REPRODUCTION AND RECRUITMENT IN FRESHWATER FISH 285 



(Mortimer & Worthington, 1942), but the area less than 20 m depth, and 

 thus that normally inhabited by perch, is 7-65 km^. On the basis of the latter 

 figures the production of adult perch was 3 1 kg/ha, and shedding of perch 

 eggs and milt 27-5 kg/ha. These figures must be considered very approxi- 

 mate, but very similar figures were obtained some years ago using a rather 

 different method of computation and the ratio between growth production 

 and shed gonad products is hkely to be fairly accurate. Thus, the annual 

 shedding of gonad products for a perch population similar to that in Winder- 

 mere is nearly as large as the total adult production. This proportion will 

 depend more than anything else upon the growth rate of the fish because the 

 gonad weight as a proportion of the body weight does not seem to vary 

 very much — at least for perch. Before 1941, the perch in Windermere 

 were stunted but the growth was not as slow as several other perch growth 

 rates that have been reported (Aim, 1946). Since 1941 the growth rate has 

 increased greatly, the population decreased and the mortahty rate changed 

 as well (Le Cren, 1958). Very rough calculations suggest that production is 

 now about 10 per cent, and gonad products shed about 6 per cent, of what 

 they were before 1941, so that with a fast growth perch gonad shedding 

 would be about half the adult production. 



Some of the best data on fish production are those given by Allen (1951) 

 for the brown trout {Salmo trutta) of the Horokiwi River. The total produc- 

 tion for all but O-group fish from May 1940 to May 1941 was 531 kg. Allen 

 does not give data on gonad weights, only fecundity, but a few data from 

 brown trout from various Enghsh sources indicate that the ripe ovary weight 

 is fairly constantly 12-5 per cent of the body weight and the testes vary 

 round 2- 5 per cent. If these data are applied to Allen's Horokiwi population 

 the gonad shedding can be estimated at 54 kg or about 10 per cent of the 

 production (of adults). Frost & Smyly (1952) give data on growth and 

 survival in Three Dubs Tarn from which the production of brown trout can 

 be calculated (though the estimate for total population in the tarn is no better 

 than an informed guess). The average production by mature and maturing 

 trout (i.e. Ill and older) is 1-75 kg and the gonad products shed is 0-42 kg 

 or about 24 per cent of the adult production. 



These few examples are enough to show that the annual production of 

 gonad products can be a quantity of significant size, and in some cases as 

 large as the annual growth production of adults. This gonad shedding can 

 be thought of both as a demand upon the productivity of the environment 

 and as a contribution towards recruitment. From many points of view the 

 annual growth of fish should be considered to include the amiual spawning 

 of gonad products, and in estimating the food consumption the demands of 

 gonad development should certainly be taken into account, as in terms of 



