EARLY MARINE LIFE OF ATLANTIC SALMON, SALMO SALAR, 

 POSTSMOLTS IN THE NORTHERN GULF OF ST. LAWRENCE 



J -D DUTIL AND J.-M. COUTU' 



ABSTRACT 



Postsmolts of Atlantic salmon, Salmo salar, having spent some 2-4 months in the marine environ- 

 ment were collected in the nearshore zone of the northern Gulf of St. Lawrence. From a back- 

 calculated smolt length of 135 mm they had increased in length to 265 mm (212 g) on 1 September, 

 and 306 mm (320 g) on 30 September. The rate of increase in length averaged 1.65 mm/day over more 

 than 2 months. Individuals reached 35 cm and 500 g in late autumn. Postsmolts moved in small shoals 

 near the surface and were possibly more active at dawn and dusk. In midsummer, stomach contents 

 changed from insects and gammarids to sand lance, 40-100 mm in length; vertically migrating 

 crustaceans also occurred in the stomachs in autumn. River origin of these postsmolts is not known. 

 The possibility of their belonging to a particular subgroup of some north shore stocks is examined in 

 relation to scale patterns and size of gonads. This occurrence of postsmolts near shore in late summer 

 and presumably their late movement out of the Gulf of St. Lawrence indicate the directional nature 

 of smolt migration to distant feeding areas should be reconsidered. Low sea temperature is hypothe- 

 sized to trigger the movement out of the Gulf of St. Lawrence. Continual presence of postsmolts in a 

 shallow layer at the surface could prove to be extremely valuable in forcasting movements and 

 production. 



Many papers have been published concerning the 

 biology of Atlantic salmon, Salmo salar, but very 

 little has been said concerning the postsmolt 

 stage. This stage has been defined as ". . .the 

 juvenile salmon from the time that it leaves the 

 river as a smolt until the onset of wide annulus 

 formation on the scales at the end of the first 

 winter in the sea" (Allan and Ritter 1977). This 

 paper presents new data on the Atlantic salmon 

 postsmolts (hereafter referred as postsmolts) in 

 the northern Gulf of St. Lawrence, reviews our 

 current knowledge on the biology of postsmolts, 

 and points to biological and environmental fac- 

 tors potentially limiting the success of their early 

 life in marine environments. 



Published data on postsmolts are mainly lim- 

 ited to stocks in the Baltic Sea. Routes of migra- 

 tion have been described based on the locations 

 and time of early recapture from smolt releases in 

 Sweden and Finland (Carlin 1959; Larsson and 

 Ateshkar 1979; Ikonen and Auvinen 1984, 1985; 

 Jutila and Alapassi 1985). Data on predators are 

 limited (Soikkeli 1973; Valle 1985), and most of 

 the material concerns predation on smolts in 

 streams and estuaries (Larsson 1985). Many 

 analyses of stomach contents have been pub- 



'Ministere des Peches et des Oceans, Gouvernement du 

 Canada, 850, route de la Mer, C.P. 1000, Mont-Joli, Quebec, 

 Canada G5H 3Z4. 



lished, particularly on smaller postsmolts (re- 

 viewed by Christensen and Larsson 1979; Jutila 

 and Toivonen 1985). Data on rate of growth 

 (Ikonen and Auvinen 1985) and rate of mortality 

 (Carlin 1959) are lacking. However, Baltic 

 salmon spend their entire sea life in the brackish 

 waters of the Baltic and nearly 80% of smolt pro- 

 duction originates from hatcheries (Anonymous 

 1984). Thus the information derived from salmon 

 in the Baltic should be extended to other stocks 

 only with caution. 



Publications on postsmolts in the northern At- 

 lantic and Gulf of St. Lawrence mentioned small 

 salmon as bycatches of commercial fisheries and 

 described the distribution of recaptures from 

 smolt release programs. The earliest report on 

 postsmolts in the Gulf of St. Lawrence claimed 

 that small salmon, referred to as "ouananiche" by 

 local fishermen, were regularly taken near shore 

 in herring nets in autumn (Comeau 1909). 

 Kendall (1935) also reported such incidental 

 catches for the New England coast. Elson (1953) 

 recorded a bycatch of more than 1,000 marked 

 postsmolts from one locality in the Bay of Fundy 

 in the period 1951-53 and reported their mean 

 length. There are also limited records of 

 postsmolts taken off France (Vibert 1953) and in 

 the Gulf of St. Lawrence (Caron 1983) from 

 smolts tagged in streams. Recently, information 

 on movements has been derived from tag returns 



Manuscript accepted December 1987. 

 FISHERY BULLETIN; VOL. 86, NO. 2, 1988. 



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