Dietary Problems

A diet deficient in either the major or minor nutrients will ultimately result in health problems in the stock. Feed formulation is such that pelleted or other manufactured diets provide stock with a good nutritionally balanced and sufficient food supply. However things can go wrong if environmental factors exceed normal tolerance limits for the stock (e.g., higher than expected water temperatures, poor quality raw ingredients are used or feedstuffs are incorrectly stored).

Dietary problems concerned with carcass quality rather than fish health through the production cycle have on occasion come to the fore in recent years. Among the problems manifesting themselves have been excess, running/flowing oil in flesh, gaping of fillets and a loss of fillet colour/pigmentation. Fish farmers blamed all these on the feed on the grounds that nothing else within the industry had changed other than the relatively rapid increase in the percentage oil content of diets. This was rather unfair in that, over the same period of time, there had been additional vast improvements in husbandry techniques and overall fish health through increased availability and use of vaccines.

One of the major outlets for farmed salmon is the fish smoking industry. Fish smokers complained in the early 1990s that fillets were difficult to slice, colour was poor and oil was leaching from fillets into packs compromising seals, shelf life and freshness. Product traceability within quality control systems meant the product could be traced back to the farms where they were reared. Results of inquiries did not present a very clear picture but the finger was pointed at diet as the main culprit. This was on the basis of a 10% or more increase in the oil content of pelleted diets (22/24% to 30/33%). The link of high oil in diet and high oil in fillet was easily made.

Whilst dietary fats will be utilised by fish as an energy source, fish will accumulate fat if they are present in excess of its needs. This is a natural biological reaction by fish - these energy (fat) stores can be utilised in times of scarcity. However it did not and does not fit in with what the fish farmer was trying to produce, namely a low fat (14%), lean fish for smoking. Investigations revealed that it was not the high oil in the diet per se that was the problem but rather the way in which these feeds were being administered to stock. Satiation feeding was the norm to promote appetite, better growth and feed conversion efficiency. As energy expenditure in a cage is fairly limited, stock were being presented with an excess of oil at each meal leading, over time, to accumulation in the flesh. Additional associated problems included gaping in fillets and apparent less strong pigmentation.