Transition metals are essential for health, forming integral components of proteins involved in all aspects of biological function. However, in excess these metals are potentially toxic, and to maintain metal homeostasis organisms must tightly coordinate metal acquisition and excretion. The diet is the main source for essential metals, but in aquatic organisms an alternative uptake route is available from the water. This review will assess physiological, pharmacological and recent molecular evidence to outline possible uptake pathways in the gills and intestine of teleost fish involved in the acquisition of three of the most abundant transition metals necessary for life; iron, copper, and zinc.

P. Carriquiriborde (from the Environmental Research Centre, National University of La Plata-CONICET, La Plata, Argentina), R. D. Handy, and S. J. Davies (School of Biological Sciences, University of Plymouth, UK), have published a report entitled: Physiological modulation of iron metabolism in rainbow trout (Oncorhynchus mykiss) fed low and high iron diets. From the abstract:

Iron (Fe) is an essential element, but Fe metabolism is poorly described in fish and the role of ferrireductase and transferrin in iron regulation by teleosts is unknown. The aim of the present study was to provide an overview of the strategy for Fe handling in rainbow trout, Oncorhynchus mykiss.

J. Burke and R. D. Handy (again, from the School of Biological Sciences, University of Plymouth, UK), have published a report entitled: Sodium-sensitive and -insensitive copper accumulation by isolated intestinal cells of rainbow trout Oncorhynchus mykiss. From the abstract:

The pathway for copper (Cu) uptake across the mucosal membrane into intestinal cells has not been elucidated in fish. Copper accumulation in freshly isolated intestinal cells from rainbow trout Oncorhynchus mykiss was measured after exposure to 0–800 µmol l^–1 CuSO₄ for 15 min.

Bui Xuan Men, Brian Ogle, and T R Preston have published research findings entitled Use of restricted broken rice in duckweed based diets for fattening Common and Muscovy ducks (Livestock Research for Rural Development, Volume 8, Number 3, September 1996). From the abstract:

A feeding system of restricted levels of broken rice (60 to 80 g/day) with free access to fresh duckweed appears to be appropriate for the Common type of duck typically used for foraging on rice fields throughout Vietnam. More research is needed in order to ascertain if the apparent capacity of Common ducks to eat large quantities of duckweed really is a comparative advantage and, if so, how this can best be used to improve the economic benefits to small scale poor farmers.

The authors have also published Duckweed (Lemna spp) as replacement for roasted soya beans in diets of broken rice for fattening ducks on a small scale farm in the Mekong delta (Livestock Research for Rural Development, Volume 8, Number 3, September 1996). From the abstract:

There was a slight indication (P=0.1 for females and 0.34 for males) that carcass yield was reduced on the duckweed treatments but differences were small. There were no differences in weights of chest and thigh muscle nor in heart and liver weights.

For farmers growing the duckweed there were economic benefits on all duckweed diets with best results from the complete substitution of the soya beans.

Paul Skillicorn, William Spira, and William Journey have published an extensive report entitled Duckweed Aquaculture – A new aquatic system for developing countries (The World Bank – Emena Technical Department, Agricultural Division). From the foreword:

Although duckweed species are familiar to most people who have seen the tiny aquatic plants covering stagnant water bodies, few people realize their potential. Until a few years ago, man made little use of duckweed species. Their unique properties, such as their phenomenal growth rate, high protein content, ability to clean wastewater and thrive in fresh as well as brackish water, were only recognized by a few scientists.

Prior to 1988 duckweed had been used only in commercial applications to treat wastewater in North America. In 1989 staff of a non-governmental organization based in Columbia, Maryland, The PRISM Group, initiated a pilot project in Bangladesh to develop farming systems for duckweed and to test its value as a fish feed. An earlier project in Peru investigated the nutritional value of dried duckweed meal in poultry rations.

The results of the pilot operations were extremely promising; production of duckweed-fed carp far exceeded expectations, and dried duckweed meal provided an excellent substitute for soy and fish meals in poultry feeds. Duckweed could be grown using wastewater for nutrients, or alternatively using commercial fertilizers.

During start-up of the pilot operations it also became apparent how little is known about the agronomic aspects of producing various species of the duckweed family, and exactly why it is so effective as a single nutritional input for carp and other fish.

Although these pilot operations were located in South Asia and Latin America, the results suggested that the plant would be important as a source of fish and poultry feed and simultaneously as a wastewater treatment process in selected areas of the Middle East, particularly in Egypt and Pakistan.

Technical and agronomic information about duckweed culture and feed use, and details of farming duckweed and fish in a single system, are not easily available to the general public, let alone to fish farmers in developing countries. The pilot operations in Bangladesh demonstrated that duckweed and fish culture can succeed commercially, although such ventures would initially require technical assistance and information. In many other areas of the world pilot operations linked to applied research may be required to review production parameters before commercial operations should be initiated. This Technical Study was therefore designed to bring together, in one publication, relevant information on duckweed culture and its uses to make people worldwide aware of the potential of this plant, to disseminate the currently available technical and agronomic information, and to list those aspects that require further research, such as duckweed agronomy, genetics and use in animal feeds.

This Technical Study is aimed at the following audiences: (a) established fish farmers who would like to experiment with duckweed as a fish feed, and staff of agricultural extension services involved in fish culture; (b) scientists of aquaculture research institutes who may initiate pilot operations and applied research on duckweed; (c) staff of bilateral and multilateral donor agencies who may promote funding for duckweed research and pilot operations; and (d) wastewater specialists in governments and donor agencies who may promote wastewater treatment plants based on duckweed in conjunction with fish culture.

A new market has emerged for live black sea bass, Centropristis striata L., in fish markets of the northeast United States and Canada. Efforts to culture black sea bass have been hampered by the lack of information regarding optimal grow-out conditions. This research was designed to determine optimal commercial diet, feeding ration, temperature, and salinity for growth of hatchery-reared, juvenile black sea bass. Optimal diet was Zeigler Salmon Starter (compared to Nelson and Son’s Silver Cup Salmon Crumbles, Trout Crumbles, and Rangen Trout and Salmon Starter). Optimal daily feeding ration was 5% (compared to 2.5% and 7.5%). Optimal water temperature was 25º C (compared to 15º C, 20º C, and 30º C), and optimal salinity was 20 ppt or 30 ppt (compared to 10 ppt). Additionally, growth rates, feed conversion ratio, and mortality were calculated in each experiment. This information will be a valuable guide for culturing juvenile black sea bass.

The Marine Resources Research Institute notes that the first reported spawning of black sea bass occurred in 1884 with fish captured off South Carolina and shortly afterwards, ripe fish captured off Massachusetts were spawned. Since then there has been increasing information developed on the culture of this typical reef dwelling species. It is unclear if the 1884 date is correct. There is no doubt about the value of the species, and the, as yet undeveloped, potential for aquaculture.

Randal L. Walker and Deborah A. Moroney from the Shellfish Aquaculture Laboratory, Marine Extension Service, University of Georgia, USA; have published their findings entitled Growth of juvenile black sea bass, Centropristis striata, fed either a commercial salmon or trout diet. They observe:

An opportunity exists to expand Georgia’s commercial fishery by developing aquaculture techniques for the black sea bass, Centropristis striata. Black sea bass fillets sell for $1.25 to $1.50 per 0.45 kg wholesale. However, live, 0.9 to 1.13 kg fish are sold on the sushi market at a wholesale price of $3.50 to $8.00 per 0.45 kg. Little biological information exists concerning the culture of this potential aquaculture species. Six 600-L tanks on a flow-through system were stocked with 12 juvenile fish each (ranging in total length from 153 to 235 mm). Fish were pot-trapped from an offshore population. Tanks (3 for each diet) were randomly assigned a diet of either a commercial trout or salmon chow. Fish were fed a two percent daily ration (grams dry weight of food to grams wet weight of fish). Rations were adjusted biweekly to account for fish weight increases per treatment. Fish fed the salmon chow were significantly heavier after 10 weeks (P = 0.0209) and after 14 weeks (P = 0.0003) than fish fed the trout chow. Fish fed trout chow increased from 196 grams to 304 grams (55% increase) in 14 weeks, while fish fed the salmon chow increased from 163 grams to 386 grams (137% increase). Based upon fish growth and cost of feed, salmon chow is the preferred diet over trout chow for rearing black sea bass.

Again, from the Shellfish Aquaculture Laboratory, Marine Extension Service, University of Georgia, USA; Richard W. Kupfer, Dorset H. Hurley and Randal L. Walker have published their findings entitled A Comparison of Six Diets on the Growth of Black Sea Bass, Centropristis striata, in an Aquacultural Environment. The abstract notes:

Two studies of juvenile black sea bass, Centropristis striata, were conducted to address questions concerning the biological feasibility of aquaculture of C. striata. Juvenile C. striata and rock sea bass, C. philadelphica, were trapped inshore, while sub-adult C. striata were trapped in nearshore waters of coastal Georgia, south of Savannah. Catch composition of inshore trapping was dominated by Centropristis philadelphica (80%). Few C. striata were caught. A comparative growth study of juvenile C. philadelphica and C. striata fed a 3% daily ration (gram dry weight feed/gram wet weight of fish) was performed over 27 days. Overall juvenile C. striata mean increase in growth was 42% ± 5.3% (SE), with 10% mortality; whereas, C. philadelphica mean growth was 14% ± 4.7%, with 65% mortality.

Sub-adult C. striata (164 g mean weight ± 1.74) were subjected to six ration treatments for 18 weeks. Three replicate 500-L tanks were used per treatment, and each tank was stocked with 15 fish. Ration treatments consisted of a high-protein trout feed, a lower-protein trout feed, and an equal mixture of both delivered daily in rations of 2% and 3% dry weight feed/wet weight fish. All rations were adjusted biweekly based upon mean fish wet weight per treatment. At week 16, ANOVA revealed no significant differences (p=0.4096) in mean fish weight among treatments with a pooled treatment mean fish weight increase of 48%. Based on cost effectiveness, the lower-protein 2% ration was judged the optimum choice among the diets tested. This study reinforces the biological feasibility of rearing trapped C. striata on a commercial diet of 2% dry weight feed/wet weight fish.

Further, from the Shellfish Aquaculture Laboratory, Marine Extension Service, University of Georgia, USA; Alan Power, Tiffany Lee, Todd Recicar, Mary Sweeney-Reeves, and Randal L. Walker have published their findings entitled The Growth and survival of juvenile black sea bass, Centropristis striata, on an artificial (Salmon chow) versus a natural (grass shrimp) diet. The abstract notes:

This study examined growth and survival rates of juvenile black sea bass Centropristis striata (Linnaeus, 1758), that were fed two distinct dietary treatments, – one, a commercially available salmon chow and the other a diet consisting of natural live grass shrimp (Palaemonetes pugio, Holthuis, 1949). The diets were fed at a 2.5% ration (grams dry weight of food/ grams wet weight of fish). Fish were reared in replicated (N=3 per dietary treatment) 65-liter flow-through tanks for six weeks between May 7 and June 25, 2001. The fish provided with live shrimp had a mean survival of 69 ± 5.9% and increased in size from an initial mean wet weight of 20.7 grams to 30.4 grams – a growth rate of 0.194 grams per day. A higher mean survival of 80 ± 6.7% and a slightly lower growth rate of 0.184 grams per day (mean wet weight increase from 20.4 grams to 29.6 grams) were recorded for fish fed the salmon pellet diet. However, there were no statistically significant differences detected in either growth (p=0.7849) or percent survival (p=0.4999) between dietary treatments. Based on these results, we recommend using the commercially available salmon chow because of its convenience and supplementing it occasionally with grass shrimp to provide essential amino acids common in natural diets.