Stock structure and connectivity of capelin (Mallotus villosus) in Greenland: Otolith microchemistry used to track past whereabouts of a small, pelagic fish

The overall aim of the research presented in this PhD thesis is to provide information on inshore stock structure, connectivity, and spatial dynamics of the small, pelagic fish species capelin (Mallotus villosus) in Greenland. Capelin is one of the most abundant species in the Arctic and has a central role in Arctic food webs. In Greenland, large schools of capelin appear at shallow water spawning sites in late spring and early summer. Their whereabouts the rest of the year are largely unknown. This study applies otolith (earstone) chemistry to investigate past whereabouts of capelin that spawn inshore in Greenland. Otoliths may serve as chronological records of ambient water chemistry exposure, and can therefore be used to distinguish between residence in environments that are chemically distinct. Chemical patterns of 14 different elements are examined for capelin otoliths collected from 19 different spawning localities along Greenland east and west coast. The project is divided into three studies that combine to address the projects main objectives. The first study identifies large-scale, regional differences in otolith chemistry, particularly for elements Li and Ba, which can be attributed to oceanographic differences in salinity along the coastline. This is used to form distinct chemical baselines for two west coast regions. In the second study, these baselines form the basis for examination of capelin whereabouts in the past. Multivariate classifications at high temporal resolution are used to evaluate if individual capelin have resided in one region or the other in the past. The results indicate that capelin have generally resided in the region where they were eventually caught, although seasonal variations in Li concentrations complicate the use of the baselines. Significant ambient Pb contrasts allow for comparison of otolith Pb between adjacent localities. The second study therefore progressed by zooming in on the use of otolith Pb as a tracer of ambient Pb. Differences in Pb concentrations for fish caught at different localities indicate lifelong separation of spawning segments, with spatial separation ranging from 150-300 km down to as little as 15-50 km in one fjord. In the third study, chronological profiles of 12 elements are used to evaluate stock structure and spatial dynamics of adult fish. 8 of the elements vary significantly with season. For salinity proxies Li and Sr, this is attributed to seasonal variations in freshwater influx from glaciers and streams, which is confirmed in salinity profiles from Godthåbsfjord (Nuup Kangerlua). For Mn, Rb, and Si, seasonal variation is likely tied to differences in sediment influx, whereas seasonal patterns of K and Zn can most likely be attributed to seasonal differences in somatic growth. Population structure and connectivity is examined for fish caught in two fjords, Uummannaq Fjord and Godthåbsfjord. Significant chemical contrasts are detected between the different spawning segments, indicating different life histories. These results reenforce the conclusions of the second paper, that within fjords spatial segregation may exist between subpopulations during a major part of their post-larval life. Furthermore, the results suggest that the sampled capelin have not migrated to offshore areas, and likely have remained inside the same fjord throughout their life. The study highlights the use of otolith chemistry to track past whereabouts of fish, and confirms the use of elements, such as Li, Sr, Mn, Pb, and possibly Rb and Si, as tracers of environmental exposure. Our results are directly applicable to future monitoring and management of inshore capelin in Greenland. Potential fishery should ideally be managed at high spatial resolution, e.g., on a fjord-by-fjord level, to ensure a sustainable and optimal harvest.