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European eels (Anguilla anguilla) undertake a approximately 5000-kilometer (km) spawning migration from Europe to the Sargasso Sea. The larvae are transported back to European waters by the Gulf Stream and North Atlantic Drift. However, details of the spawning migration remain unknown because tracking eels in the Atlantic Ocean has, so far, eluded study. Recent advances in satellite tracking enable investigation of migratory behavior of large ocean-dwelling animals. However, sizes of available tags have precluded tracking smaller animals like European eels. Here, we present information about the swimming direction, depth, and migratory behavior of European eels during spawning migration, based on a miniaturized pop-up satellite archival transmitter. Although the tagging experiment fell short of revealing the full migration to the Sargasso Sea, the data covered the first 1300 km and provided unique insights.

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Swimming performance was compared among groups of juvenile rainbow trout Oncorhynchus mykiss (5-10 g) with and without intraperitoneally implanted radio transmitters. The generally accepted rule of 2% body weight : transmitter weight was extended to a 6-12% ratio, and swimming performance was not altered by the presence of the transmitter or effects of the operation. Also, no relationship was found between the weight of the fish and its swimming performance among the groups examined. Although we found swimming performance was not affected by implantation of transmitters weighing up to 12% of the body weight, changes in behavior were not evaluated. We suggest further research be done so that the ''2% rule'' can be replaced by an index with a more scientific basis. Instead of using a scale based on percentage of transmitter to body weight in air, preferred indices could be weight in water, volume of the tag, or both.

Fishways have traditionally been designed to provide safe passage for jumping fish and only recently have nonjumping species been considered. Concern over dwindling populations of lake sturgeon, Acipenser fulvescens, has focused attention on fishway designs that accommodate its swimming abilities. The objective of this study was to derive a model that relates swimming endurance of lake sturgeon to length and flow characteristics of fishways. Endurance at sustained and prolonged swimming speeds (those maintainable for more than 20 s) increased with water temperature but was independent of temperature at higher burst speeds. Endurance increased with total length at all swimming velocities. Swimming performance of lake sturgeon, relative to body length, is inferior to that of most salmonids, particularly at burst speeds. Fishway designers need to consider swimming ability, space requirements, and behavior of lake sturgeon to ensure that they can ascend potential migratory obstacles safely.

Paleomagnetic measurements of meteorites 1-5 suggest that, shortly after the birth of the solar system, the molten metallic cores of many small planetary bodies convected vigorously and were capable of generating magnetic fields 6. Convection on these bodies is currently thought to have been thermally driven 7,8 , implying that magnetic activity would have been short-lived 9. Here we present a time-series paleomagnetic record of the field recorded by the Imilac and Esquel pallasite meteorites, derived from nanomagnetic images 10 of their metallic matrices. The results reveal a history of long-lived magnetic activity on the pallasite parent body, capturing the decay and eventual shut down of the magnetic field. We demonstrate that magnetic activity driven by progressive solidification of an inner core 11-13 is consistent with our measured magnetic field characteristics and cooling rates 14. Solidification-driven convection was likely common among small body cores 15 , and, in contrast to thermally driven convection, will have led to a relatively late (hundreds of millions of years after accretion), long-lasting, intense and widespread epoch of magnetic activity among these bodies in the early solar system. The pallasites are slowly cooled (2-9 K Myr-1) 14 stony-iron meteorites 16 , which originated from the mid-to upper-mantle of a ~200-km-radius body 1. The slow cooling rate of these meteorites allowed for characteristic microstructures to form in their metal matrix 17 , a key feature of which are regions of intergrown nanoscale islands of tetrataenite (ordered FeNi) 18,19 and an ordered Fe 3 Ni matrix 20 , collectively known as cloudy zones (CZ) 21. During parent body cooling, these tetrataenite islands exsolved and subsequently coarsened over tens of millions of years 22. The island diameter decreases systematically across the CZ, reflecting a decrease in the local formation age of the islands 20. Each island adopted one of three orthogonal magnetic easy axes as it formed 18,23 , thus could display any one of six magnetisation directions. Variations in the intensity and direction of an external magnetic field led to measurable differences in the populations of each magnetisation direction 20. Crucially, the temporal evolution of an external field is recorded by the variations in the relative proportions of these directions across the CZ 10 , which can be quantified using high-resolution nanomagnetic imaging. These images were captured for the Imilac and Esquel pallasites, utilising X-ray magnetic circular dichroism 24,25 at the X-ray photoemission electron microscope (XPEEM) 26 at the BESSY II synchrotron, Berlin, which provides the spatially resolved magnetisation of a sample surface with a resolution down to 40 nm over a 5 µm field-of-view 10. Four and six non-overlapping, 450-nm-wide regions across the CZ (decreasing age) were extracted from the XPEEM images of Imilac and Esquel meteorites, respectively (Fig. 1). The field recorded by each region was deduced by comparing the experimental XPEEM signal to tha...

Anadromous Atlantic salmon (Salmo salar) were collected during their spawning migration along the Exploits River, Newfoundland, in 1996 and analyzed for stable-isotope ratios (δ13C and δ15N) and tissue concentrations of protein and lipid. Fish became progressively more 13C enriched during the spawning migration (about -24 to -19‰), and the degree of enrichment was dependent on tissue type (red muscle: +4.1‰, liver: +2.6‰, white muscle: +1.3‰). Only liver showed consistent changes in δ15N, as overwintering kelts (13.5 ± 0.8‰) were about +2‰ more enriched than upstream migrants (11.4 ± 1.5‰). Isotopic enrichment in migrating salmon appeared to result from mobilization, reorganization, and catabolism of stored lipid and protein reserves associated with the cessation of feeding upon entering freshwater. The most significant correlations existed between lipid content and δ13C in red muscle (r2 = 0.67) and protein content and δ15N in liver (r2 = 0.32). This study shows that fasting affects the stable-isotope ratios of both carbon and nitrogen in anadromous fishes and that nutritional status should be considered when inferring food web relationships from the isotopic compositions of migrating salmonids.

Freshwater and early marine migration and survival of endangered Cultus Lake sockeye ( Oncorhynchus nerka ) salmon were studied using the Pacific Ocean Shelf Tracking (POST) array. Smolts were acoustically tagged in 2004–2007, and their migration was recorded within the lower Fraser River and coastal southern British Columbia waters. Most smolts showed rapid directional movement (swimming speeds of ~15–30 km·day–1). Average exit time from the Fraser River was 4.0–5.6 days after release, and average residence time within the Strait of Georgia was 25.6–34.1 days. Most individuals migrated northward, generally close to the mainland coast. Survival rates, assessed using standard mark–recapture methods, were generally high during the downstream migration (50%–70%), except in 2005 when survival was <20%, possibly because of a late release. Marine survival rates were stable among years, between 10%–30% at a subarray sited 500 km away from the release site. Movement rates were similar to those of previously published work, but the POST array provided direct measurements of movement and estimates of survival and demonstrated the feasibility of establishing continental-scale acoustic arrays for management and conservation of marine species.