Since inferring spawning areas from larval distributions in the Sargasso Sea a century ago, the oceanic migration of adult American eels has remained a mystery. No adult eel has ever been observed migrating in the open ocean or in the spawning area. Here, we track movements of maturing eels equipped with pop-up satellite archival tags from the Scotian Shelf (Canada) into the open ocean, with one individual migrating 2,400 km to the northern limit of the spawning site in the Sargasso Sea. The reconstructed routes suggest a migration in two phases: one over the continental shelf and along its edge in shallow waters; the second in deeper waters straight south towards the spawning area. This study is the first direct evidence of adult Anguilla migrating to the Sargasso Sea and represents an important step forward in the understanding of routes and migratory cues.

Halifax Harbour is located on the Atlantic coast of Nova Scotia, Canada. It is one of the world's largest, ice-free natural harbours and of great economic importance to the region. A good understanding of the physical processes controlling tides, flooding, transport and dispersion, and hydrographic variability is required for pollution control and sustainable development of the Harbour. For the first time, a multinested, finite difference coastal ocean circulation model is used to reconstruct the three-dimensional circulation and hydrography of the Harbour and its variability on timescales of hours to months for 2006. The model is driven by tides, wind and sea level pressure, air-sea fluxes of heat, and terrestrial buoyancy fluxes associated with river and sewage discharge. The predictive skill of the model is assessed by comparing the model simulations with independent observations of sea level from coastal tide gauges and currents from moored instruments. The simulated hydrography is also compared against a new monthly climatology created from all available temperature and salinity observations made in the Harbour over the last century. It is shown that the model can reproduce accurately the main features of the observed tides and storm surge, seasonal mean circulation and hydrography, and wind driven variations. The model is next used to examine the main physical processes controlling the circulation and hydrography of the Harbour. It is shown that nonlinear interaction between tidal currents and complex topography occurs over the Narrows. The overall circulation can be characterized as a two-layer estuarine circulation with seaward flow in the thin upper layer and landward flow in the broad lower layer. An important component of this estuarine circulation is a relatively strong, vertically sheared jet situated over a narrow sill connecting the inner Harbour to the deep and relatively quiescent Bedford Basin. Local wind driven variability is strongest in winter as expected but it is also shown that a significant part of the temperature and salinity variability is driven by physical processes occurring on the adjacent inner continental shelf, especially during storm and coastal upwelling events.

Assessing the performance of a multi-nested ocean circulation model using satellite remote sensing and in situ observations. © 2016 Shiliang Shan et al. This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial 4.0 International License (http://creativecommons.org/licenses/by-nc/4.0/), permitting all non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. 39 RESEARCH ARTICLEAssessing the performance of a multi-nested ocean circulation model using satellite remote sensing and in situ observations

Both the American eel (Anguilla rostrata) and European eel (Anguilla anguilla) undertake long-distance migrations from continental waters to their spawning sites in the Sargasso Sea. Their migration routes and orientation mechanisms remain a mystery. A biophysical particle tracking model was used in this study to simulate their oceanic migration from two release areas: off the Scotian Shelf (Canada) and off the Irish continental shelf. Two plausible swimming-directed behaviours were considered for simulating two different migratory paths: true navigation to specific spawning sites and innate compass orientation towards the vast spawning area. Several combinations of swimming speeds and depths were tested to assess the effect of ocean circulation on resulting migratory pathways of virtual eels (v-eels), environmental conditions experienced along their oceanic migration, and energy consumption. Simulations show that the spawning area can be reached in time by constantly swimming and following a readjusted heading (true navigation) or a constant heading (compass orientation) even at the lowest swimming speed tested (0.2 m s−1) for most v-eels. True navigation might not be necessary to reach the spawning area. The ocean currents affect mainly the migration of American v-eels, particularly for swimming speeds lower than 0.8 m s−1. The ocean circulation increases the variability in the oceanic migration and generally reduces the efficiency of the v-eels, although positive effects can be possible for certain individuals. The depth range of diel vertical migration (DVM) significantly affects the total energy expenditure due to the water temperature experienced at the various depths. Model results also suggest that energy would not be a limiting factor as v-eels constantly swimming at 0.8 BL s−1 spent <25 and 42% of energy available for migration for American and European v-eels, respectively.

Dissolved hydrogen measurements were made at high resolution in surface waters along a tropical north Atlantic transect between Guadeloupe and Cape Verde in 2015 (Meteor 116). Parallel water samples acquired to assess the relative abundance of the nifH gene from several types of diazotrophs, indicated that Trichodesmium and UCYN‐A were dominant in this region. We show that a high degree of correlation exists between the hydrogen saturations and UCYN‐A nifH abundance, and a weak correlation with Trichodesmium. The findings suggest that nitrogen fixation by UCYN‐A is a major contributor to hydrogen supersaturations in this region of the ocean. The ratio of hydrogen released to nitrogen fixed has not been determined for this symbiont, but the indications are that it may be high in comparison with the small number of diazotrophs for which the ratio has been measured in laboratory cultures. We speculate that this would be consistent with the diazotroph being an exosymbiont on its haptophyte host. Our high resolution measurements of hydrogen concentrations are capable of illustrating the time and space scales of inferred activity of diazotrophs in near real‐time in a way that cannot be achieved by biological sampling and rate measurements requiring incubations with 15N2. Direct measurement of high resolution spatial variability would be relatively challenging through collection and analysis of biological samples by qPCR, and extremely challenging by 15N‐uptake techniques, neither of which methods yields real‐time data. Nonetheless, determination of fixation rates still firmly depends on the established procedure of incubations in the presence of 15N2.