Fluctuating recruitment and variable life history of migratory brown trout, Salmo trutta L., in a small, unstable stream

The recruitment dynamics and life history of migratory brown trout, Sulmo trutta L., were investigated in a small Baltic coast stream subject to recurring drought. Spawning males consisted of both mature male parr (101–206 mm t.l. ) and migrant males (205–780 mm t.l. ). Spawning females were all migrants which delayed maturity until reaching a significantly greater size on average (424–805 mm t.l. ) than migrant males. Male: female ratios were very high in spawning aggregations (9–12 males: 1 female) with males representing up to five year-classes or more. Gametes from several generations of males per spawning event may be important for maintaining the genetic viability of this population with few female spawners per year. The amount of spawning was dependent on precipitation just prior to and during the spawning period since migrants could not enter the stream under drought conditions. Migrants did not overwinter in the stream.
Drought also caused variable fry mortality following emergence in early summer. Recruitment of 0+ parr in autumn varied from c . 175 to 3000 during 3 years. Smolts were relatively young (ages 1–2) and small (≥8 cm), and were significantly longer on average than sibling parr. Yet emigration of 1-year-olds was not related to 0+ parr size the previous autumn because of overlapping growth rates.
Persistence of the migratory brown trout in this unstable environment may be the consequence of (i) life history adaptation (e.g. short freshwater residence of both juveniles and spawners), and (ii) a complementary set of individual life histories where variation in age of migrant spawners and the occurrence of mature male parr result in a stable spawner population despite inconsistent recruitment of migrants to the sea.