Koi - show fish

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Koi, Japanese Carp, show fish (youtube upload). These were taken at the Brunswick Community College Aquaculture center, and are currently being kept in a recirculating system.

The fish are large. Though related to Goldfish, they grow up to over a ft in length. These particular fish measure over ft in length.


One student remarked today, though large enough for food-fish, Koi are more valuable alive (for show fish).


John Baka, an instructor at BCC said a couple of these large koi are carrying eggs and that's the reason for the branches floating in the pool, for the fish to lay their eggs. I presume the eggs will be attached to the branches and can be gathered for culture.

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Yellow Perch

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Yellow Perch Production
Modified in parts from class notes by Dr. Doug Holland, Aquaculture Program, Brunswick Community College

Yellow perch (Perca flavescens) is a relatively new species to aquaculture. This species is widely distributed, ranging from Nova Scotia to South Carolina along the Atlantic seaboard and throughout the Great Lakes region and as far west as the Dakotas and Nebraska. Commercially, it has been cultured in the upper Midwest, and in the Eastern US from Pennsylvania to South Carolina.

The best culture method for yellow perch is widely debated. The material presented here is based on the extensive experiences of staff at Brunswick Community College and yellow perch farmers in Brunswick County, North Carolina.

Broodstock Management and Egg Collection
Eggs may be collected from the wild, but as with all cultured aquatic species, preferred stock comes from domesticated broodstock. In North Carolina, one of the best places to collect wild eggs is from the Perquimmans River, a tributary of Albemarle Sound. The NC Division of Marine Fisheries manages the waters below the old bridge in downtown Hertford, and there is a healthy commercial fyke-net fishery for yellow perch in these waters. The perch are trapped in the nets during their annual spawning runs in February and March, and fertilized egg ribbons may be easily collected from the nets where the fish spawn. Egg ribbons may also be collected in submerged vegetation along the edges of the river.

Domesticated yellow perch broodfish may be held in ponds at densities up to about 500 lbs/acre. The fish should range from ¼ lb to 1 lb+ in size, and there should be a 50/50 mix of males and females. Good egg production generally requires an annual increase in body weight of at least 50%. Broodfish that are feed-trained will grow and reproduce well on pelleted feeds containing about 40% crude protein and 10% crude fat. Yellow perch generally prefer live food and use of a forage species such as fathead minnows in brood ponds, which will likely improve egg production and success with spawning.

Spawning
Spawning in ponds begins in mid-February. It is a good idea to encourage growth of submerged aquatic vegetation around the edges of brood ponds for the perch to use as a spawning substrate. If there no submerged vegetation exists around the edges, discarded Christmas trees work for spawning substrate. A wetsuit and snorkeling mask may be necessary to recover egg masses in deeper water. Perch often spawn wherever bottom vegetation occurs, across the bottom of shallow ponds. In ponds where the entire bottom is covered with rooted submerged vegetation, only a small percentage of the egg ribbons may be recovered. Egg ribbons should be placed in a cooler full of well-oxygenated water and transported to the hatchery within thirty minutes. Eggs consume oxygen, therefore if the oxygen is depleted the eggs will die.

Fry Production
We have found that standard catfish egg baskets suspended in ordinary aluminum catfish hatching troughs work very well for yellow perch egg incubation and hatching. Instead of rotating paddles we simply place large air stones in the troughs to provide water agitation and aeration. Water from a small header pond is circulated through the troughs at about 1gpm per 100 gallons of trough capacity. During periods of very cold weather, the flow of pond water should be slowed down, and well water (66ºF in Brunswick County) may be added along with pond water or large aquarium heaters may be used to increase the temperature in the troughs. Be very careful using well water in yellow perch hatching troughs. Well water often contains hydrogen sulfide and other toxic compounds. The amount of hydrogen sulfide required to kill yellow perch eggs is barely detectable, and may not be noticeable to anybody but those with the most sensitive noses.

At water temperatures in mid-60'sºF, yellow perch eggs usually hatch in 5-7 days. Hatching time may be up to two or three weeks at colder temperatures. After the fry hatch out, they may be concentrated using light. A 100-watt incandescent "heat lamp" outfitted with a clamp and an 8" diameter reflector, works well and is available at most hardware and builder's supply stores. The light can be clamped to the side of the hatching trough. If all doors and windows in the hatchery are covered and the lights are turned off, the fry will collect under the light in a couple of hours. They are then siphoned into a five-gallon bucket and stocked into fingerling production tanks or prepared nursery ponds.

Fingerling Production in Tanks
Yellow perch fingerlings may be produced in tanks supplied with pond water containing adequate densities of wild zooplankton (a mixture of rotifers and small microcrustaceans is best). 7 days post-hatch, fry should be offered Fry Feed Kyowa B (<250 microns). This feed is available in the US through BioKyowa in Cape Girardeau, Missouri. A 500-gram packet costs about $45, but very little is required to get the fry started on an artificial diet. Once fry are feed-trained, they should be slowly changed over to a finely ground (500 microns or less) freeze-dried krill. After 3-4 weeks, slowly substitute a good quality #0 soft-moist crumble feed, with at least 45% crude protein and 20% crude fat. Rangen Feeds makes an excellent soft-moist crumble feed. After another 3-4 weeks, dry crumbles containing at least 45% crude protein and 12-16% crude fat may be substituted for the soft-moist crumbles.

Yellow perch fry held in troughs or tanks may experience problems with swim-bladder inflation, and settle on the bottom of the tank where they eventually smother and die.

Perch suffering from swim bladder inflation failure (sinkers) are reported from wild populations of Lakes Constance, Geneva, Lucerne, Sempach, and Zurich in central Europe. Sinkers were found at all 7 locations considered in a survey. Relative abundance of sinkers varied from 0.1% to 7.9% of local perch populations. Sinkers from wild populations and sinkers reared in the laboratory showed similar behaviour and malformations. Information on sinker syndrome in 26 physoclistous species is compiled. It is found to be a widespread problem in aquaculture, but is reported here for the first time from natural populations.
Springerlink, Zoologisches Museum, Universität Zürich, Winterthurerstrasse 190, 8057 Zürich, Switzerland

This is often caused by an ultra-thin layer of lipid (fatty material) that forms on the surface of the water in hatching troughs as the eggs hatch out (the hatching eggs are the source of the lipids). This can be avoided by adding a tiny drop of dishwashing liquid to the surface of the water when the eggs are about to hatch. This may be repeated as necessary. The soap breaks up the surface tension of the water and allows the fry to get the "gulp of air" needed to inflate their swim bladder. Don't overdo it with the soap. It only takes a tiny drop to do the job. More than this will harm the fish.

Fingerling Production in Ponds
Most yellow perch fingerlings are produced in prepared nursery ponds. Small ponds up to three acres in size and 3-5 feet deep work best. The ponds should be drained and thoroughly dried during autumn. Spread about 300 lbs/acre of cottonseed meal over the pond bottom prior to flooding. Begin flooding the ponds 3-4 weeks before anticipated egg hatching. It takes much longer for an adequate zooplankton bloom to develop in winter than in spring or summer, when only a few days may be required to obtain adequate densities of rotifers and early instar microcrustaceans (the preferred food of newly hatched yellow perch). Begin examining the water for zooplankters after 10-14 days. If very few organisms are observed at this time, try inoculating the pond with water from another pond that contains high densities of zooplankters.

Fingerling production ponds should also be fertilized with liquid ammonium polyphosphate (10-34-0 fertilizer grade) or phosphoric acid (0-54-0 fertilizer grade) to promote the development of a phytoplankton bloom. Inorganic fertilizers generally don't work very well to promote phytoplankton blooms at the low water temperatures experienced in January. Begin adding the liquid fertilizer around mid-February at a rate of 1 gallon/acre of ammonium polyphosphate in soft-water ponds, or up to 3 gallons/acre of phosphoric acid in hard-water ponds. Don't use ammonium polyphosphate at the high rates required for hard water in yellow perch fingerling ponds. Yellow perch are very sensitive to ammonia and higher rates may be very detrimental to the fry. Repeat fertilizer application each week until a bloom appears on the pond. Fertilize as needed to maintain a Secchi disk depth of 12-18".

Stock the prepared nursery ponds with 200,000 to 500,000 fry. Lower stocking densities will result in larger fingerlings at harvest. Higher stocking densities may result in greater numbers of fingerlings, but they will be smaller at harvest. Yellow perch fingerlings may be harvested from ponds after 50-70 days. The fingerlings may be trapped with up to 40-50% success, but the remaining fish must be seined or concentrated by draining the pond into pre-constructed catch basins or kettles around the pond drains. Be very careful handling the fish and plan to harvest on cool days.

Yellow perch fingerlings produced in ponds must be feed-trained. Harvested fingerlings should be placed in raceways or tanks with flowing water and good aeration at a rate of 35-40 lbs per 1000 gallons of water. Following stocking of the fingerlings, the water in the tanks or raceways should be stopped and treated with oxytetracycline at a rate of 100g active ingredient per 1000 gallons of water to prevent fin rot. This bacterial disease will kill 30-40% or more of the fingerlings if this is not done.
Begin offering the perch finely ground freeze-dried krill at a rate of 5% of body weight per day. Once the perch are actively feeding, slowly switch the diet over to a #0 soft-moist crumble containing 45% crude protein and 20% crude fat. After about two weeks, dry crumbles should be substituted for the soft-moist feed. Yellow perch are not completely feed-trained until they have been taking artificial feed for 3-4 weeks. By this time, fish that are not feed- trained will starve to death and may be cannibalized by the other fish.

At this stage of their lives, yellow perch will consume up to 15% of their body weight each day. If they are not fed to satiation, they will cannibalize their siblings, and large losses (as much as 50-75%) have been attributed to this.

Frequent grading may also reduce cannibalism. Be careful not to grade the fish too frequently. Excessive handling may stress the fish and cause an outbreak of fin rot or other bacterial diseases. Yellow perch should not be handled at all at temperatures above 26ºC.

Fin rot
Fin rot in perch is generally related to stressful handling and/or severe water quality stress during harvesting. With mild cases can be treatment in a salt bath of 5 ppt will assist the fish in fighting the infection. Severe cases should be quarantined where possible and a veterinarian called to prescribe registered chemicals.
The tolerance for perch with fin rot for live or whole fresh chilled is:
• Minor cases: 5%
Major cases: nil - Fish should be processed for the fillet market.
From Size Gradings, on Fin Rot

Grow-Out to Market Size
There are many opinions among both producers and researchers about how best to produce market-size yellow perch. The market size for yellow perch is quite small compared with other cultured species, ranging from about ¼ pound (115 grams) up to about 1/3 pound (150 grams). Canadian markets will take some yellow perch that are both larger and smaller than this, but 115-150 grams is the preferred range in most markets. At Brunswick Community College and at yellow perch farms in Brunswick County, we have attempted to grow yellow perch to market size in a variety of ways. We have grown perch in open ponds, and in cages placed in ponds. We have successfully produced market-size perch in indoor recirculating systems, and in "outdoor recirculating systems", which utilize tanks supplied with water that is recirculated through ponds which function in particulate removal and biofiltration.

Pond Culture: This has proven to be a rather unreliable way to grow yellow perch to market size. We have tried many different stocking regimes, and have generally found that perch feed poorly, grow slowly (and with great variability in growth rates), and cannibalize each other heavily in open pond culture. When 1-2" feed-trained fingerlings are stocked into ponds, it generally takes 18-24 months to produce market-size fish, and a large number of the harvested fish will be outside the optimum range of size for foodfish markets. There is also low survival (40-60%) due mainly, we believe, to cannibalism. It also appears that pond-reared fish generally have a lower dressing percentage (41-45%) compared with tank-reared fish (45-51%). Total production in ponds ranges from 500 to 2500 lbs/acre per production cycle. This translates to an average production of 700-1000 lbs/acre per year, which is probably not a profitable production rate under most circumstances.

Cage Culture: Yellow perch feed and grow well in cages, as long as water quality is maintained at high levels. Yellow perch are very sensitive to ammonia. When cages are placed in shallow water, fecal material and uneaten feed tend to build up beneath the cage and cause localized problems with high ammonia concentrations in the waters around the cages. For this reason, cages should be placed in deep water (at least twice the depth of the cage) or the cages should be designed so that they can be easily moved. Aeration should be available at all times to maintain water quality around the cages. Large fingerlings (at least 4") must be stocked into cages. Smaller fingerlings necessitate the use of small mesh sizes for cage materials. Small mesh cages do not provide adequate water circulation, and may become completely clogged with algae and debris.

Indoor Recirculating Systems: This has become the most popular method for producing market-size yellow perch. Most market-size perch are produced in the upper Mid-Atlantic and upper Midwest states, where winter temperatures are cold enough to dramatically slow growth rates. Perch are maintained on feed and actively grow year-round in indoor climate-controlled recirculating production systems. They are routinely stocked in these systems at densities up to four fish per gallon, with production of up to 0.5 lb per gallon of tank capacity. Perch may be stocked in these systems at any size, but many producers prefer large stockers, in the range of 3-5". These fish are very expensive, on the order of $0.20 to $0.35 per fingerling (at an average cost of $0.07 per inch for feed-trained fingerlings). There is no reason perch cannot be stocked at smaller sizes, around 1-2", which would cost about $0.07-0.14 per fish. Since it takes 3-4 fish to weigh one pound at harvest, this would provide considerable savings to the producer. Also, yellow perch are prone to significant variability in growth rates, with resultant cannibalism. Cannibalism rates of up to 50% are not uncommon when fish are underfed and un-graded. Feeding to satiation and frequent grading, especially at smaller sizes, will greatly reduce problems with cannibalism. Be careful not to handle fish so frequently that they become stressed, especially at temperatures above 26ºC.

Outdoor Recirculating Systems: The use of outdoor tanks with water supplied by gravity flow from an adjacent pond has been shown to be a very cost-effective method for producing yellow perch in southeastern North Carolina. The water is returned to the pond by a low-head sewage pump. The pond also serves as a particulate settling chamber and biofilter. One example of the various systems now in place in Brunswick County is one that is able to produce at least 5000 lbs of market-size yellow perch in three 5000-gallon tanks supplied with water from a 1-acre pond. By separating solid fecal matter and uneaten feed into a separate small retention pond, we believe this system may be able to produce perch at much higher levels on a per-acre basis. Such a system is currently being planned for construction at Brunswick Community College.



The smaller sizes should be fed as often as is practical, up to three or four times each day. Fish more than 4 or 5 inches only need to be fed once or twice each day.

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Rainbow Trout Culture II

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Rainbow Trout Culture II
Notes modified from Dr. Doug Holland, Aquaculture, Brunswick Community College.

Feeds and Feeding
Many companies manufacture high quality trout feeds. Feeds in all particle sizes are available, beginning with crumbles #00, 0, 1, 2, 3, 4 which are best suited for fry and small fingerlings. Advanced fingerlings, larger fish and broodstock should be fed floating or sinking pellets 1/16", 1/8", 3/16", etc.

Homemade diets should be avoided, especially unsterilized trash fish. The risk of disease combined with a diet that is usually nutritionally unbalanced, may result in a variety of nutritional disorders.

Feed utilization by the fish is of utmost importance.
Feed should be distributed in a manner that all fish get something to eat. To do so reduces variations in growth rate genetically inherent in fish. Such genetic predisposition are made worse by uneven distribution of feed and aggressive feeding instinct among individual fish.

Feeding can be made less efficient and/or wasted because:
1. Low temperatures
2. Excessive flow-through rates
3. Excessive water turbidity (trout feed by sight)

Automatic feeders may reduce labor costs, but also may result in poor feed conversion and/or uneven feed distribution, which contributes to higher feed cost. The cost of feed is generally higher than labor costs, therefore automatic feeders may represent a "false economy".

The best way to keep track of feed conversion is to keep accurate records of daily consumption rates and frequent sampling of fish for average size and weight. Fish sampling should be done preferably every two weeks but at least once per month.

Grow-Out Techniques
"The least-cost producer wins"
To develop a production strategy with any culture, available markets should be investigated first, then "working backwards" to determine a workable production strategy. By knowing the particular requirements of any market, the producer can develop methods to produce at the lowest unit cost.

Most consumers want an even supply of food-size fish throughout the year. Fluctuations in demand occur during certain times throughout the year:
- Thanksgiving and Christmas. Less people eat fish and seafood during the Holiday season.
- During Lent many people give up meat for religious reasons. Lent begins on Ash Wednesday the day following "Mardi Gras" and ends at Easter. The demand for fish and seafood increase during this time of year.

Fertilized eggs/embryos of trout, called "eyed trout eggs" are available from various parts of the world during most of the year.

For grow-out to market size fish, differential growth rates in fingerlings make stocking almost any size during most of the year possible. Differential growth rates during grow-out to market size further contributes to overall size variability, making market-size fish available throughout the year.

Growth rates of fish may be controlled by any combination of:
1. Genes. Natural variability exists between different strains and individuals within strains.
2. Feeding rates
3. Temperature which is not under the control of the farmer, but may be anticipated and utilized in overall production strategy.
4. Current/flow rate - swimming against rapid currents requires more energy which reduces growth rate at a constant feeding rate.
5. Grading. While it is best to have every fish growing at an optimum rate, naturally the rate will differ from individual to individual and from strain to strain. This can be used to ensure that market size fish are available at any and all times of the year.

Grading the fish
Grading should be done on a regular basis, but if done too frequently it risks increased stress and reduction in production levels.
A variety of grading methods are available. Producers should choose carefully to ensure the greatest grading efficiency with the least stress on the fish as possible.

Effluent Management
Types and amounts of allowable effluents from trout farms are governed by state and federal regulations. The most important of these is the NPDES permit, National Pollutant Discharge Elimination System. An NPDES permit is required of trout farms that produce more than 30,000 lbs annually.

Trout streams and other coldwater receiving waters are likely to be more profoundly affected by effluents than warmer waters, due to the low natural nutrient levels of most coldwater streams.

Suspended solids are the most serious effluent problem:
1. Contribute to Biochemical Oxygen Demand (BOD).
2. Can completely cover the bottom of receiving streams.

Settleable Solids = Suspended solids that settle out of standing water in one hour.

0.3 lb. of settleable solids are produced for every lb of feed offered to the fish in trout raceways.

Settleable solids are typically removed from effluents through the use of sedimentation basins.

Total pollutants in trout farm effluents come from many sources, but most originate from feed offered to the fish.

Levels of effluent pollutants due to feed can be calculated using the equation:

Average ppm pollutant = Pollutant Factor x Lbs Feed
Water Flow (gpm)

Pollutant factors for this equation:

Total ammonia 2.67
Nitrate 7.25
Phosphate 0.417
Settleable solids 25.0
BOD 28.3

Example
250 lbs of feed are offered each day in a hatchery with 1,000 gpm of water flow. What is the concentration in ppm of settleable solids in the effluent?

25.0 x 250
------------- = 6.25 ppm
1,000


An example of a sedimentation basin design from arizona.edu on Filtration and Biofiltration
Sedimentation basin design: Wide inlet (to reduce velocity), a surface area of .7 to 1.4 sq. ft. of basin per gpm flow (for feces with a specific gravity of 1.01 or greater), wide outlet weir (never a stand pipe), no baffles (which increase velocities) and a simple waste drain. A depth of just a few inches is enough for most designs.
Source: University of Arizona
Sedimentation Basins
These are usually tanks, ponds, lagoons, etc. which serve the purpose of slowing velocity of the water, and allow suspended solids to settle to the bottom.

Four factors taken into account for design of sedimentation basins:
1. Retention time
2. Density of waste solids
3. Water velocity and flow distribution
4. Water depth

Retention time = average period that a unit of water remains in the basin.

Retention time ranges from 15 minutes to 2 hours. For a given rate of flow, retention time increases with area and depth of the basin.

If not carefully engineered, a sedimentation basin will have an area of rapid flow down the middle, with backwater "dead zones" where water stagnates and is replaced very slowly. A system of baffles should be incorporated into the design to ensure even flow through the basin.

The basin should be about 1.5 feet deep. A shallower basin promotes scouring of the bottom, keeping solids suspended throughout the basin. There may not be enough time for solids to settle out completely in a deeper basin.

There are several types of sedimentation basins:

1. Linear clarifier - a modified concrete raceway.

Water should enter the raceway through a series of screens to distribute flow and reduce turbulence.

2. Lagoons - usually a shallow earthen pond.

The larger the pond, the more effluent it can accommodate.

3. Commercial Settling Systems

There are many types and designs of these systems available. They all generally incorporate baffles and settling tubes. This type requires less space and retention time than linear clarifiers or lagoons. They are expensive, and usually impractical in commercial aquaculture.

Solid Waste Disposal
Over half of all nutrients released by trout farms are in the form of settleable solids. The sludge from sedimentation basins is a high quality organic fertilizer. It may be composted and made available to organic farmers, gardeners, etc. It may be possible to market such material to help offset costs of waste management.

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Feeding the Catfish

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Chad at BCC told us he was going to feed the catfish, and if we wanted to see them come up to eat... to follow. I captured some neat shots today, with the catfish surfacing for food pellets.




Image enlargement
Catfish surfacing...

Image enlargement
Catfish surfacing... and food pellets floating nearby in the water.
Catfish video - Windows Media Player
feeding_catfish.wmv
4967 kilobytes / 4.96 meg

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Can fish farming save depleted cod?

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February 07, 2007
By Simon Rabinovitch
LONDON (Reuters) - Cod, a mainstay food from Britain to Brazil, all but disappeared from Canadian waters in the 1990s after years of overfishing, and scientists say a similar fate awaits the shoals of the North Sea.

But fish farms are putting cod back in North Sea water, at least within enclosed sea pens, easing the strain on wild fisheries and, fish farmers say, protecting a species that would otherwise be fished into extinction.

Off the Shetland Islands in northeast Scotland, Johnson Sustainable Seafoods is providing what it says is a model of good farming practice.

Given more space to roam around their pens and fed a natural diet, the Shetland cod farm has won the backing of Britain's Organic Food Federation.

"Fish farming can be the saviour," said Karol Rzepkowski, managing director of the company. "It takes a little bit of left-field thinking, having the right ethic and the right ethos, and it can be done right," Rzepkowski said.

RESTOCKING THE WATERS?

The Shetland farm expects to harvest 2,500 tonnes of cod this year and aims to double its output in 2008. Other producers include Pan Fish in Norway, which recently acquired Marine Harvest to become the world's leading fish farming group.

Much more is needed, though, if cod farming is help redress the decline in the wild population. Globally, the wild catch has plunged to about 1 million tonnes a year from 4 million in the 1960s. Stocks in northern waters, especially the Barents Sea, remain strong, but the World Wildlife Federation and others warn that overfishing is changing that.

Experts say it will be a long time before farmed cod production rivals the wild catch. "I don't think at this stage we are anywhere close to that," said Barrie Deas, chief executive of the Britain's National Federation of Fishermen's Organisations (NFFO).

Farmed cod will also be hard pressed to match the popularity of farmed salmon, more than one million tonnes of which were consumed last year, say aquaculture analysts at Norway's Kontali Analyze.

Salmon is better suited to aquaculture and its distinctive pink hue offers a marketing advantage over cod's white anonymity, Kontali Analyze noted.

TRULY SUSTAINABLE?

For now, attracting more attention than production figures is Johnson's claim that it is raising the world's first organic, sustainable cod.

One staunch opponent of the aquaculture industry is Bruce Sandison, chairman of the Salmon Farm Protest Group, based in Scotland. Barely pausing for breath, he reels off a list of problems: diseases have spread in crowded sea pens; farmed fish have escaped and damaged wild stocks; the farmed product is less healthful for consumers.

"The same thing is going to happen with cod," he said, pointing out that a disease called Francisella decimated about half the cod in a Norwegian fish farm in 2005.

"What we're playing with here is a wild species that has existed on the planet since probably the end of the last ice age. We're pushing that toward extinction, and we're going to replace it with a totally artificial species."

Questions have also been raised about the sustainability of fish farming. It takes a huge cull of smaller wild fish, about four tonnes worth, to feed every single ton of the captive population.

The Shetland farm has found a way around this problem. All its cod are fed with the "off-cuts" -- scraps destined to be discarded -- of wild fish already caught for human consumption.

"I doubt if that would be practical if the kind of expansion that is envisaged takes place," said Deas of Britain's National Federation of Fishermen's Organisations.

Aware of this limitation, researchers are beginning to consider alternative food sources, raising a distant prospect of truly sustainable fish farming. And if organic farms are also successful in curtailing harm to the broader environment, aquaculture could win over more of its critics.

"There would not be a lot left for us to moan about," said Tom Pickerell, a fisheries policy officer at the World Wildlife Fund-UK.

FROM SEA PEN TO TABLE

In the meantime, Johnson's Shetland cod has been able to lure a growing number of customers. Sold under the No Catch brand name, it is available in hundreds of Tesco and Sainsbury's supermarkets across Britain.

"It allows people to enjoy cod but without having any sort of guilty conscience about where that cod is coming from," said Joanna Keohane, spokeswoman for Tesco.

The avoidance of guilt is clearly a powerful influence in some markets: British shoppers are happy to pay a premium for the farmed cod over its wild brethren.

Copyright © 2007 Reuters, All Rights Reserved.

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