Western Indian Ocean JOURNAL OF Marine Science Volume 18 | Issue 2 | Jul – Dec 2019 | ISSN: 0856-860X Chief Editor José Paula Western Indian Ocean JOURNAL OF Marine Science Chief Editor José Paula | Faculty of Sciences of University of Lisbon, Portugal Copy Editor Timothy Andrew Editorial Board Lena GIPPERTH Aviti MMOCHI Sweden Tanzania Serge ANDREFOUËT Johan GROENEVELD France Cosmas MUNGA South Africa Kenya Ranjeet BHAGOOLI Issufo HALO Mauritius South Africa/Mozambique Nyawira MUTHIGA Kenya Salomão BANDEIRA Christina HICKS Mozambique Australia/UK Brent NEWMAN Betsy Anne BEYMER-FARRIS Johnson KITHEKA South Africa USA/Norway Kenya Jan ROBINSON Jared BOSIRE Kassim KULINDWA Seycheles Kenya Tanzania Sérgio ROSENDO Atanásio BRITO Thierry LAVITRA Portugal Mozambique Madagascar Louis CELLIERS Blandina LUGENDO Melita SAMOILYS Kenya South Africa Tanzania Pascale CHABANET Joseph MAINA Max TROELL France Australia Sweden Published biannually Aims and scope: The Western Indian Ocean Journal of Marine Science provides an avenue for the wide dissem- ination of high quality research generated in the Western Indian Ocean (WIO) region, in particular on the sustainable use of coastal and marine resources. This is central to the goal of supporting and promoting sustainable coastal development in the region, as well as contributing to the global base of marine science. The journal publishes original research articles dealing with all aspects of marine science and coastal manage- ment. Topics include, but are not limited to: theoretical studies, oceanography, marine biology and ecology, fisheries, recovery and restoration processes, legal and institutional frameworks, and interactions/relationships between humans and the coastal and marine environment. In addition, Western Indian Ocean Journal of Marine Science features state-of-the-art review articles and short communications. The journal will, from time to time, consist of special issues on major events or important thematic issues. Submitted articles are subjected to standard peer-review prior to publication. 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ISSN 0856-860X Cover image: Macrophthalmus depressus at Saco da Inhaca, Mozambique (© Jose Paula, 2014) Western Indian Ocean JOURNAL OF Marine Science Volume 18 | Issue 2 | Jul – Dec 2019 Table of Contents Mineral content in local feed ingredients used by fish farmers in four different regions of Tanzania Francis P. Mmanda, Jan E. Lindberg, Anna N. Haldén, Torbjörn Lundh 1 Small-scale milkfish (Chanos chanos) farming in Kenya: An overview of the trends and dynamics of production David O. Mirera 11 Natural recovery of mangroves in abandoned rice farming areas of the Rufiji Delta, Tanzania Emmanuel Japhet, Mwita M. Mangora, Carl C. Trettin, Judith A. Okello 25 Assessing the effectiveness of LED lights for the reduction of sea turtle bycatch in an artisanal gillnet fishery – a case study from the north coast of Kenya Timothy Kakai 37 Biophysical factors influencing the choice of nesting sites by the green turtle (Chelonia mydas) on the Kenyan coast Fridah D. Obare , Robert Chira, Dorcus Sigana, Andrew Wamukota 45 Spirulina (Arthrospira fusiformis) as a potential protein source in practical diets for fry mariculture of Rufiji tilapia Oreochromis( urolepis urolepis) Deogratias P. Mulokozi, Matern S. Mtolera, Aviti J. Mmochi 57 Stock status and some biological aspects of Lethrinus lentjan (Lacapede, 1802) from the south coast of Kenya Nathan Mrombo, Chrisestom Mlewa, Cosmas Munga, Julius Manyala 69 The biology of Goldsilk Sea Bream (family: Sparidae) from the inshore waters of north coast Kenya Rashid O. Anam , Cosmas N. Munga, James R. Gonda 77 Sometimes hard to swallow: Attempted feeding on a porcupinefish results in death of both predator and prey Bart Shepherd , Hudson T. Pinheiro, Luiz A. Rocha 87 Broadcast spawning in Porites lutea at Reunion Island (Western Indian Ocean) Lola M. Massé, Armand Daydé, Mathieu G. Séré , Florence Trentin 91 Re-description and new records of a poorly known leucosiid crab, Merocryptus boletisculpta Zarenkov, 1994 (Crustacea, Decapoda, Brachyura) Bella S. Galil 93 Instructions for Authors WIO Journal of Marine Science 18 (2 ) 2019 1-9 Original Article 1 Mineral content in local feed ingredients used by fish farmers in four different regions of Tanzania Francis P. Mmanda 1, 2, Jan E. Lindberg 2, Anna N. Haldén3, Torbjörn Lundh 2* 1 Institute of Marine Sciences, 2 Department of Animal Nutrition 3 Department of Biomedical Sciences PO Box 668, Zanzibar, and Management, Swedish University and Veterinary Public Health, Tanzania; of Agricultural Sciences, Swedish University of Agricultural Sciences, PO Box 7024, Uppsala, PO Box 7028, Uppsala, Sweden; Sweden *Corresponding author: [email protected] Abstract This study investigated the content of selected minerals (P, Ca, K, Na, Mg, Fe and I) in local feed ingredients used by tilapia fish farmers in Tanzania. Analyses were performed on 26 local feed ingredients collected at four different geographical locations in Tanzania (Dar es Salaam, Morogoro, Mbeya and Mwanza). The samples were taken ran- domly from fish farmers, fish feed producers, fingerling producers and animal feed shops or centers near fish farms in each region. The results showed a wide range of mineral concentrations. The highest levels of P was found in fish skeletons (17.8 g kg-1), of Ca in limestone (107.3 g kg-1), of K in gallant soldier (51.0 g kg-1), of Na in marine shrimp (Exhippolysmata oplophoroides) (11.7 g kg-1), of Mg in prawn head waste (4.2 g kg-1), of Fe in azolla (2355 mg kg-1) and of I in full fat soybean (447 mg kg-1). The data on mineral content in feed ingredients can be used as a platform for better-targeted feed formulation for tilapia farming systems. In conclusion, the data suggest that if more than two ingredients are used in the diet, this may be sufficient to meet the mineral requirements of all cultured tilapia species and their hybrids, without inclusion of any mineral premix. Keywords: mineral requirements; feedstuffs; fish pond; aquaculture; tilapia Introduction In Tanzania, fish farmers raise tilapia (Oreochromis Minerals play an important role in the maintenance of niloticus) in extensive systems and/or semi-intensively normal metabolic and physiological functions in fish through fertilisation of the fish culture system (ponds and other animals, which include muscle contraction, and tanks) and provision of supplementary feeds blood clotting, enzyme activity, transmission of nerve (Chenyambuga et al., 2014). To date, the majority of impulses, metabolism and electrolyte balance (Tacon, tilapia fish farmers in Tanzania feed their fish with 1987; Dato-Cajegas and Yakupitiyage, 1996; Assey et al., locally available feedstuffs of both plant and animal 2009). Moreover, lack of essential minerals in the fish origin. However, in order to formulate nutritionally diet may lead to mineral deficiency, resulting in con- appropriate supplementary feeds for fish, accurate ditions such as anaemia, osteoporosis, stunted growth data on the nutrient content of different dietary com- and genetic disorders (Tacon, 1987; Dato-Cajegas and ponents are required. There are some limited data Yakupitiyage, 1996; Fumio et al., 2012; Bhandari and available (based on proximate analysis) on the major Banjara, 2015). nutrient composition (i.e. crude protein, crude fat, crude fibre and ash) of commonly used feed ingredi- Iron (Fe) and iodine (I) are limited in large areas of ents in Tanzania (FAO, 1987). However, information Tanzania and deficiency of these elements causes anae- on the mineral content of locally available feed ingre- mia and goitre in humans. Therefore, Fe and I were dients in Tanzania is lacking. analysed in addition to the macro-minerals calcium http://dx.doi.org/10.4314/wiojms.v18i2.1 2 WIO Journal of Marine Science 18 (2 ) 2019 1-9 | F. Mmanda et al. (Ca), phosphorus (P), magnesium (Mg), sodium (Na) taken and mixed. This process was repeated until the and potassium (K). In fish, Fe is involved in regulatory two quarters selected comprised the desired amount mechanisms controlling blood formation and res- of 100-200 g. The sub-samples were then sun-dried piration, hormone synthesis, fatty acid mechanism for 48 h, packaged and transported to the laboratory (Brody, 1994) and maintaining the integrity of the for mineral analysis. Prior to mineral analysis, sam- epithelium (Naser, 2000). Fish can absorb some trace ples were milled in a blender ( JYL-D020 Powerful minerals such as Fe not only from the diet offered, but Multifunctional Blender Food Processor, Joyoung, also from their external aquatic environment through China) and sieved by hand to pass through a sieve with the gills (Watanabe et al., 1997). The effects of limited 1.0mm circular openings. access to I in fish are not well described, but the effects of I deficiency in fish are suggested to be similar to For analysis of Ca, P, Mg, K, Na and Fe, 1.0 g of milled, those in humans (NRC, 2011). homogenised sample was placed into a weighed por- celain crucible, which was placed in an incinerator and The content of Fe and I, as well as other minerals may ignited at 450 oC until white or grey ash was obtained vary in the fish diet and in the aquatic environment, ( Jorhem, 2000).