Is there a threshold size regulating seaward migration of brown trout and Atlantic salmon?

We investigated the influence of variation in body size and growth rate on age of smolting in Atlantic salmon and brown trout in four different Norwegian rivers. In Atlantic salmon smolt ages varied between 2 and 6 years, and in brown trout between 2 and 7 years. Smolt age was negatively correlated with parr growth, and positively correlated with smolt size. Age at smolting was more variable in the two northern than the two southern rivers. Smolt sizes and ages were also more variable in brown trout than in Atlantic salmon. Based on the observed variation in smolt size and age, we reject the hypothesis that a threshold size alone regulates age at smolting. Within populations smolt age depends on growth rate so that fast-growing parr smolted younger and smaller than slow-growing parr. We hypothesize that smolt size and age is a trade-off between expected benefits and costs imposed by differences in individual growth rate.     

Importance of ontogenetic habitat shifts to juvenile output and life history of Atlantic salmon in a large subarctic river: an approach based on analysis of scale characteristics

Juvenile and adult scale characteristics were used to compare two juvenile groups of Atlantic salmon in a large subarctic river in northern Scandinavia: individuals that have migrated from the main stem into small tributaries and those which remain in the main stem. Body size and scale measurements indicated enhanced growth in migratory parr as compared to their resident main stem counterparts. Analysis of adult salmon scale characteristics using maximum likelihood estimators revealed that 20% of the adults had been in the tributaries before the end of their second year of life, and more than 30% more had moved into the tributaries in the third year. Tributary fish matured at a smaller size and younger age (one-sea-winter salmon) than those rearing in the main stem which included a higher proportion of multi-sea-winter salmon. In addition, when smolt ages and ages at maturity were compared, older female smolts often resulted in smaller spawners and younger smolts, larger spawners. Small female spawners were more likely to survive to become repeat spawners.     

Life histories of Atlantic salmon altered by winter temperature and summer rearing in fresh- or sea-water

Increased growth during winter increased the incidence of age 1+ Salmo salar smolts in spring. High condition factor in spring and good growth in summer was associated with a high incidence of sexually mature males in autumn. In two experiments, groups (n=160–237 per group) of individually identified parr, either ungraded (lower and upper modal groups: LMG, UMG) or size-graded (LMG only), were reared at either 10, 6 or 3 °C overwinter (Nov to May). At 10 °C, up to 51% of parr originally in the LMG became smolts in spring at age 1+. In contrast, at either 6 or 3 °C (control), < 6% of LMG parr became smolts. The probability of being recruited into the UMG overwinter was positively related to initial body size, and was increased by size-grading. Smolt recruitment was two-fold higher among females compared to males; a proportion of males by age 0+ had already opted to mature at age 1+ rather than smolt at age 1+. In contrast, smolting at age 1+ was not inhibited in males previously mature at age 0+. During summer (May to Nov), all experimental groups were reared at ambient temperature, each subdivided between fresh water (max 21 °C) or seawater (max 15 °C). Good growth in seawater of winter recruits into the UMG confirmed they had completed smolting. Mortality in seawater among parr was 41–83%, and among smolts was 10–22%. Specific growth rate during summer was inversely related to winter rearing temperature. The incidence of sexual maturity in autumn at age 1+ among male parr was positively related to winter rearing temperature, fork length and condition factor in May, but there was large variation among individuals with respect to body size and maturity. Summer rearing in seawater reduced growth and the incidence of maturation. Parr and post-smolt maturity was 84–99% and 100% in fresh water respectively, 21–58% and 0% in seawater.     

Variation in some biological characteristics of British sea trout, Salmo trutta L.

Almost 50 assessments of British sea trout are available in the literature and the objective in this work is to examine variation in the parameters by which the fish are usually described and to discover biological criteria on which stocks may be classified. Eight statistics are common to the majority of stock evaluations: mean smolt age, proportion of finnock to spawn, mean age of a stock at first maturation, size achieved by the smolt at migration, rate of growth at sea, survival at sea, diversity of age categories in a stock and condition factor of sea-run fish. When the influence of each factor on the others is tested two characteristics emerge as being of key importance in the biology of adult sea-trout; the life expectancy and the weight: percentage previous spawners in a sample. The distribution of stocks is discussed in this context and two main groupings of British sea trout, corresponding geographically with the Irish sea and the Atlantic sea-board, are proposed.     

Sea growth, smolt age and age at sexual maturation in Atlantic salmon

Relationships between growth at sea, smolt size and age at sexual maturation of Atlantic salmon Salmo salar were tested. The fish were offspring of brood stocks sampled in eight Norwegian rivers at latitudes between 59° and 70° N, hatchery reared and released at smolting at the mouth of the River Imsa (59° N). Smolt size influenced the subsequent growth rate of Atlantic salmon. The larger the fish were at release, the slower the yearly length increment at sea. Mean sea age at sexual maturity, measured as proportion of the returning adults attaining sexual maturity at sea age 2 years, was significantly correlated with mean growth rate during the first year at sea and mean smolt size ( r ²= 0·74, P < 0·001). Fish attaining maturity at a relatively high sea age were more fast growing during their first year at sea than those maturing at a younger age. The results indicate that high sea age at sexual maturation is a population-specific characteristic and associated with high early growth rate at sea.     

Ecological Forms of Black Sea Brown Trout (<Emphasis Type="Italic">Salmo trutta labrax</Emphasis>) in the Mzymta River as Manifestation of Ontogenetic Plasticity

Populations of brown trout in the Mzymta River and its tributaries include anadromous (mainly female) and resident (mainly males) fish. Some resident males in the basin of the Mzymty River attain sexual maturity at the age 1+, and resident females mature at the age 2+ or 3+. The maximum age of resident fish is 4+ in the samples studied. Migrations of anadromous brown trout to the sea occur at the ages 1+, 2+, or 3+. Future spawners spend from 1 to 4 years at feeding grounds in the sea. Smolts of the population are characterized by performing not only spring but also autumn migrations to the sea. One smolt specimen has been detected upstream from the dam in the river where spawners of anadromous brown trout do not migrate; this means that the capability for sea migrations persists long in the population represented only by resident specimens of brown trout. The diversity of life cycles and ecological forms in populations of brown trout is not lower than in populations of brown trout in Northern and Western Europe. The comparison of the data obtained with published data makes it possible to come to the conclusion about the high plasticity of ontogenesis of Black Sea brown trout.     

Early sexual maturation in males of wild sea trout, Salmo trutta L., inhibits smoltification

Precocious maturation among wild sea trout, Salmo trutta (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.     

Relative production of Atlantic salmon from fluvial and lacustrine habitats estimated from analyses of scale characteristics

Empirical and back-calculated growth of Atlantic salmon parr were compared between fish reared in fluvial and lacustrine habitats of Conne River, Newfoundland. Length at age was significantly higher for lacustrine parr. Various classification and maximum likelihood estimators indicated that 75% or more of the fish used lakes for rearing. Lacustrine use is another aspect of the inherent variability and plasticity of Atlantic salmon life-history traits. As most Newfoundland river systems include lakes, estimates of regional spawning targets and potential smolt production will need to take lake habitat into account.     

Temporal variation in abundance, return rate and life histories of previously spawned Atlantic salmon in a large subarctic river

The 30 year time series analyses revealed large temporal variation in the return rates and a recent increase in abundance of previous spawning Atlantic salmon Salmo salar in the River Teno, northern Scandinavia. The mean proportion of repeat spawners was 7 and 4% in the total Atlantic salmon catch and 9 and 22% in multi‐sea‐winter (MSW) catch component for females and males, respectively. Previous spawners constituted on the average 7% of the catch in mass but up to 20%(31 t) and 30%(19 t) in 2003 and in 2004, respectively. In 1975–2000, the proportion of previous spawners varied between 1 and 6%(3–12% of MSW Atlantic salmon), whereas in 2001–2004, they accounted for 8–21%(16–35% of MSW Atlantic salmon) of the total Atlantic salmon catch. The number of previous spawners in the catch correlated significantly with the preceding numbers of respective 1–3 sea‐winter (SW) maiden Atlantic salmon 2 years earlier. The recent increase in the numbers of 1S1 and 2S1 (1 or 2 years at sea followed by first spawning and 1 year reconditioning period at sea) alternate spawning Atlantic salmon was a consequence of higher numbers of maiden 1SW and 2SW Atlantic salmon in the catches and increased sea temperatures. Similarly, the return rate of 1SW Atlantic salmon to second spawning has improved in recent years. Most previous spawners ascended and were captured early in the fishing season. The smolt and sea‐age combinations of repeat spawners comprised 68 age groups contributing with the annual mean of 15 age groups to the great diversity of the River Teno Atlantic salmon population complex.     

Life-history effects of migratory costs in anadromous brown trout

Mean size of sexually mature anadromous brown trout (sea trout) Salmo trutta in south-east Norway increased significantly with migratory distance ( D ) between the feeding area at sea and the spawning area in fresh water, from 32 cm for those spawning close to the river mouth to 43 cm at the spawning grounds 40 km inland. This was largely due to an increased size of the smallest anadromous spawners with increasing D . The raised mean size of the long-distance migrants is paralleled by an increased age at sexual maturity. Body mass at the same length of sea trout decreased with D in fresh water, meaning that the fish moving far inland was slimmer than those spawning near the coast. Gonadal mass of first-time spawning anadromous males declined significantly with D , and the fecundity and the ratio of fecundity over mean mass of the individual eggs adjusted for variation in fish mass, increased with D . There was no clear relationship between the ratio of anadromous to resident fish and D , probably because more variables than D , influence this relationship in the study streams.     

Marine post-smolt growth and age at maturity of Atlantic salmon

The annual variation in sea-age of maturation for a hatchery dependent stock of Atlantic salmon was compared to variation in post-smolt growth as evidenced by circuli spacing patterns. The proportion of returns of 1-seawinter (1 SW) and 2 SW salmon and the fraction of the smolt year class or cohort that maturated as 1 SW fish, were compared to seasonal growth indices determined from circuli spacing on the scales of smolt class survivors returning as 1 SW and 2 SW spawners. Using image processing techniques, we extracted inter-circuli distances from scales from 2244 recaptured fish. Spacing data for the first year at sea were collected and then expressed as seasonal growth indices for the spring period, when post-smolts first enter the ocean; the summer, when growth appears maximal; and winter, when growth appears to be at a minimum. In general, circuli spacings were wider for 1 SW than for the 2 SW returns of the same smolt cohort. The 1 SW fraction was significantly and positively correlated with late summer growth, suggesting that growth during this season is pivotal in determining the proportion of a smolt class that matures early.     

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.     

Time series covering up to four decades reveals major changes and drivers of marine growth and proportion of repeat spawners in an Atlantic salmon population

1. Wild Atlantic salmon populations have declined in many regions and are affected by diverse natural and anthropogenic factors. To facilitate management guidelines, precise knowledge of mechanisms driving population changes in demographics and life history traits is needed.
2. Our analyses were conducted on (a) age and growth data from scales of salmon caught by angling in the river Etneelva, Norway, covering smolt year classes from 1980 to 2018, (b) extensive sampling of the whole spawning run in the fish trap from 2013 onwards, and (c) time series of sea surface temperature, zooplankton biomass, and salmon lice infestation intensity.
3. Marine growth during the first year at sea displayed a distinct stepwise decline across the four decades. Simultaneously, the population shifted from predominantly 1SW to 2SW salmon, and the proportion of repeat spawners increased from 3 to 7%. The latter observation is most evident in females and likely due to decreased marine exploitation. Female repeat spawners tended to be less catchable than males by anglers.
4. Depending on the time period analyzed, marine growth rate during the first year at sea was both positively and negatively associated with sea surface temperature. Zooplankton biomass was positively associated with growth, while salmon lice infestation intensity was negatively associated with growth.
5. Collectively, these results are likely to be linked with both changes in oceanic conditions and harvest regimes. Our conflicting results regarding the influence of sea surface temperature on marine growth are likely to be caused by long‐term increases in temperature, which may have triggered (or coincided with) ecosystem shifts creating generally poorer growth conditions over time, but within shorter datasets warmer years gave generally higher growth. We encourage management authorities to expand the use of permanently monitored reference rivers with complete trapping facilities, like the river Etneelva, generating valuable long‐term data for future analyses.

     

Trade-off between egg mass and egg number in brown trout

Individual egg mass and fecundity increased with somatic mass in first time and repeat spawning wild anadromous and freshwater resident brown trout Salmo trutta . The egg mass was larger for similar-sized trout in south (58° N) than mid Norway (63° N), whereas fecundity was higher in mid- than in south Norway, making total gonadal investment similar in the two areas. Repeat spawners had heavier eggs than similar-sized first time spawners. The egg mass of residents was c. 10% larger than that of similar-sized first time spawning anadromous trout. Common garden experiments with offspring of wild anadromous trout showed no significant correlation between egg and somatic mass in first time spawners in two of the three populations studied. In the third population, a slight positive correlation was found. Similar results were found for repeat spawners. In the three populations, fecundity increased significantly with somatic mass in both first time and repeat spawners. Wild and hatchery-reared trout showed negative correlation between egg mass and fecundity when the effect of body size was excluded, indicating a trade-off between the two parameters. In wild trout, this was caused by variation among populations, whereas in hatchery fish, within population variation was observed in egg mass over fecundity. Furthermore, the egg mass of first time and repeat spawners were positively correlated, when adjusted for fish size. Size-specific gonadal investment was significantly higher in wild anadromous than resident trout. There was no significant difference in gonadal investment between first time and repeat spawners in wild anadromous trout. However, in the hatchery-reared trout, gonadal investment was significantly higher at repeat than first time maturation. The hatchery trout did not spawn naturally, but were artificially stripped. Among populations, a part of the variation in egg mass and fecundity is phenotypically plastic, a part appears genetically determined.     

Survival and growth of sea trout parr in fresh and brackish water

There was no di.erence in survival or growth rate over 63 days for young-of-the-year sea trout Salmo trutta in fresh and brackish Baltic Sea water (6·7 psu). Hence, such sea trout parr that migrate from the freshwater habitat in which they hatch to the Baltic coastal zone, without smolting, should experience little or no physiological cost in survival and growth.     

Smolt size in relation to age at first maturity of Atlantic salmon (Salmo salar): the role of lacustrine habitat

Back-calculated growth and size of Atlantic salmon smolts were compared in two groups of river systems in Newfoundland. One group consisted of rivers dominated by lacustrine habitats while the other had rivers characterized by fluvial habitats. Back-calculated length at each age and smolt size was significantly higher for the rivers dominated by lacustrine habitats. Associated with this was a lower proportion of maiden large salmon in adult returns. These findings are discussed in relation to density in fresh water, environmental conditions at sea, and life-history strategy.     

Net ground speed of downstream migrating radio-tagged Atlantic salmon (Salmo salar L.) and brown trout (Salmo trutta L.) smolts in relation to environmental factors

The downstream migration of Atlantic salmon (Salmo salarL.) and sea trout smolt (S. trutta L.) was investigated using radio telemetry in the spring of 1999 and 2000. Forty wild sea trout smolts, 20 F1 sea trout smolts, 20 hatchery salmon smolts and 20 salmon smolts from river stockings were radio tagged and released in the Danish River Lilleaa. The downstream migration of the different groups of fish was monitored by manual tracking and by three automatic listening stations. The downstream migration of radio tagged smolts of both species occurred concurrently with their untagged counterparts. The diel migration pattern of the radio tagged smolts was predominantly nocturnal in both species. Wild sea trout smolt migrated significantly faster than both the F1 trout and the introduced salmon. There was no correlation between net ground speed, gill Na⁺,K⁺-ATPase activity or fish length in any of the different groups. The migration speed of wild sea trout smolts was positively correlated with water discharge in both years. In F1 sea trout smolts, migration speed was positively correlated with temperature in 1999. The migration speed of salmon smolts did not correlate to any of the investigated parameters.     

Altitudinal variation of demographic life-history traits does not mimic latitudinal variation in natterjack toads (Bufo calamita)

In anuran amphibians, age- and size-related life-history traits vary along latitudinal and altiudinal gradients. In the present study, we tested the hypothesis that altitudinal and latitudinal effects cause similar responses by assessing demographic life-history traits in nine Bufo calamita populations inhabiting elevations from sea level to 2270 m. Skeletochronologically determined age at maturity and longevity increased at elevations exceeding 2000 m, but female potential reproductive lifespan (PRLS) did not increase with altitude, as it did with latitude. Integrating the available evidence, it was found that lifetime fecundity of natterjacks decreased at the upper altitudinal range because PRLS was about the same as in lowland populations but females were smaller. In contrast, small size of northern females was compensated for by increased PRLS which minimised latitudinal variation of lifetime fecundity. Thus, this study provides evidence that altitudinal effects on life-history traits do not mimic latitudinal effects. Life-history trait variation along the altitudinal gradient seems to respond directly to the shortening of the annual activity period. As there is no evidence for increasing mortality in highland populations, reduced lifetime fecundity may be the ultimate reason for the natterjacks' inability to colonise elevations exceeding 2500 m.     

Migratory timing, marine survival and growth of anadromous brown trout Salmo trutta in the River Imsa, Norway

The aim of the paper was to study sea migration, growth and survival of brown trout Salmo trutta of the River Imsa, 1976–2005. The migratory S. trutta were individually tagged and fish leaving or entering the river were monitored daily in traps located near the river mouth. The mean annual duration of the sea sojourn was 6–9 months for first-time migrants moving to sea between January and June. It was 8–18 months for those migrating to sea between July and December. Veteran migrants stayed 12 months or less at sea and most returned to the river in August. Early ascending fish stayed the longest in fresh water because most returned to sea in April to May. The day number of 50% cumulative smolt descent correlated negatively with mean water temperature in February to March and the February North Atlantic Oscillation index (NAOI). Mean annual sea growth during the first 2 years after smolting was higher for S. trutta spending the winter at sea than those wintering in the River Imsa. First year's sea growth was lower for S. trutta descending in spring than autumn. For first-time migrants, it correlated negatively with the February NAOI of the smolt year. Sea survival was higher for spring than autumn descending first-time migratory S. trutta with a maximum in May (14·9%). Number of anadromous S. trutta returning to the river increased linearly with the size of the cohort moving to sea, with no evidence of density-dependent sea mortality. Sea survival of S. trutta smolts moving to sea between January and June correlated positively both with the annual number of Atlantic Salmo salar smolts, the specific growth rate at sea, and time of seaward migration in spring. This is the first study indicating how environmental factors at the time of seaward migration influence the sea survival of S. trutta .     

Relationships between smolt size, postsmolt growth and sea age at maturity in Atlantic salmon ranched in the Baltic Sea

Tagging data were used to examine the relationships between smolt size, post-smolt growth and sea age at first maturity for the short-migrating Neva strain of Atlantic salmon ( Salmo salar L.) ranched in the Bothnian Sea and the Gulf of Finland. The results provided evidence that post-smolt growth was influenced by both relative and absolute smolt size. For both sea-areas, 2-year smolts of small relative size within a release group grew more rapidly in the sea than did smolts of higher relative, but equivalent absolute size. The negative influence of increasing relative smolt size on marine growth was, however, outweighed by the stronger positive influence of increasing absolute smolt size. A 160-mm increase in smolt size (140–300 mm) resulted in an overall growth advantage of about 1 year. In the Bothnian Sea, the predicted mean length after 1 year in the sea was 288 ± 25 mm for 140-mm smolts and 560 ± 16 mm for 300-mm smolts. Under the more favourable conditions of the Gulf of Finland, the respective mean lengths were 369 ± 15 mm and 613 ± 12 mm. The sea age at first maturity was inversely related to both freshwater and marine growth rates. For both sea areas, large smolts yielded proportionately more grilse than did small ones. Smolt years with good post-smolt growth rates yielded more grilse than did years with poor growth rates. The overall level of grilsing was higher in the Gulf of Finland than in the Bothnian Sea. These results suggest that the relationships between smolt size, post-smolt growth and age at first maturity in the sea are influenced by the environmental conditions of the respective sea area. A framework explaining the links between smolt size, marine growth, survival and sea age at maturity in Neva salmon is presented for the Gulf of Finland and the Bothnian Sea.