Aarestrup, K. | bioRxiv Science

bioRxiv2026

Smolt migration in relation to latitude, season and migratory distance

Downstream migration and sea entry are periods of high mortality for sea trout smolts and migration timing is a critical aspect of survival. We aimed to investigate migration timing of PIT-tagged sea trout smolts across a latitudinal gradient in five freshwater systems across Europe: Norway, Sweden, Denmark, France and Portugal. In two systems, smolt migration was further examined to assess a) differences in sex, body size and condition between autumn and spring migrants; b) the influence of spatial origin within the stream; and c) relationships between individual body size and migration date. Tagged smolts were not detected migrating in the Portuguese watershed. Spring migration timing differed significantly between watersheds in Norway, Sweden, Denmark and France. Generally, there was a trend of earlier migration at lower latitudes. Autumn vs spring migration was examined in Gudso-Denmark. Autumn migrants were larger in both length and mass, with no differences were found in sex ratios or body condition. In terms of spatial origin, fish originating from an upstream site were more likely to migrate in the autumn compared to the spring and vice versa. Size dependent migration was found in the Swedish system, Haga [a]-Sweden, with larger individuals migrating earlier in the spring than smaller individuals. Outward-migrating smolts were also more likely to originate from a downstream site than an upstream site. Overall, these results show both large-scale geographic and fine-scale individual influences on migration timing. Given that climate change may have large impacts on migration patterns in sea trout, understanding variability in migratory patterns across a latitudinal gradient is an important tool for predicting responses to environmental changes.

ecology↗

bioRxiv2017

Improved management facilitates return of an iconic fish species

Species declines and losses of biota are often associated with shifting baselines in perceived historical abundances, and/or neglect or abandonment of recovery actions aimed at ecological restoration. Such declines are frequently accompanied by contractions in the geographical distribution of the species, with associated negative ecological impacts and diminishing socio-economic benefits. Here we show using citizen science and other data that after 50-60 years of near total absence, the iconic top predator and highly migratory species bluefin tuna, Thunnus thynnus, returned by the hundreds if not thousands in waters near Denmark, Norway and Sweden during August-October 2015-2017. The re-utilisation of this former habitat is part of a geographically more widespread expansion of the summer foraging area to the northern part of the northeast Atlantic Ocean, encompassing waters from east Greenland to west Sweden. The remarkable return to the Skagerrak, Kattegat and North Sea has been facilitated by improved fishery management for bluefin tuna and its prey. Bluefin tuna biomass in the northeast Atlantic and Mediterranean has been increasing since a recovery plan was implemented in the late 2000s, and biomasses of two key prey species (herring, Clupea harengus; mackerel, Scomber scombrus) recovered during the late 1980s-1990s. The reappearance of bluefin tuna in the Skagerrak-Kattegat and other waters of northern Europe, despite a recent history of mismanagement and illegal fishing in the northeast Atlantic and Mediterranean which led to a critical population decline, offers hope that other marine ecological recoveries are possible under improved management of fisheries and ecosystems. One Sentence SummaryImproved management promotes the return of an ocean icon to northern Europe. Significance StatementCommercial fisheries are often perceived being in a state of decline and collapse, putting food and economic security at risk. Such declines are frequently accompanied by contractions in stock distribution, negative ecological impacts and diminishing socio-economic benefits. Here we present an example based on one of the worlds most valuable and iconic fish species, bluefin tuna, which demonstrates that effective management of both bluefin tuna and its prey has been a key factor leading to a remarkable reoccupation of formerly lost habitat. This reappearance, following decades of absence, occurred despite the bluefin tuna stock having had a recent, long history of unsustainable and illegal exploitation. Marine ecological recovery actions can be successful, even in situations which may initially appear intractable.

ecology↗