 160 1.9057 y = 0.3154x 160 140 y = 0.555x1.5282 140 140 2.1983 weight(g) 2 y = 0.0946x Weight(g) R = 0.5569 140 Weight (cm) 120 R2 = 0.4842 120 Weight (g) 120 R2 = 0.7663 120 100 100 100 100 80 80 80 80 60 60 60 60 40 40 40 40 20 20 20 20 0 0 0 0 0 5 10 15 20 25 30 0 5 10 15 20 25 30 0 5 10 15 20 25 30 0 5 10 15 20 25 30 35 Total length (cm) Total length (cm) Total length (cm) Total length (cm) Figure 6: Length-Weight relationship of fish cultured in earthen ponds under different treatments at the end of the culture experiments: e– stocking at 3fish/m 2, f- stocked at 2fish/m2, g-stocking at 2fish/m2 and feeding at 10% body weight, h-stocking at 2fish/m2 and feeding at 5% body weight. When comparing the whole culture period, the regression equations obtained were: W = -1.28 *10 -2 L2.8961 (9) – graph i W = -5.8 * 10 -2 L2.4304 (10) – graph j W = -2.68 *10 -2 L2.6566 (11) – graph k W = -6.14 *10 -2 L2.4011 (12) – graph l

The “b” value obtained was lower than 3.0 which indicate that the fish grew narrower of girth with growth. These observations indicated that the cultured fish did not put on weight isometrically with increase in length. However the general growth trend of fish indicated that there was a relatively strong relationship between the total length and weight throughout the experimental period (figure 7).

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300 300

300 280 300 280 280 280 260 260 260 260 240 240 240 240 220 220 l 220 j 220 200 200 y = 0.0268x2.6566 200 200 i 180 k 180 R2 = 0.9536 2.4011 180 180 y = 0.0614x 160 160 2 160 2.4304 160 R = 0.7872 140 y = 0.0581x 140 Weight(g) 140 2.8961 weight (g) 2 140 Weight (g) 120 Weight (g) y = 0.0128x 120 R = 0.8713 120 2 120 R = 0.9309 100 100 100 100 80 80 80 80 60 60 60 60 40 40 40 40 20 20 20 20 0 0 0 0 0 5 10 15 20 25 30 0 5 10 15 20 25 30 0 5 10 15 20 25 30 0 5 10 15 20 25 30 T otal length (cm) Total length (cm) Total length (cm) Total length (cm) Figure 7: Length-Weight relationship of fish cultured in earthen ponds under different treatments during the whole culture period: i– stocking at 3fish/m 2, j- stocked at 2fish/m2, k-stocking at 2fish/m2 and feeding at 10% body weight, l-stocking at 2fish/m2 and feeding at 5% body weight.

c. Water quality The results of dissolved oxygen and water column dissolved inorganic nutrients (ammonia-N, Nitrogen and phosphate-P) concentrations are provided in table 2. Relatively constant water quality parameters were observed in all the treatments. The fish stocked at 3 fish/m2 recorded the lowest concentration of dissolved oxygen levels though not significantly different from the others. Ammonia-N and phosphate-P increased in the supplementary feed treatments as opposed to no feed treatments though not significantly different. Oxygen fluctuations were between 2.8 mg/l in the morning to10.4mg/l in the late afternoon in all the treatments. Dissolved oxygen concentrations were relatively higher in the late afternoon compared to the morning.

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Table 2: Comparison of water quality parameters (range of concentrations ± SD) in earthen ponds used for culture of milkfish at different stocking densities Treatment Parameter Oxygen (mg/l) Phosphates (mg/l) Nitrogen (mg/l) Ammonia-N(mg/l)

3 fish/m 2 5.24 ± 0.18 0.26 ± 0.05 0.11 ± 0.03 0.19 ± 0.08 No feed

2 fish/m 2 6.71 ± 0.88 0.24 ± 0.07 0.12 ± 0.07 0.313 ± 0.23 No feed

2 fish/m 2 6.42 ± 0.26 0.31 ± 0.08 0.15 ± 0.10 0.28 ± 0.03 +10% feed

2 fish/m 2 6.64 ± 0.32 0.28 ± 0.06 0.13 ± 0.05 0.21 ± 0.10 + 5% feed

15 Discussion According to Bagarinao (1999), growth rates of milkfish depend on the initial fish size, stocking density, food and feeding, water and soil quality, and farm management. Farmers in South East Asia will always anticipate getting a growth rate of 2-3 g/day for their production using pelleted feeds but this has not yet been achieved while such trials have not been made in the western Indian Ocean (WIO) region. While using milkfish larvae from hatchery and wild from nursery to maturity, Bagarinao (1991 and 1994) obtained a sigmoid (S-shaped) growth curve with the steepest slope which indicates faster growth being in the juvenile stage (1.2 cm -10 cm). This might have been the cause of the varied growth rates (Table 1) within treatments in the current experiments since multi-size classes were used for culture due to inability to get similar since classes from the wild.

The study recorded varied growth rates per site and treatment ranging from 0.19g/day as the lowest at 3 fish/m 2 and no supplementary feed to 1.58g/day as the highest at 2 fish/m2 and no supplementary feed. The observed growth rates were higher than those recorded by Swanson (1998) in his salinity experiments using plastic tanks (0.018 g/day & 0.011 g/day) though at different stocking densities. Though, the results were similar to those by Eldani and Primavera (1981) who stocked milkfish at 0.2 fish/m 2 in a mixed culture with shrimps where all formed a stocking density of 0.6 individuals/m 2 with no supplementary feed; but higher than those observed by James et al (1984) where a milkfish fish was cultured together with mullets at 0.83 milkfish/m 2 (0.64 g/day) and 1.6 fish /m 2 for combined milkfish and mullets and rice bran as supplementary feed.

Fish that were cultured at stocking density of 2 fish/m 2 with no supplementary feed provided statistically significant different growth rates compared with those that were cultured with similar stocking density with a supplementary feed of wheat bran at 5% of their body weight though similar to those feed at 10 % of body weight. The results tends to put forward the aspect of feeding quantity as a factor that may make a difference, whereby the former could not make a break through with what the fish were getting from their natural environment. This makes a strong statement for mariculture development as a business; a fish farmer needs to be given the right feeding rate that will not make him use more resources without increasing production.

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Milkfish is a recent introduction to aquaculture research in the region (Mwangamilo and Jiddawi, 2003), therefore needs more research dedicated to feed and stocking density to justify its promotion for aquaculture. From the present study, admirable growth rates were obtained with both the no feed and supplementary feed trials. Though the supplementary feed in this study was not formulated (Plain wheat bran) to balance nutrients as in Jiddawi and Mwangamilo (2003). The results of the present study might have been boosted by the natural production in the earthen ponds unlike the case of plastic tanks used in the laboratory experiments that gave relatively lower growth rates in a period of 64 culture days (0.15 g/day to 0.19 g/day).

According to Brummett et al (2008), mariculture in African is at its earlier stages and looks complicated to be achieved due to the cannibalistic nature of most marine fish that requires feed formulation such as fishmeal that may cause competition with what is available for local consumption. In his review Brummett has not had a mention of potential herbivorous fish like milkfish and mullets that may boost food availability in Africa at minimal costs. From the results of the present study, it has been observed that growth rates was more influenced by stocking density (66.82 %) compared to feed (33.18 %). This is an indication that is milkfish culture is given the right stocking density, farmers will be able to gain profits without much worries as to cost of feeds that is a challenge in aquaculture development. Bagarinao (1999) also supports development of milkfish culture due to its dependency on lablab and filamentous algae that can be obtained through organic manure fertilization if the right stocking density is maintained; a fact which has been established by Mirera (2008-in press).

The results of body weight verses total length of fish cultured (b between 1.53 and 3.38) compared well with those of obtained in other culture studies (Grover and Juliano, 1976; Arroyo et al 1976). Kumagai et al (1985) established that wild juvenile milkfish in their natural mangrove pools had a “b” value of 3.2388 suggesting that they became wider of girth with growth. Most of the fish in this study did not but on weight isometrically with increase in length but rather became narrower in girth with growth. However only the fish that were stocked at 2 fish/m 2 and fed with 10 % body weight with wheat bran showed became wider of girth with growth compared to all the others. It therefore follows that for improved production of milkfish

17 in earthen ponds such combination could give the best results and hence recommended for use y farmers. However, the cost implications need to be considered before it can be fully adopted.

Conclusion Stocking density of 2 fish/m 2 is recommended in this study if production is based on the natural productivity which is dependent on algal mat as food for milkfish that is generated by nutrients in the water column. However better results in terms of length – weight relations were obtained when wheat bran was introduced at 10 % of body weight at the same stocking density. The observations create a challenge for more development of diet that could fasten growth of fish in earthen ponds which are also easily available to the famers. It’s out of the present results that different stocking densities can be worked on to enable profitability while using affordable supplementary feeds which are locally formulated.

Acknowledgement This research was supported by a Marg 1 research grant from WIOMSA, which is greatly appreciated. The water quality analysis of this study was done by colleagues from Kenyan Marine and Research Institute (KMFRI) whom I appreciate. My special thanks are extended to the community groups (Majaoni and Makongeni) where the research was conducted in a participatory way. I also appreciate CORDIO East Africa and Kwetu Training Centre for providing the office space during the experimental period.

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18 Bagarinao, T. (1991). Biology of milkfish (Chanos chanos Forsskal). SEAFDEC Aquaculture Department, Illoilo, Philippines, 94 pp.

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Ling, S.W. (1977). Aquaculture in Southeast Asia: A historical view. Univ. of Washington Press, Seattle Washington. Mirera, O. D. (2008). Experimental polyculture of milkfish (Chanos chanos) and mullet (Mugil cephalus) using earthen ponds in Kenya. (In press).

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Manuscript Abstract Community mariculture; Influence of stocking density/ supplementary feed on wild caught milkfish ( Chanos chanos ) fingerlings cultured in earthen ponds Mirera Oersted David Box 98422-80100, Mombasa Email: [email protected]

Abstract Growth rates and length-weight relations of milkfish cultured at two stocking densities (2 fish/m 2 & 3 fish/m 2) with no supplementary feed in earthen ponds constructed in the mangrove sandy flats over one culture period were determined. The best stocking density in terms of growth was subjected to supplementary feed at 5 % and 10 % body weight daily. There were significant differences within and between treatments (p < 0.05). The no feed treatment with stocking density of 2 fish/m 2 recorded the highest growth rate by weight (1.10 ± 0.91 g/day) and length (0.087 ± 0.01 cm/day). Supplementary feed of 10 % body weight influenced the highest growth rate. Length – weight relations (W = a x 10 -x Lb) provided an initial “b” value of between 2.47 and 2.75 and a strong correlation between length and weight of the fish (R 2 of between 0.82 & 0.96) stocked. The value of “b” shifted greatly at the end of the culture period giving values of between 1.53 and 3.38 with weak length – weight correlations (R 2 of between 0.48 & 0.98). The results of this study indicated that most fish grew narrower of girth with growth (“b” value less than 3.0). Stocking density had a strong influence on fish growth (66.82 %) as opposed to supplementary feed (33.18%). Key words: Milkfish, stocking density, feed, growth rate, earthen pond

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Assessment of Some Key Success Indicators in Community Milkfish (Chanos chanos ) Mariculture in Kenya

David Oersted Mirera P. O. Box 98422-Mombasa [email protected]

Abstract The study investigated the effect of milkfish stocking density at 2 fish/m2 and 3 fish/m2 on growth rate in earthen intertidal fish ponds that depended on natural water productivity and tidal influence. Fingerlings for stocking were seined in the mangrove stream channels and pools during spring low tides using push nets. The fingerlings were then measured for total length and weight before being stocked in ponds. Water quality parameters were measured weekly while fish sampling was once a month and 10% of the fish were sampled to avoid bias. There was a significant difference (p < 0.01) in growth rate between fish at stocking density of 2 fish/m2 and 3 fish/m2 with the former having a higher growth rate of 0.504 g/day compared to 0.058 g/day at 3 fish/m2. No significant difference was recorded in growth of fish with respect to site for the same stocking density. There was a strong relationship between weight and length of fish at stocking in both treatments (R2 = 0.8554– 3 fish/m2 and R2 = 0.8833 — 2 fish/m2). The strength of the relationship weakened during the culture for the 3 fish/m2 (R2 = 0.7104) compared to the 2 fish/m2 (R2 = 0.886) where the strength was maintained. General observation indicated that fish in the former treatment were weak and slender with less vigour compared to their counterparts. Water quality parameters measured in the study were within the required tolerance limit for milkfish. Hence could not be anticipated to cause retarded growth in any of the treatments.

Keywords: Culture, Mangroves, milkfish, stocking density

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