Growth performance of the rabbitfish Siganus sutor raised at outdoor rearing ponds in Toliara, SouthWestern Madagascar

In Madagascar, decrease of fish stock and poverty lead small scale fishermen to deploy destructive fishing gear. At Toliara Reef, fishermen using mosquito seine net catch up to 50% of juvenile fish and up to 42% of these juveniles belong only to the rabbitfish Siganus sutor species.This prohibited fishing practice disturbs the structure of adult fish populations and aquaculture may be one of the effective solutions to alleviate the problem of stock depletion. Rabbitfishes possess most of the desirable characteristics for aquaculture (e.g. herbivory and responsive to artificial food, high survival in high rearing densities) and the culture of several Siganus species have been tested in various countries. However, no study has been conducted on the rearing of S. sutor in Madagascar. Here, we investigated such a possibility regarding the growth performances and survival rate of S. sutor. Juveniles (initial mean body weight of 6.06g ± 2.43 and length of 6.23cm ± 0.8) were successfully retrieved from fishermen catches and we applied fish grow-out experiments in outdoor rearing pond (16m3) for a duration of five months. The rearing density was 8 fish/m3 and the performance of different diets were tested. A locally produced industrial fish food (composed of corn, soya flour, wheat, vitamins, oil…) at a feed ration of8% of fish biomass were compared with cooked rice at a feed ration of 25% of fish biomass. Dissolved oxygen, temperature, salinity, turbidity and pH were recorded three times a day along the experiment. Fish fed with industrial fish food had a significant higher growth (p<0.05) with a final mean body weight of 68.11g ± 26.71 and a final mean length of13.91cm ± 1.69. At the end of the experiment, fish fed with cooked rice displayed a mean body weight of 34.81g ± 15 and a mean length of 11.15cm ± 1.76. Feed conversion ratio is 2.99 for industrial food while it reaches up to 20.67 for the cooked rice. Surprisingly, fish fed with industrial food showed a significant lower survival rate (p<0.05) of 47.2%, compared to 95.7% for the cooked rice. Our study highlights the possibility of rearing juvenile rabbitfish in outdoor rearing pond using essentially locally produced industrial food. Controlling the rearing of juvenile rabbitfishes can be suggested as a practical approach to improve the survival rate of these juveniles through sea ranching and for improving livelihood of local communities through alternative activity

The potential nursery areas and recruitment seasons of Siganus sutor in Madagascar

editorial reviewedBackground Seagrass beds support the productivity of coastal fisheries by ensuring the survival of juvenile fishes and securing their recruitment in adult populations. This habitat is a preferred fishing area targeted by small-scale fishermen in developing countries, using Mosquito seine nets as observed in SW Madagascar. Previous studies revealed that seines nets caught large proportions of juveniles, especially dominated by Siganus sutor up to 42% of catches. In a conservation perspective, recruitment monitoring of this highly affected species is needed to provide key information related to their spatial and temporal distribution along the coastal habitats. This research aims at understanding recruitment patterns of S. sutor in critical habitats of SW Madagascar. The specific objectives are (a) to detect the recruitment seasons of S. sutor; (b) to identify their potential nursery areas; and (c) to understand why S. sutor select specific zones as a nursery area. Methods Catches from small scale fishers from mangroves, seagrass meadows, intermediate areas (i.e. sandy bottoms between seagrass patches and coral reefs) and seagrass associated to coral reefs were sampled with three stations per habitat and three days per month from July 2021 to June 2022. In meantime, ecological data related to surface temperature and salinity were recorded In the laboratory, all individuals were identified, photographed in order to automatically measure their size with ImageJ software. Another survey was carried out to address biological data (food availability) and habitat characteristics (depth, seagrass cover and diversity) in order to understand their influence on nursery areas selection for S. sutor. Results The catch of 5714 individuals shows that the abundance and size of rabbitfish exhibited a spatial and seasonal variability pattern. Rabbitfish size increased from habitats located near the coast compared to those near the reef. The smallest S. sutor size only occurred in the mangroves and seagrass meadows with 2.2 ±1.46 cm, against 4.2±1.48 for the intermediate areas and 5.1±1.47 seagrass associated with coral reefs. Considering abundance, juveniles of S. sutor is significantly more abundant in seagrass meadows (with 59.4%) while this species appeared to be rare in mangroves (<1%). This is indicated that this species did not colonize mangroves at an early stage. The small sized S. sutor were most abundant in seagrass meadow, suggesting this habitat could be the main nursery area for this species. Two recruitment seasons corresponding to the smallest mean size and the most abundant occurred in August 2021 and in January 2022 so far. This highlights the seasonality of S. sutor recruitment in SW Madagascar during warm and cool seasons. Conclusion To conclude, seagrass meadows constitutes the main nursery areas for S. sutor. Their massive settlement occurred in January (warm season) and August (cool season). Madagascar is characterized by two main recruitment season so far. As sampling will be completed in June 2022, results explaining why S. sutor choose seagrass meadows as a potential nursery will be available by August 2022 and will be presented. Such information will be useful for the decision making to develop management measures for the sustainable use of S. sutor resources and coastal habitats.Fish juvenile recruitment in coastal habitats of Western Indian Ocean14. Life below water11. Sustainable cities and communitie

Overview and progress of consortium research related to the biology, ecology and aquaculture of rabbitfish

editorial reviewedCoastal habitats support global fisheries by ensuring the survival of juvenile fishes. These habitats constitute one of the fishing areas targeted by small-scale fishermen in the least developed countries, including Madagascar. The accessibility of these habitats at low tides makes it an ideal fishing area for mosquito seine nets as observed for instance in SW Madagascar and beach seining in Kenya. However, this practice negatively impacts fisheries production due to catches of high numbers of juveniles, in Madagascar and Kenya mostly composed of shoemaker spinefoot rabbitfish (Siganus sutor). Despite Malagasy laws that forbid the deployment of mosquito seine nets, fishermen continue with their use. In Kenya, the beach seines have been outlawed but enforcement remains a challenge. This context highlights the need for management measures and alternative sources of income for a sustainable use of marine resources and for improving the fishermen livelihood. The ongoing consortium research entitled “Fish juvenile recruitment in coastal habitats of western Indian Ocean” was funded by MASMA program administered by WIOMSA. It is an interdisciplinary research program intending to understand recruitment patterns of shoemaker spinefoot rabbitfish (Siganus sutor) in coastal habitats of Kenya and Madagascar. It explores evidence-based solutions for improving the welfare of coastal communities and sustainable use of marine resources. Research activities were divided into four work packages. In WP1, Siganus sutor recruitment patterns in coastal habitats were targeted to identify the nursery ground and recruitment periods. It is based on juvenile fish sampling at four coastal habitats (mangroves, seagrass meadows, intermediate areas and seagrass associated with the coral reef) in Madagascar during twelve months. In WP2, sampling for the analysis of the population connectivity of S. sutor for detecting the sources of juveniles in the coastal habitats at five sites along the western coast of Madagascar was completed. In WP3, ecological models for predicting the arrival of newly settled S. sutor will be based on historical and newly collected data (WP1) using the random forests algorithm. Predictors are composed of remotely sensed oceanic conditions and a post-larval supply index calculated from post-larval sampling in the coastal habitats using light-traps. Like juvenile sampling, post-larval sampling was performed three nights per month which will cover all the juvenile sampling periods. In WP4, fish feeding behavior is studied and experiments on capture-based juvenile fish grow-out are ongoing at the Belaza aquaculture facilities (Toliara, Madagascar). Eight fish grow-out treatments focusing on three stocking densities, three fish diets, and pond dimensions are being tested. In WP1, about 5,720 juvenile individuals were obtained from 120 juvenile fish samples. The standard length of each of these individuals were measured for analyzing the spatial distribution of S. sutor. The nursery ground and recruitment seasons for S. sutor emerged from our research. The findings will be presented orally by PhD student Helga Berjulie Ravelohasina during the symposium. In addition, about 360 epifaunal community samples were obtained between July 2021 and April 2022. The spatial distribution of abundance, diversity and richness of epifauna associated with seagrass will be presented in a poster by MSc student Mory Justino. In WP2, at each location, 45 individuals were sampled, for a total of 225 adults for Madagascar and 180 from Kenya. Genotyping is in progress. In WP3, monitoring of the newly settled fish, in parallel with post-larval sampling, is in progress and should be completed by June 2022. In meantime, the extraction of remotely sensed oceanic conditions covering the sampling periods is being processed with R programming for the period. In WP4, preliminary results on fish grow-out identified the best fish diet and the most optimal stocking density at the smallest size (about 2 cm of standard length). More details related to these findings will be presented in a poster by master student Nandrianina Maminantenaina. In addition, the gut content and stage isotopes of three ontogenetic stages (i.e. post-larvae, juvenile, and adult) of S. sutor were analysed the natural trophodynamics. The findings are based on 1160 gut contents and muscle tissue samples collected during the warm (October 2021 to February 2022) and cool season (May to August 2022). The potential nursery areas and the main recruitment seasons of Siganus sutor were identified. The oral presentation entitled “The potential nursery areas and recruitment season of S. sutor in Madagascar” by Helga Berjulie Ravelohasina will provide further details of our findings. The variability of food availability related to epifauna concentration will be known in the poster on Mory Justino. The most optimal fish diet as well as the optimal stocking density will be presented in a separate poster entitled “Density and fish diet effect on rabbitfish growth in controlled systems” presented by Nandrianina Maminantenaina.Biology, ecology and aquaculture of rabbitfish14. Life below water2. Zero hunger11. Sustainable cities and communitie

Spatial trophic plasticity of two dominant seagrass-associated fishes in Toliara lagoon (SW Madagascar)

In the seagrass zone of Toliara (South West of Madagascar), the local fishermen deployed seine nets with small mesh size that likely affect the structure of seagrass meadow systems and associated fishes. Nevertheless, the responses of fish communities living in this ecosystem to these threats are currently unknown for the WIO region. This present study aims to evaluate the trophic diversity of fishes living in seagrass beds and to assess their trophic plasticity. More specifically, along this work, we compare the diet of two dominant species (the rabbitfish Siganus sutor and the goby Oplopomus oplopomus) found in the catches of local fishermen on seagrass beds and we evaluate a potential variation in their trophic ecology. The sampling of fishes was achieved by collecting the catches of traditional fishermen using bottom seine nets. Fish sampling occurred in front of the Ankilibe fisherman village in December 2017 and 2018, and in front of the village of Sarodrano (December 2018). This sampling strategy allowed us to make spatial and temporal comparisons. In order to assess the diet of these two fish species, stomach content analyses was performed on 40 specimens/species/village/year. Using a binocular microscope, animal preys were identified to the level of class and assigned to the planktonic or the benthic compartment. Plant items are classified as either phytoplankton, fragments of algae, or seagrasses. The trophic niche of each fish populations was quantified in two ways: as a percentage of occurrence and as a mean percent composition of each item in the gut content. The rabbitfish has gut filled by 50% of detritus, 40% of fragments of seagrass, and 10% of phytoplankton. The diet of the goby was more diversified and composed of 40% of detritus, 25% of copepods, 20% of operculum, 10% of gastropods and 5% of ostracods. There was no significant difference between the two sites along the two years of campaign. An isotopic analyses will be effectuated soon to know the trophic position of the studied fishes

Variation of the trophic ecology in two teleost species associated to seagrass beds at Toliara Reef (Madagascar)

editorial reviewedSmall scale fishermen in Toliara (South West of Madagascar) extensively use small mesh seine nets in the seagrass zones and this practice may affect the structure of seagrass-associated fishes by catching small fish, juvenile and even post-larvae. The general objective of the present study is to evaluate the trophic ecology of two dominant teleost species (the rabbitfish Siganus sutor and the goby Oplopomus oplopomus) from catches of local fishermen using bottom seine nets. More specifically, this study aims to evaluate the temporal and spatial variation of the trophic ecology of those species by using stable isotopes and stomach contents. The sampling was carried out in seagrass beds in front of two villages: Ankilibe on December 2017 and 2018, and in Sarodrano on December 2018. To assess variation in diet, stomach content analyses were performed on 40 specimens/species/village/year. Preys were identified to the level of class and then assigned to different food groups. Stable isotope analyses were performed on muscles to complete information on variation in feeding habits. The combination of these methods revealed that the diet of species varies according to the year and the site of collection. This probably results from plasticity and accommodation to the availability of food sources and changes in environmental factors. In both species, the type of food consumed remains the same but the difference is observed in the frequency and the abundance of prey. Trophic variation is also observed over relatively long period of time as it is confirmed by stable isotope analyses. This study clearly demonstrates the trophic ecology of these two seagrass-associated fishes vary across time and space but additional works are now needed to understand which factors drive this variation.2. Zero hunger1. No povert

Density and fish diet effect on rabbitfish growth in controlled systems

editorial reviewedBackground Decrease of fish stock and poverty lead small scale fishermen to deploy destructive fishing gear. In South West region of Madagascar, fishermen using mosquito seine net catch up to 50% of juvenile fish and up to 42% of these juveniles belong only to the rabbitfish Siganus sutor. This fishing practice certainly disturbs the structure of adult fish populations and aquaculture may be one of the effective solutions to alleviate the problem of stock depletion. Rabbitfish possesses most of the desirable characteristics for aquaculture (e.g.: herbivory and responsive to artificial food, survive in high rearing densities…) and the culture of several Siganus species (S. rivulatus, S. canalicullatus, S. sutor,…) have been tested in many countries. However, no study has been conducted on rearing S. sutor in Madagascar. Here, we investigated the possibility to develop the aquaculture of S. sutor regarding the growth performance of juveniles reared with different food and at different stocking density. Methods We collected catched post-larvae from fishermen at a lagoon south to Toliara. Fish grow-out experiments were conducted in outdoor rearing tanks with an initial mean body weight of 0.75 ±0.16 g and length of 3.06±0.17 cm. The 200l rearing tanks were filled with 150l of seawater that was renewed once a day up to 80% of its total volume. Three densities (120, 320 and 520 fish/m3) were tested over a period of two months (from February to April 2022). In parallel, the performance of two fish diets was tested: an artisanal fish food (composed of fish meal, soya flour, rice bran, corn bran, cassava flour and peanut meal) and a locally produced industrial fish food (composed of corn, soya flour, wheat, vitamins, oil…). Fish were fed five times a day every two hours from 8 a.m. to 4 p.m. at 12% of fish biomass per day. Physico-chemical parameters as dissolved oxygen, temperature, salinity, turbidity and pH were recorded three times a day to explain their effect on livestock growth and survival. Results The industrial fish food is significantly more efficient than the artisanal one (p<0.05). We observed an increase of length and weight of 2cm and 3g for fishes fed with the industrial food and 0.85cm and 0.85g for those fed with the artisanal food. Feed conversion ratio is 1.28 for industrial food while it reaches up to 4.78 for artisanal food. Nevertheless, fish fed with industrial food represent a lower survival rate (values). Significant difference between the three rearing densities was also observed (p<0.05) with the best growth rate in terms of length and weight observed at the tank with the density of 320 fish/m3 closely followed by the density of 120 fish/m3. The highest survival rate was observed at the density of 320 fish/m3 and the lowest survival rate was observed at the density of 520 fish/m3. Moreover, all fish in two out of four higher density tanks died after 31 and 54 days of rearing. Conclusion To conclude, the present study suggests the possibility of rearing rabbitfish post-larvae in rearing tank using essentially locally produced food. Controlling the rearing of juvenile rabbitfishes can be suggested as a practical approach to improve the survival rate of these juvenile fishes through sea ranching and for improving livelihood of local communities through alternative activity. Studies are continuing on the grow-out of juveniles in tanks to really develop aquaculture of this speciesFish juvenile recruitment in coastal habitats of Western Indian Ocean2. Zero hunger14. Life below wate