Size at maturity as a management tool

by

Jón Kristjánsson and Tumi Tómasson

Introduction

Management of the fishery in char lakes is based on population parameters such as age, size and growth of the individuals in the population and the relation to other fish species and food items in the lake. The most common problem in Icelandic char lakes is overcrowding of char, resulting in slow growth, small size and bad quality of the fish. The remedy is to increase fishing pressure, especially on small fish. The usual method to evaluate the result of recommended actions is to study the changes in the growth of the fish. However, length at age is accumulated growth, and it will take some time for the population to stabilize under the "new" fishing strategy. Therefore, some time will elapse, sometimes several years, until the effect of a new fishing regime will be known. Then, perhaps, the outcome may be contrary to what was expected, and years may have been wasted in wrong management. Tómasson (T. Tómasson 1987) studied several arctic char populations and found that size at sexual maturity was strongly linked to maximum attainable size.

Using size at maturity as a management tool

If the size at the onset of sexual maturity is linked to growth rate, it can be used as a measure of the growth conditions in a population. Changing growth conditions will quickly be reflected in changes in the size at sexual maturity. On the basis of the changes in size at maturity, harvesting strategy (fishing pattern) can be adjusted.

Size at maturity in Arctic Char

It is a well documented fact that there are great differences in growth rates and size obtained in different char populations (Johnson 1980). Some authors (Alm 1959, Iles 1974) have argued that sexual maturity was brought about by reduced growth rates, but many authors have stated the opposite. Changes in food supply by removal of large proportions of the populations of arctic char and transplantations experiments show that density, and thereby food supply, is the main factor determining growth and size at sexual maturity.

Figure

Figure 1. Relationship in size at sexual maturity and asymptotic length in arctic char populations in north Iceland (from Tómasson 1987).

Tómasson (1987) concluded that feeding conditions were dominating genetical factors in determining size at sexual maturity in different populations or ecotypes of arctic char. He also concluded that his observations supported theoretical considerations on (Stearns & Crandall 1984), that when reduced growth rate is associated with increased natural mortality, this will result in reduced size at sexual maturity.

Discussion

Is maturity set on by retarded growth rate? This has been debated. In stunted populations, most fish are mature and of a small size. Progeny from such stocks, when put into new environment, grow fast, become bigger and mature at a larger size than their parents. When the population increases growth is retarded and the individuals mature at a smaller size. In fish farms, it has been observed that if former spawners are fed at libitum, they do not mature the following spawning season. Also, small chars, fed poorly in their second year of life, mature at a size of 15-18 cm. This indicates that food supply/ growth rate is a determining factor for the onset of maturity. The change in the growth rate is probably important: When growth rate decreases as the fish becomes larger (food supply limits the maximum size), maturity sets in.

Contrary, it has been maintained by salmon researchers and fish farmers that fast growth results in precocious males and reduced feeding would help in reducing precocious maturing in fish farms. It has to be remembered that in fish farms, other factors such as density and space affect the fish.

The ability to adjust the spawning size quickly to variations in growth conditions must be vital for the success of the population.

References

Alm, G. 1959.Connection between maturity, size and age in fishes. Rep. Inst. Freshwater Res. Drottningholm. 40: 5-145.

Iles, T.D. 1974. The tactics and strategy for growth in fishes. In F.R. Harden Jones (ed.)). Sea fisheries research. John Wiley and Sons. New York.

Stearns, S.C. & Crandall, R.E. 1984. Plasticity for age and size at sexual maturity: a life-history response to unavoidable stress. p. 13-33. In Potts, G,W, & Watton, R.J. (eds.). Fish reproduction: strategy and tactics. Academic Press. London.

Tómasson, T. 1987. The connection between growth and size at sexual maturity in Icelandic arctic char (Salvelinus alpinus). A paper prepared for the International Conference on Alternative Life History Styles of Fishes and Other Organisms. Grahamstown, South Africa.

Authors adresses:

Jón Kristjánsson: jonkr@mmedia.is

Tumi Tómasson: tumi@hafro.is


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