RESULTS OF STOCKING WITH BROWN TROUT (Salmo trutta m. lacustris) IN FINNISH LAKES AND RIVERS

J. Toivonen, H. Auvinen, E. Ikonen, T. Alapassi and U. Kokko
Finnish Game and Fisheries Research Institute
Helsinki, Finland

ABSTRACT

The paper deals with the tagging experiments with brown trout (Salmo trutta m. lacustris) in Finnish lakes in the sixties and seventies. These were performed on fish in different kinds of lakes in order to monitor profitability and to find the best fish size and date for stocking. The yield per 1 000 stocked smolts varied between 0 and 600 kg. The recommended minimum size for stocking is 18 cm, and 25 cm in most of the regulated lakes. Stocking in spring has given considerably better results than stocking in autumn. The best results have been obtained in large, clear, oligotrophic lakes with dense stocks of vendace (Coregonus albula).

RESUME

Au cours des années 60 et 70, des expériences de marquage de truites (Salmo trutta m. lacustris) ont été effectuées en Finlande dans différents types de lacs. Il s'agissait de suivre l'évolution des rendements et de déterminer quelles étaient les conditions optimales pour le repeuplement (date et taille des poissons). On a constaté que les rendements variaient entre 0 et 600 kg pour 1 000 juvéniles mis à l'eau et que la taille minimale de ces juvéniles devait être de 18 cm en général et de 25 cm dans la plupart des lacs aux eaux régularisées. Le repeuplement de printemps s'est révélé bien plus efficace que celui d'automne. Les meilleurs résultats ont été obtenus dans de grands lacs oligotrophes, aux eaux claires, où vivaient des populations denses de Coregonus albula.

1. INTRODUCTION

In Finland, stocking with brown trout (Salmo trutta m. lacustris) was started in the fifties in all main waters in order to maintain a fishable stock. Natural reproduction was prevented by power-plant constructions, pollution and dredging for timber floating. In spite of the fact that the price of smolts is quite high and the stocking results are unknown, the number of stocked smolts has increased rapidly. In many cases money is badly invested in such stocking. This paper gives a brief report of the results of brown trout stocking in Finnish lakes in the sixties and seventies based on tagging experiments. Some ideas on fish size and stocking time and place are given.

2. MATERIAL AND METHODS

The material includes tagging experiments in Finnish lakes, rivers and impoundments in 1959–76. The total number of tagged smolts was 211 062 (73 635 in the sixties, 137 427 in the seventies) in 503 experimental groups (210 in the sixties, 293 in the seventies). The mean number of tagged fish in each experimental group was 350 in the sixties and 469 in the seventies. The tag most commonly used was the Carlin tag (Carlin, 1955). The age of the tagged fish was usually two years. The conditions under which the fish were tagged varied considerably.

3. RESULTS

Table 1 gives the poorest, best and mean catches per 1 000 stocked smolts in tagging experiments in the main water courses of Finland. The best result was 538 kg/1 000 stocked smolts in 1966 in Lake Puruvesi, a large, clear, oligotrophic lake in the Vuoksi watercourse. The mean catch was lower in the seventies than in the sixties (Fig. 1).

Figs. 2a–2c show that stocking in spring usually gives better results than stocking in autumn. The same figures also show that bigger smolts have a better chance of survival until fished.

4. DISCUSSION

Tagging results do not represent the results obtained by stocking with untagged fish. It is obvious that untagged fish give much better results than tagged ones. This is due to the higher mortality and slower growth rate of tagged fish, tag losses, unreported tags, etc. Neither are we necessarily comparing the different experimental groups because smolt-rearing conditions, taggers, transporters and other factors differ greatly.

The 1982 price of one brown trout smolt is about 4.6 Finnish marks. The value of the brown trout catch for the fisherman in 1980 was 30 marks/kg. On this basis, a yield of more than 150 kg/1 000 stocked smolts is the limit for the profitability of brown trout stocking. The costs of fishing have not been taken into account in this estimate. They may be as high as 50 percent. However, fishing for brown trout is mainly recreational, which is why it is difficult to evaluate the profitability of stocking. Usually the prospects of catching brown trout are sufficient to start and maintain a recreational fishery.

The trend toward poorer results (Fig. 1) is not very clear but raises many questions. It is not believed that the reporting rate in inland waters has fallen off, because no limitations on fishing (TAC or others) have been imposed. There may be a change in the condition of the stocking material. In Finland most brown trout eggs are produced from hatchery-reared spawners, which may cause severe genetic problems due to inbreeding (Kincaid, 1976). However, a special study is required before this can be verified. There have been no major change in either food or tagging methods.

As may be expected from the need of brown trout, the best results have generally been obtained in large oligotrophic lakes. A dense stock of vendace (Coregonus albula) will further increase the yield. When the brown trout are stocked at a size large enough to eat fish, the result will be better than with smaller fish which have to feed on benthos for a time. This is especially true in lakes where water-level regulation causes a poor bottom fauna in the littoral (Nilsson, 1961). Very good catches have been reported for stocking with a minimum smolt size of 25 cm in regulated lakes (Salojarvi et al., 1981). In unregulated lakes the bottom fauna is better and supplies the smolts with adequate food. Here good results can be obtained even with smaller smolts.

5. REFERENCES

Carlin, B., 1955 Markning àv utvandringsfärdiga laxungar i Lagen. Vandringsfiskutredningsmedd., 1955:5

Kincaid, H.L., 1976 Inbreeding in salmonids. In Salmonids genetics: status and role in mariculture, edited by T.Y. Nosko and W.K. Hershberger. Seattle, University of Washington, Sea Grant Report (WSG-WO-76-2):33–7

Nilsson, N.-A., 1961 The effect of water-level fluctuations on the feeding habits of trout and char in the lakes Blåsjön and Jormsjón, North Sweden. Rep.Inst.Freshwat.Res., Drottningholm, (42):238–61

Salojärvi, K. et al., 1981 Oulujoen vesistön kalatalouden hoitosuunnitelma. Riista- ja kalatalouden tutkimuslaitos, kalantutkimusosasto, Monistettuja julkaisuja, 1–277

Table 1 Results of stocking with brown trout of age 1+ to 3+ years in five main watercourses in Finland
(kg/1 000 stocked fish)

Stocking year	Vuoksi				Kymijoki				Kokemäenjoki				Oulujoki				Kemijoki
Stocking year	worst	best	mean	No.	worst	best	mean	No.	worst	best	mean	No.	worst	best	mean	No.	worst	best	mean	No.
1961	0	228	64	6		-				-			26	26	26	1		-
1962	3	93	48	10	102	102	102	1		-			30	30	30	1	0.5	0.5	0.5	1
1963		-				-				-			53	53	53	1		-
1964	4	84	48	6	136	292	204	4		-			11	11	11	1		-
1965	43	324	184	2	3	496	175	6		-			4	23	14	2	19	120	70	2
1966	538	538	538	1	82	199	130	3	18	262	114	3		-			5	185	81	11
1967	390	448	419	2	31	147	76	7	16	50	32	3	17	17	17	1	1	70	20	35
1968	153	372	278	4	7	78	28	6		-				-			1	32	15	6
1969	1	289	61	10	1	59	24	10	24	24	24	1	1	25	13	2	44	44	44	1
1970	0	126	20	11	0	139	26	21	12	14	13	2	1	139	47	3	0	94	45	4
1971	20	78	50	3	0	67	15	15		-			9	10	9	1	0	37	8	8
1971	28	280	140	6	4	251	124	14	0	45	17	4	5	7	6	2	0	123	33	9
1973	3	101	36	9	52	387	206	4	114	114	114	1	0	15	9	4	39	124	78	9
1974	118	118	118	1	19	105	58	4		-			4	60	17	5	134	134	134	1
1975	103	132	117	3	1	177	60	9		-			0	47	15	12	15	96	43	7
1976		-			0	223	71	11	1	66	34	2	2	2	2	2	159	159	159	1
Mean	100	229	151	74	31	194	93	115	26	82	50	16	12	33	19	39	32	94	56	95