Once a fish begins to mature, an increasing proportion of its energy reserves are devoted to building gonads, which inevitably means that less energy is available to maintain body condition.
Early maturation is a major problem for the New Zealand salmon industry.
The onset of maturation is usually noticeable, even to a casual observer, as early as 6 months beforehand. Maturing fish tend to be darker than normal, and often have a more rounded body shape, even though their gonads may be rudimentary. But these effects are secondary to maturation, and are merely a result of hormonal cycles, which, by the previous November, are already well advanced. Fish that maintain a high level of body fat are more likely to mature in the next autumn than those with lower reserves. Salmon farmers can, and do, exert some degree of control over maturation simply by reducing feeding rates at the appropriate time of year.