Fish- Beneath the Surface (An article for ‘Future of Fish’ magazine)

Natural cod stocks going down.

There has been much press coverage of the decline in natural cod stocks due to over fishing in the North Sea.

I like my cod and chips and so was quite concerned about this. It may be too late to get the numbers back up to 1970's levels but less documented by the press, is the rapid growth of mariculture, the cultivation of marine organisms for food, which retailers say could 'revolutionise the fishing industry' (2).

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I visited the Manx Mariculture fish hatchery to investigate the principles behind fish farming and soon discovered that it was not without its problems. As my guide, Rick Fullerton, explained, a bacterial problem in the live feed meant that the hatchery faced a crisis in the year 2006 when only a few hundred cod were produced instead of the target 1 million. This is a common problem in mariculture and there is the potential for exciting new developments in the production of live feed which could eliminate this problem in the future.

Use of live feed as an answer

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In order to kill bacteria, live feed may be disinfected or treated with UV rays (6). A recent study (Cutts, Sherwood and Treasurer) showed that bacterial numbers were lower in tanks of live feed treated with Pyceze, often used as a disinfectant of water and a preservative in cosmetic products, and the survival of larvae was 6.1% higher (6). Other future developments in the production of live feed may include using rotifers which match the nutritional requirements of the larvae. This could be achieved by finding new methods of enrichment. Research in collaboration with Australia's Commonwealth Scientific and Industrial Research Organisation (CSIRO) has been carried out into developing enrichment methods using vitamin C, bacteria and fungi (7).

Rotifers: These multicellular animals are 200-300�m in length and are kept at a density of around 1500 per ml (3). They supply the nutritional requirements of fish larvae by retaining nutrients which are then passed on to the larvae (5), ensuring rapid growth.

As I go round, a putrid smell laces the air. Dark and bitter, it hangs heavily in the small room. The humidity makes it difficult to breath. Four tanks prevail to one side, overbearing. This is the live feed room of the Manx Mariculture fish hatchery. I peer closer to make out the reptilian green contents of the tanks. Here, rotifers, used as live feed, are grown in aerated tanks on a diet of yeast and algae at 25�C (3), optimum conditions for growth.

The cod and turbot larvae farmed at the site are reared on these for up to 20 days during which time it is not possible to produce food pellets small enough for their consumption. To small to be seen with the naked eye, rotifers are an excellent replacement of the fish's natural diet, zooplankton (copepods) and they are enriched to supply the nutritional requirements of the larvae. Nutrition in the diet is essential in ensuring optimum growth.

Brine shrimps: With my knowledge of cod limited to its appearance in a plastic take-out box, the sight of the fish at 20 days, with their dark silken bodies deftly navigating the water, surprises me.

At this age, they are around 7-8 mm in length and are fed for a further 20 days on brine shrimp, or Artemia, 500-700 �m in length (3). By using a specially formulated diet for the Artemia, their nutritional content is increased to maximise the growth of the larvae. This also reduces pigmentation problems in the turbot, a species of fish I had not previously heard of. My first impression of them is that they bear a certain resemblance to the shape of a stingray. They line the bottom of the holding tanks, their slender bodies overlapping, producing a carpet of shimmering grey.

A tight ship: Nevertheless, live feed production is extremely expensive. To improve efficiency at Manx Mariculture, rotifer production is carried out as a continuous process. Future developments are needed in the production and reliability of rotifer and Artemia whilst reducing the cost of the feeds. Recent studies (Lubzens E, Zmora O, Barr Y, 2001) have shown that the rotifers, B. plicatilis and B. rotundiformis, are two species of rotifer most suited to mariculture.

Fish and chips - a change of diet

As cod are cannibalistic, grading is carried out at 2 to 4 months using filters of increasing width to separate out different sizes into different tanks. This increases the survival rate of smaller, weaker cod which may not have survived in the wild.

As it is not economically viable to produce live feed in large amounts as the fish mature, they are weaned off this and onto a diet of hard fishmeal pellets. This is achieved by gradually increasing the ratio of hard pellets to live food, fed once a day using automated feeders suspended above the tanks.

Made from smaller species of fish and crustaceans unfit for human consumption, standard fishmeal has a content of 65-67% protein and 12% fat (8). There may be some dispute over the validity of this data as it is from an internet site and the source is not clear, therefore it cannot be guaranteed that the information is entirely accurate. Fishmeal contains proteins, lipids, minerals and vitamins but very little carbohydrate. Its close composition to that of the fish's natural diet makes it an excellent alternative (8).

Fishmeal increases feed efficiency and growth due to a greater nutrient uptake and absorption. It also provides a high amount of energy per unit weight (8). Lipids in the diet provide phospholipids and fatty acids required for optimum growth and development. They are also important in the structure and function of cell membranes. They can be easily digested and have a high energy content which is important as the diet contains little carbohydrate. This is most likely because there is limited availability of carbohydrates in marine environments (9). If the diet does not provide enough energy, protein needed for growth will be broken down instead which increases the costs of fish farming.

Protein is needed in the diet as it provides the 10 essential amino acids which the fish cannot synthesis. Fishmeal also contains minerals such as calcium, phosphorus and magnesium and vitamins including the B-complex (8). There are, however, ethical concerns over the use of fishmeal as some argue that for every tonne of farmed fish produced, 5 tonnes of fishmeal are used in the process. On the other hand, fishmeal can be seen to reduce pollution to the environment as there is increased nutrient uptake by the fish (8).

Green issues

Further environmental concerns which surround fish farming include the pollution caused by waste being discharged into the sea. However, made up of fish waste and undigested food, this is natural and it does not cause major problems or damage to marine environments (3). This likely to be accurate and precise information as Nick Fullerton is a professional with a degree in biology and has had many years experience in mariculture.

The energy cost of running the hatchery which includes pumping water 24 hours day and the continuous monitoring probes must also be taken into account when considering the environmental implications of fish farming.

A breath of fresh air

Continuing my tour, I am shown the large round tanks where the fish are held. A strange looking square device sits on the surface of the water. Consisting of a flimsy plastic frame, it looks unimportant, however, it has the potential to raise the survival of the fish by 90% (3). This is a surface skimmer which cleans the surface of the water by blowing air over it to remove the oily film which would otherwise build up.

When the surface is clear, young fish are able to gulp air in order to inflate the swim bladder, an organ in bony fish used for buoyancy. As the fish rely on the water in order to breathe and grow, the water quality can have a huge impact on the success of fish farming (10). The seawater in the hatchery is filtered through filters only 1 �m thick in order to remove most harmful bacteria and parasites and the continuous flow maintains the water quality (3). Bacteria such as Pseudomonas, Oceanospirillum, Marinobactera and Paracoccus are common in marine environments (11).

Much more impressive are the electronic probes which measure oxygen, carbon dioxide and ammonia levels and pH. However, water is not recirculated otherwise very strict controls would have to be enforced on water quality (3). Oxygen, required for aerobic respiration, is kept at a concentration of 8 mg per litre, or almost 100% saturation (At 20 �C, normal pressure and in freshwater, 9.1 mg/L = 100% saturation (12)). Fish require a high concentration of oxygen because they are very active and have a high metabolism. The oxygen concentration is increased when stress levels are elevated such as during transport. This causes the heart rate to increase and so the fish to take in more oxygen. Carbon dioxide levels are kept below 5 parts per million (ppm) and ammonia which is excreted by the fish is kept below 0.02 ppm. PH is ideally kept between 6.5 and 9.0 (10).

More advanced probes are being developed but these advancements are limited by cost. Temperature is kept around 7-8�C for cod and slightly warmer for turbot. These temperatures are maintained by 3 water inlets to each tank of cold, heated and ambient water. Hot water is not used due to the high cost, especially as the water is not recirculated.

Food for thought

The ethics of keeping the fish at high stocking densities needs to be considered, however, it can also be seen that their survival is much higher than in the wild. At 50 days, this is 20-30% compared with less than 1% in the wild (3). Turbot even appear to thrive at high densities (13) as this is natural to them in a marine environment on the ocean floor. This is likely to be valid information as it comes from an educational source although, published in 1992, it is not up-to-date.

Another concern is that if farmed fish escape and breed with wild stock, the genetic strain may be weakened as farmed cod have lost much of their natural awareness (3).

Help or hindrance?

Mariculture is growing worldwide at a rate of 5% annually (14) but is it actually accomplishing its aim of increasing fish stock? The use of fishmeal in fish farming means that it relieves pressure on one species only to transfer it to others. It can be argued that the numbers of other species of fish removed from the ocean in order to feed farmed fish mean that the problem is not solved. Furthermore, depleted stocks of other species may disrupt complex food webs in marine ecosystems as 'population dynamics, competition for food and patterns of predation' are changed (15).

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