Maps of the spatial distribution of stable isotope ratios across wide geographic areas (isoscapes) are increasingly used to study mechanisms of nutrient flux, movements of animals, and to improve trophic information derived from stable isotope analyses. Isoscapes are usually constructed from reference samples collected from known geographic positions, a time consuming and costly process. In this study, we test the temporal stability of isoscapes of carbon and nitrogen isotopes across the North Sea over a ten-year period. Using jellyfish tissues as reference organisms, we show that hydrodynamic and biogeochemical processes controlling the distribution of carbon and nitrogen isotope values, and thus that the underlying isoscapes, are temporally stable. Remarkably, broad geographic variations in stable carbon and nitrogen isotope distributions across the North Sea are consistent with previously published variations seen in archaeological cod bones from the ninth to seventeenth centuries, despite dramatic changes in land use in the past 1,000 years. Stable isotope-based studies of trophic interactions or movements in animals with lifespans of a year or more in shelf ecosystems can consequently be referenced to previously published isoscape models, and do not require construction of temporally explicit isotope baseline corrections. Scyphomedusan jellyfish are excellent reference organisms for ecological stable isotope analyses in pelagic ecosystems, due to their widespread distributions, well-defined life histories, and fast tissue growth.

The study of the morphological and chemical characteristics of otoliths have recently been proposed as complementary tools for fish stock identification. However, their effectiveness remains to be fully assessed, especially in species whose life history is still poorly understood. The relative efficiency of otolith microchemistry and otolith shape analysis in discriminating samples of the deep-sea macrourid Coryphaenoides rupestris collected in different areas in the North Atlantic was examined. Otolith microchemistry based on LA/ICP-MS found significant differences in the concentrations of lithium, manganese and barium among sample sites. This allowed for very high classification accuracy (92%), when using discriminant function analysis. Otolith shape analysis based both on linear shape measurements and elliptical fourier analysis revealed a contrasting weak discrimination, with only 43% classification success. Otolith microchemistry appears to be a more effective tool in identifying individual fish from different locations. The implications for the study of population structure are discussed taking into account the limitations of the methodologies employed.

Little is understood about connectivity of deep-sea fish populations. Analysis of the geochemical properties of fish otoliths is one way to draw inferences regarding their movements and habitat use in the marine environment. Trace element and stable isotope analyses of otoliths were undertaken to assess patterns of spatial and temporal population structure of a wide-ranging deepwater fish, the roundnose grenadier Coryphaenoides rupestris. Fish were sampled from 4 locations across the distribution range of the species in the northeast Atlantic. Multivariate analyses of elemental ratios (Li/Ca, Mn/Ca, Ba/Ca, Zn/Ca, Cu/Ca) revealed strong geographic separation at each life stage, and an overall significant difference between life stages. Otolith oxygen (δ 18 O) and carbon (δ 13 C) stable isotope analysis indicated a depth migration (and reduction in metabolic activity) from relatively shallow in the juvenile phase to much deeper in the adult phase at all locations. The results suggest that roundnose grenadier are comprised of geographically distinct population units that persist throughout their life-history, migrating deeper as they get older.

As human exploration and harvesting moves to the high seas, ecological understanding of the deep sea has become a priority, especially in those commercially exploited species whose life cycle, habitat use, and demographic structure remain poorly understood. Here we combine otolith trace element and stable isotope analyses with microsatellite data to investigate population structure and connectivity in the migratory deep-sea black scabbardfish (Aphanopus carbo), sampled along a latitudinal gradient spanning much of the known species range in the Northeast Atlantic. In each sampled life stage, otolith trace element and oxygen isotope compositions are similar among fish from different capture locations, but otolith compositions vary greatly between life stages. Oxygen isotope compositions indicate ontogenetic migrations from relatively warm water conditions during larval growth to cooler waters with increasing age. Analysis of microsatellite DNA also suggests lack of genetic structure among the areas sampled. The multidisciplinary approach employed collectively suggests that A. carbo individuals undergo an ocean-scale ontogenetic migration, beginning with spawning in southern, warm-water Macaronesian areas (potentially dominated by Madeira), followed by a large proportion of immature fish moving to and feeding on the continental slope in northern areas. The results lend the first conclusive evidence for defining the life-history circuit of this species and the perception of its stock structure across the North Atlantic.