No abstract

Precocious maturation among wild sea trout, Sulmo irurru (L.), was studied in two small streams in south-western Sweden. The proportion of parr males varied between 17.9 and 57.0%. Parr males and immature individuals of the same age were compared with respect to length distribution, migratory tendency and seawater tolerance. Precocious males were, on average, longer than immature fish. Marking and recapture showed a reduced or inhibited migration tendency of parr males. The seawater tolerance of these was also consistently lower than for juveniles. The seasonal pattern in osmoregulatory ability was similar for both categories, with a peak during the smolt run.

A tagging programme, in which wild sea trout (Salmo trutta) were marked at the parr stage with internal tags in winter and recaptured during smolt migration in a trap in the following spring, was conducted for three seasons in a small stream in southwestern Sweden, individuals that were initially smaller migrated later, grew more in length, and were smaller at migration than those initially bigger. Body length at migration decreased with migration time during the season for tagged as well as nontagged trout. A model for optimal time of and size at migration is proposed, based on the assumption that the expression (body length at the end of the season) × (survival over the season) is maximized and that migratory and postmigratory survival is positively size dependent. The qualitative predictions on the relationship between initial size and time of and size at migration are supported by the tagging data. Pre- and postmigratory growth rates are predicted to have opposite effects on migration time, so the migration timing is expected to depend strongly on the degree of dependence between these.

Wild, one-and two-summers-old sea-trout parr in a small stream, were tagged with PIT-tags in late autumn and recaptured in a smolt trap during the following spring. One-summer-old smolts migrated later than older ones and were smaller at migration. ANCOVA indicated that migration date was negatively related to body length at tagging, but we found no significant effect of age or condition factor at tagging on this relation. The difference in timing between the age-classes was thus an effect of body size rather than age. 1996 The Fisheries Society of the British Isles

In three years, in late autumn, underyearling wild sea-run brown trout (Salmo trutta) in a small stream were captured with electric fishing and tagged with microchips (passive integrated transponders). They were recaptured during the following season either as migrating smolts in a trap or by electric fishing in the stream just after the migration period and during spawning (late autumn). At spawning, they were distinguished as sexually mature male parr or immature juveniles. The smolts were longer and heavier at tagging than the mature male parr and the immature parr, which were similar in initial length and weight. The mature male parr had a significantly higher condition factor at tagging than the immature parr. The probability of parr maturation was positively associated with initial condition factor but not with initial body length. Growth of mature male parr and immature parr was similar in the early season but higher for immature parr in the late season. The result indicated that the choice of strategy was made earlier in life than previously recognized.