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Korea’s northeastern coastal waters (NECWs) represent the southwestern range of chum salmon ( Oncorhynchus keta ), spanning temperate and boreal zones. However, understanding their migration to the NECWs in association with environmental changes remains challenging. Using tagging and tracking techniques, we studied salmon migration behavior and timing during spawning in ascending rivers. When sea levels rose, resulting in a decrease in sea surface temperature, salmon exhibited active vertical movement through the water column and migrated to the surface layer. The number of salmon ascending rivers between early October and early December increased when the sea surface temperature decreased below 18 °C, peaking when the water column was mixed during spring tides, and decreased when sea surface temperature dropped below 14 °C. In rivers, salmon favored gravelly riverbeds over sand/silt substrates, stayed in deep and shaded areas during the day, and advanced upstream at night. Our findings emphasize that water temperature and tidal elevation are key factors affecting salmon distribution in coastal waters and the timing of river entry. Riverbed composition, water depth, and photoperiod also influenced migration speed and timing in rivers. This research enhances our understanding of salmon behavior during spawning in the NECWs and adjacent rivers at their southern limits.

Blocked riverine spawning migration routes constitute one of the main threats of potamodromous fish that require well-connected habitats of high quality to maintain their unique life-history form. Alburnus vistonicus, a potamodromous species inhabiting the Vistonis freshwater system in northern Greece, manages to cope with wide salinity fluctuations in Vistonis Lake, but the impact of the fragmented connectivity of the lake’s two main freshwater suppliers, Kosynthos and Kompsatos rivers, has not been assessed. We analyzed the species whole reproductive cycle and we divided it into phases. Cyclicity was observed in gonadosomatic index and oocyte diameter, while total fecundity remained constant prior to the onset of the spawning season. Stomach fullness and condition factor differed among the surveyed habitats. Successful spawning and multiple spawning were verified, as well as spawning migration and potamodromy. However, mature females with markers of recent spawning activity were captured also in the lake, suggesting spawning occurrence in the lake or a previously undocumented oscillatory behavior between the lake and the rivers during the spawning season and between sequential spawning events. Either behavior may serve as a mechanism to overcome the restricted spawning migration routes due to the fragmented river connectivity.

Chinook salmon (Oncorhynchus tshawytscha) is invading South America. Both the high plasticity and genetic diversity of introduced propagules have been hypothesized to be responsible for the success of this species’ invasion. Yet, the influence of environmental variability on the expressed phenology of the adult spawning migration has been overlooked in this region. Here, we examined the consistency in timing, duration, and relative abundance of adult salmon migrants and their associations with environmental river conditions and surrounding ocean in a regulated river system in Patagonia. We conducted monthly long-term snorkeling fish surveys (2010–2019) and collected associated environmental information from the river and ocean. We observed a recent increase in duration of the spawning migration and a decline in the relative abundance of adult migrants. A warming phase of the Southern Pacific Ocean (during the two previous years) was associated to an extended migration season, whereas a colder river in fall was associated to a lower number of adult migrants. Collectively, our findings suggest that rapid phenological shifts could occur in a recently established salmon population (circa 1980). This process could be explained by novel selective pressures and expression of life history traits in response to novel environmental regimes. Further long-term surveys of introduced salmon can aid in parsing the relationships between environmental regimes and the biology and persistence of these self-sustained populations.

“Soggy meat”, which is abnormally softened crushed sardine meat, is often detected in canned sardines seasoned with tomato chili sauce. As soggy meat represents lysed and decomposed tissue, we examined protease activity in the sardine meat during the spawning season. Abnormally soft sardine meat is often found in canned sardine meat from fish caught during the spring and landed at Choshi Fishing Port in Japan, located on the Pacific Ocean. We measured cathepsin L activity in the muscle and protein, lipid, and moisture, as well as the trace elemental concentrations. High cathepsin L activity was detected in February and March during the Pacific sardine spawning season. In addition, 4 of 20 fish had extremely high protease activity of more than 30 units/g. Cathepsin L activity in the muscle was negatively correlated with selenium, protein, and lipid concentrations, indicating that sardines in the spring were starving during their spawning migration. Therefore, starved physiological conditions during the spawning season may be responsible for soggy meat.

During their once-in-a-lifetime transoceanic spawning migration, anguillid eels do not feed, instead rely on energy stores to fuel the demands of locomotion and reproduction while they reorganize their bodies by depleting body reserves and building up gonadal tissue. Here we show how the European eel (Anguilla anguilla) breaks down its skeleton to redistribute phosphorus and calcium from hard to soft tissues during its sexual development. Using multiple analytical and imaging techniques, we characterize the spatial and temporal degradation of the skeletal framework from initial to final gonadal maturation and use elemental mass ratios in bone, muscle, liver, and gonadal tissue to determine the fluxes and fates of selected minerals and metals in the eels’ bodies. We find that bone loss is more pronounced in females than in males and eventually may reach a point at which the mechanical stability of the skeleton is challenged. P and Ca are released and translocated from skeletal tissues to muscle and gonads, leaving both elements in constant proportion in remaining bone structures. The depletion of internal stores from hard and soft tissues during maturation-induced body reorganization is accompanied by the recirculation, translocation, and maternal transfer of potentially toxic metals from bone and muscle to the ovaries in gravid females, which may have direct deleterious effects on health and hinder the reproductive success of individuals of this critically endangered species.

To examine the habitat usage of adult Pacific bluefin tuna (PBF), electronic tagging was conducted in the Sea of Japan during May and June of 2012−2017. Archival tags were internally implanted and pop-up satellite archival transmitting tags were deployed; data on the horizontal movements and diving behaviours of 36 individual PBF were successfully retrieved. In the summer spawning season, the tagged PBF were concentrated near Sado Island and Oki Island in the Sea of Japan, and they were distributed widely to the southwest (near Tsushima Island) or northeast (near the Tsugaru Strait) in the autumn and winter. We obtained the first long-term tracking record (246 d) for adult PBF, and this individual exhibited residency in a known spawning region during the spawning season in the proximity of warm-core eddy features. This fish spent most of the daytime below the thermocline between 30 and 150 m depths where the surface ambient temperature was 26.0 ± 1.5°C, but at night it ventured into the warm surface layer. Its whole-body heat transfer coefficient increased when it experienced warm waters (≥24°C), which we suggest is a physiological response to avoid overheating. The mean peritoneal cavity temperature was only 1.8°C higher than the ambient temperature, compared with 6.9°C higher during the cooler autumn–winter period. Our hypothesis is that the warm surface temperatures found in the spawning grounds induce a physiology–reproduction trade-off in adult PBF, which must behaviourally and physiologically thermoregulate their body temperature to gain spatial and temporal access to oceanographic conditions that may promote larval survivorship and growth.

The results of a study of the distribution of eggs of Baikal omul Coregonus migratorius in a large foothill watercourse in Siberia in 1935–2022 are presented. The distanse of omul spawning migration in the Selenga River (some of the migration parameters are taken into account since 1920) depends on a certain combination of abiotic environmental factors, biological parameters of spawners, timing of entry into the river, and the number of their spawning herd. The distance of migration has a high inverse correlation with the degree of maturity of the spawners, which is expressed by the maturity index of omul females, as well as with the timing of herd entry into the river and water consumption at the beginning of migration (September). The higher the maturity of the reproductive products of spawners, the shorter the migration route, and vice versa. In addition, the maturity of spawners also determines the timing (date) of stock entry into the river: less mature individuals enter the river earlier and are distributed at spawning grounds higher upstream, while more mature individuals enter the river later and spawn downstream of the river. The distance of omul spawning migration increases during a decrease in the water flow of the Selenga River in September and, on the contrary, decreases with increase in water flow. Finally, The distance of the spawning route of omul in Selenga is determined by the size of the spawning herd: the higher the number of spawners, the more area they need for spawning and the longer their spawning route. Water temperature does not influence the length of omul spawning migration. The spawning run takes place against the background of a decrease in water temperature in the river.

For long-distance migrations to pay off for individual fish, the energetic costs must be counterbalanced by benefits. Understanding the fitness trade-offs associated with migration is essential for our ability to sustainably manage migratory species. Here, we investigated such trade-offs associated with the spawning migration of Northeast Arctic cod Gadus morhua, a stock of high historical and commercial value known to be able to migrate more than 1000 km southward to reach suitable spawning grounds. Reaching the more distant spawning grounds requires more energy and hence leaves less energy available for processes such as egg production. Previous studies have indicated that increased larval survival (e.g. from rapid early development in warmer southern waters) may offset the parental costs of migration. However, it was suggested that spatial variability in survival of early life stages might cancel out this survival benefit. As an alternative, we ask if the fitness benefit of long-distance migration may reside in increased offspring growth. Using an integral projection model incorporating effects of body length and migration distance, we quantified the increase in offspring growth needed to offset parental costs of long-distance migration on fitness. Our results suggest that a 12.5% length in crease of juvenile cod is required to offset parental costs of long-distance migration. This is within the estimated growth benefit of 20% suggested by drift models of early life stages of cod. These results highlight the potential importance of offspring growth as another factor explaining the benefit of long-distance migration, broadening our knowledge on spawning migration.

Marine protected areas (MPAs) are considered an effective tool for protecting marine organisms. The precise estimation of home range size, as well as diel differences in home ranges, are essential when considering the appropriate size of an MPA. In addition, behavioral characteristics of spawning migration should also be considered for species that form spawning aggregations. Our aim was to clarify the diel variation in home range size and the degree of precision of the returning ability of white-streaked grouper Epinephelus onus by acoustic telemetry. Seventeen individuals were studied, and nighttime home range sizes that were calculated by 50 and 95% kernel utilization distributions were 5.9-fold and 5.5-fold greater, respectively, than the daytime home ranges. The average inter-center distance between home ranges during the 2 time periods ranged from 3.0 to 67.9 m (22.5 m on average), suggesting that the day–night home range shift within the home ground varied individually. Returning ability for 10 individuals that showed clear spawning migration behavior was also analyzed, and the average inter-center distance between home ranges during the periods before and after spawning was 8.1 m. Eight out of these 10 individuals showed precise returning after the spawning migration to the patchy coral substrates that were used before the spawning migration. The present study suggests that appropriate setting position of the home ground can establish long-term protection of the species due to their precise returning ability after the spawning migration.

Background Movement to spawn in offshore waters provides numerous benefits to inshore marine fish and crustacean species, but there is often limited empirical data on the behaviour and ecology of this key aspect of their life history. This is the case for Portunid mud crabs of the genus Scylla , which have swimming abilities and are widely distributed throughout the Indo-West Pacific. Methods Two lines of evidence were used to quantitatively and qualitatively improve the understanding of the spawning migration of Scylla serrata , the largest of the four species of mud crab: (i) the novel application of the smallest pop-up satellite archival tag available to twelve females considered to have advanced ovarian development, and (ii) the collation of requested sightings of migrating and/or egg-bearing females. The satellite tags provided empirical data on the movement and behaviour of migrating females, whilst the collated sightings ( n  = 101) provided information on the direction and seasonality of migration. The study occurred between October 2020 and June 2024 in the waters of Queensland, Australia, that included the continental shelves of the Gulf of Carpentaria (approximately 12–18°S, 138–142°E) and the Queensland’s east coast (approximately 11–28°S, 142–153.5°E). Results Satellite tag data was obtained from nine individuals, with 30 second interval depth, temperature, and light archived data available from two individuals, including one egg-bearing female. This data indicated three types of behaviour associated with the spawning migration: (i) estuarine benthic behaviour in shallow water (<10 m) where these benthic dwelling crabs remain mostly on the sea floor of tidal estuarine habitats; (ii) active swimming behaviour when the crabs alternated between near surface positioning interspersed with sedentary behaviour at increasing depth likely indicative of swimming to deeper offshore waters; and (iii) offshore benthic behaviour in deeper waters (>20 m). Over 100 sightings of spawning females provided a broadscale insight into their movement and possible offshore destinations. Conclusions Results indicated the offshore spawning migration of giant mud crabs is variable, depending on local bathymetric and oceanographic conditions, which has consequences for larval distribution and the genetic and demographic connectivity of this species. Pop-up satellite archival tags can provide novel insights into the spawning migration of brachyuran crabs, providing additional information to inform fisheries management.