Timing Of Seaward Migration Research Articles

Early-season brown trout (Salmo trutta) migrants grow and survive better at sea.

The timing of seaward migration is a key life‐history trait for many anadromous fish species, with growth and survival at sea depending on a match/mismatch scenario between the timing of the sea entry and optimal conditions. Based on a 25‐year study with 15,226 individually tagged brown trout (Salmo trutta) in a Norwegian river, we analysed how the within‐season timing of sea migration impacted growth and survival. In both first‐time and veteran migrants, marine growth was highest for early migrating individuals, large individuals, and those with a low condition factor when entering the sea. Survival was highest for individuals entering the sea early in the season. In first‐time migrants, survival increased with body length. Survival also increased with the number of other smolts migrating simultaneously. As the early smolts were the most successful, it may seem strange that many smolts migrate later in the season. We suggest that late‐migrating smolts may not be of a size and/or physiological state suitable for early marine conditions, and may make the best of a bad situation.

Direction and magnitude of natural selection on body size differ among age-classes of seaward-migrating Pacific salmon.

Due to the mediating role of body size in determining fitness, the “bigger‐is‐better” hypothesis still pervades evolutionary ecology despite evidence that natural selection on phenotypic traits varies in time and space. For Pacific salmon (genus Oncorhynchus), most individual studies quantify selection across a narrow range of sizes and ages; therefore, uncertainties remain concerning how selection on size may differ among diverse life histories. Here, we quantify the direction and magnitude of natural selection on body size among age‐classes of multiple marine cohorts of O. nerka (sockeye salmon). Across four cohorts of seaward migrants, we calculated standardized selection differentials by comparing observed size distributions of out‐migrating juvenile salmon to back‐calculated smolt length from the scales of surviving, returning adults. Results reveal the magnitude of selection on size was very strong (>90th percentile compared to a database of 3,759 linear selection differentials) and consistent among years. However, the direction of selection on size consistently varied among age‐classes. Selection was positive for fish migrating to sea after two years in freshwater (age 2) and in their first year of life (age 0), but negative for fish migrating after 1 year in freshwater (age 1). The absolute magnitude of selection was negatively correlated to mean ocean‐entry timing, which may underpin negative selection favoring small age‐1 fish, given associations between size and timing of seaward migration. Collectively, these results indicate that “bigger is not always better” in terms of survival and emphasize trade‐offs that may exist between fitness components for organisms with similarly diverse migratory life histories.

Opportunistic spawning of tropical anguillid eels Anguilla bicolor bicolor and A. bengalensis bengalensis

Information on the spawning migration, spawning ecology and life history of tropical eels in the Indo-Pacific region is very limited. The physiological and morphological characteristics of tropical freshwater eels, Anguilla bicolor bicolor and A. bengalensis bengalensis collected in Malaysia were examined in relation to their downstream migration patterns. A total of 455 eels were collected over monthly intervals between February 2014 and January 2016 and we examined both gonadosomatic index and gonad histology features. In both species, close positive relationships between advanced maturation stages and eye, fin, gonadosomatic indexes were found in males and females. A. bengalensis bengalensis was found to be larger and heavier than A. bicolor bicolor at the time of seaward migration. The final stage of maturation for seaward spawning migration occurred throughout the year in A. bicolor bicolor, although that of A. bengalensis bengalensis was estimated to six months due to the limited number of samples. These results suggest that year-round spawning in the open ocean occurs in the tropical eel. This non-seasonal spawning ecology is notably different from that of temperate eels, which are known to follow a well-defined spawning season, with spawning migrations generally taking place during autumn months.

Accuracy of nonmolecular identification of growth-hormone- transgenic coho salmon after simulated escape.

Concerns with transgenic animals include the potential ecological risks associated with release or escape to the natural environment, and a critical requirement for assessment of ecological effects is the ability to distinguish transgenic animals from wild type. Here, we explore geometric morphometrics (GeoM) and human expertise to distinguish growth-hormone-transgenic coho salmon (Oncorhynchus kisutch) specimens from wild type. First, we simulated an escape of 3-month-old hatchery-reared wild-type and transgenic fish to an artificial stream, and recaptured them at the time of seaward migration at an age of 13 months. Second, we reared fish in the stream from first-feeding fry until an age of 13 months, thereby simulating fish arising from a successful spawn in the wild of an escaped hatchery-reared transgenic fish. All fish were then assessed from 'photographs by visual identification (VID) by local staff and by GeoM based on 13 morphological landmarks. A leave-one-out discriminant analysis of GeoM data had on average 86% (72-100% for individual groups) accuracy in assigning the correct genotypes, whereas the human experts were correct, on average, in only 49% of cases (range of 18-100% for individual fish groups). However, serious errors (i.e., classifying transgenic specimens as wild type) occurred for 7% (GeoM) and 67% (VID) of transgenic fish, and all of these incorrect assignments arose with fish reared in the stream from the first-feeding stage. The results show that we presently lack the skills of visually distinguishing transgenic coho salmon from wild type with a high level of accuracy, but that further development-of GeoM methods could be useful in identifying second-generation,fish from nature as a nonmolecular approach.

Effects of Growth and Reproductive History on the Egg Size–Fecundity Trade‐off in Steelhead

AbstractThe allocation of energy by females into different components of reproduction is critical and includes trade‐offs between egg size and number. In anadromous salmonids, it is unclear whether the allocation patterns are determined by the growing conditions experienced when the fish are in freshwater or at sea or by the female's reproductive history. We collected data on length, egg size, and fecundity from female steelhead Oncorhynchus mykiss that had spent 1 year in freshwater and then 2 (designated 1.2) or 3 years at sea (1.3) before spawning for the first time. We also collected the same data from females spawning for the second time but of the same age as those spawning after 3 years for the first time (1.1S1). The older females were longer than the younger ones, but there was considerable overlap in length. The repeat‐spawning fish were shorter than fish of the same age that were spawning for the first time, which reflected the reduction in growth associated with the previous spawning migration and reproductive event. Fish length was positively correlated with egg size and fecundity, but the 1.2 females had smaller eggs for their size than did the 1.3 fish, which implied an effect of growth rate at sea. We then measured scales from a subset of the fish as an index of individual fish size at the time of seaward migration and size after their first year of growth at sea. Fish that matured first after 2 years at sea were slightly larger as smolts but grew significantly more in their first year at sea than did those maturing a year later. However, neither measure was related to egg size or egg number after adjustment for length. The processes controlling the egg size–fecundity trade‐off in steelhead and other salmonid species remain unclear, but the present evidence is incompatible with the hypothesis that growth in freshwater as juveniles determines reproductive trade‐offs at maturity by females.

Altered smolt timing for masu salmon Oncorhynchus masou resulting from domestication

We investigated differences in the timing of seaward migration among hatchery reared progeny of first and third generation captive brood and wild Shiribetsu River adult masu salmon released into the Furuu River of south-western Hokkaido in Japan, and naturally occurring wild masu salmon. After only three generations of domestication, captive brood smolts migrated downstream later than the hatchery reared progeny of wild fish of the same ancestry and of native Furuu wild smolts. In addition, there were sometimes significant differences in the timing of smolt migration between the Furuu River wild smolts and the smolts from the hatchery reared progeny of Shiribetsu River wild fish that suggests that there may also be important inter-river differences timing of smolt migration. We caution managers against using captive brood stocks that are more than two generations removed from the wild to avoid the development of potentially negative traits such as delayed seaward migration that may reduce salmon fitness and survival. We also caution managers against the transfer of fish between different river systems where the maintenance of natural self sustaining populations is important.

Imprinting and Homing of Atlantic Salmon (Salmo salar) Kelts

Experiments were conducted to determine if a process similar to imprinting occurs during each seaward migration of Atlantic Salmon (Salmo salar). Displaced Atlantic salmon kelts homed to the site from which they had previously emigrated to sea as smolts (Big Salmon River, New Brunswick, Canada) and did not home to their release sites, or their overwintering sites 150 km away. Delaying release of kelts past the time of normal seaward migration did not cause the behavioral changes associated with interference of the imprinting process described for smolts. Delayed release kelts did not remain near their seawater or estuarine release sites, home to their release sites, or non-selectively home to suitable spawning streams near their release sites. Delayed released kelts travelled away from the release site faster than kelts released during their normal time of seaward migration, but they did not return either to Passamaquoddy Bay or their natal stream in the calendar year of their release (the normal time for this stock) or in subsequent years.

Hormone‐induced maturation in male New Zealand freshwater eels (Anguillaspp.)

Abstract Male adult Anguilla australis and A. dieffenbachii caught migrating to sea were held in sea water and induced to mature with injections of human chorionic gonadotrophin. Eye size increased during maturation by a factor of 1.5 in A. australis and 1.2 in A. dieffenbachii from the pre‐injection migrant stage. Spawning was observed in 4 of the 42 eels that matured. A. australis matured at between 12 and 28°C and A. dieffenbachii between 12 and 26°C. Minimum maturation times occurred at 26°C for A. australis and 24°C for A. dieffenbachii. Maturation rate and temperature were linearly related in both species. A. dieffenbachii matured more quickly than A. australis at all temperatures; this is probably related to the different state of sexual development of the experimental eels. At the time of seaward migration A. dieffenbachii are more sexually advanced than A. australis.

Distribution, Growth, and Survival of Sockeye Fry (Oncorhynchus nerka) Produced in Natural and Artificial Stream Environments

A comparative study was made at Babine Lake, British Columbia, of the distribution, growth, and survival of sockeye salmon fry resulting from the same parental stock but reared in natural and artificial streams. Fry produced from natural spawning in the Fulton River and from eyed eggs planted in an adjacent artificial spawning channel were marked distinctively, released, and later recovered in the lake nursery area and at the lake outlet at time of seaward migration. Both groups dispersed rapidly and widely into the main lake basin and apparently mixed extensively with sockeye produced from other main lake tributaries. Lake distribution of marked fish, and the underyearling population as a whole, was not uniform nor static and the fish were concentrated in different lake areas at different times of their first growing season. River and channel fry were comparable in mean length at time of release but subsequently channel fish were smaller. Their smaller size appeared to result from late lake entry and a slower rate of growth for a short period thereafter. Over most of the growing period (June 25–October 25) rates of growth in length were similar (instantaneous daily rates of 0.00687 and 0.00737). No significant difference in survival rates of the two groups could be detected for the first 5 months of lake residence. Production of age I seaward migrants was less for river fish than for channel fish but no significance was attached to the small difference observed. These findings are discussed with respect to a fish-cultural scheme which is aimed at increasing adult production by making fuller use of the lake's capacity to rear young sockeye.

Estimation of Sea Mortality Rates for the 1961 Brood-Year Pink Salmon of the Bella Coola Area, British Columbia

A two-stage marking experiment was designed to estimate natural mortality rates of pink salmon during their early (coastal) and later (oceanic) periods of sea life. Difficulty was encountered because of disproportionate representation of various stocks in the marked populations, the stocks being subject to dissimilar rates of exploitation. Several assumptions were made for which the error introduced could not be fully evaluated. Under these assumptions, the results indicate (approximate values) that 55.7 million fry, resulting from the 1961 Burke Channel–Bella Coola stocks brood year, entered the sea. Of these, 77% died during the initial 40-day period of life in the enclosed marine waters. The following stage, from the time of seaward migration to adult return, a period of about 410 days, 78% of the remaining population died. The adult return was subjected to a rate of exploitation of 0.85 which produced a catch of 2.5 million fish. The spawning escapement was approximately 450,000 fish. It is established that the rate of natural mortality during the initial short period was much more severe than that during the remaining period of sea life.