The Capelin: Key to Predicting Global Warming Effects on Arctic Environments

By Alethea Barbaro, Björn Birnir, and Sam Subbey

The capelin, a small planktivorous fish that lives in Arctic and subarctic waters, is a key species in many marine ecosystems on which the world’s northern fisheries depend. In these ecosystems, changes in the capelin’s life history traits—such as migration—have been linked to variability in the marine environment’s physical characteristics. Researchers have also linked variability in capelin stock dynamics to reconfiguration of the marine food webs. Thus, as the Arctic warms rapidly due to climate change, understanding the capelin’s movement and life cycle facilitates better comprehension of climate change’s impact on marine ecosystems. Since the mid-1980s, however, there has been no consistent pattern in the capelin’s migration routes. Consequently, it has become difficult for scientists to assess the state of the stock and thus the degree of ecosystem resilience in the face of a changing climate.

Our primary research focuses on simulating and predicting the migration routes and life cycles of three of the several stocks of capelin worldwide. Canada was the first country to experience the severe consequences of climate change with collapse of the Canadian Labrador-Newfoundland capelin stock around 1990. Two years later, the cod stock on the Georges Bank of Newfoundland also collapsed, as the capelin is a major part of the cod’s diet. Neither stock has fully recovered. 

We focus our efforts on the most well-studied stocks: those in the oceans around Iceland and Greenland, and those in the Barents Sea. The capelin’s migration route in Iceland was consistent prior to 2000, after which the route became unpredictable. Researchers speculate that this is due to sea ice loss and variations in salinity and temperature in the sea between Greenland and the island of Jan Mayen (north and east of Iceland). They worry that these alterations may cause the capelin stock to change spawning locations. Even more concerning, such changes could lead to the collapse of the entire stock. Either of these events could trigger a chain reaction within the trophic chain, since capelins serve as fodder fish for many larger and well-known species—such as the cod, herring, and haddock—as well as marine birds and mammals.

Figure 1. Capelin migrations before 2000 (left) and after 2000 (right). Feeding migrations are in green, spawning migrations are in red, and new drift of larvae are in blue. Blue regions denote distribution of juveniles. Figure courtesy of [2].

It is equally important to understand the types of shift that might take place in the Barents Sea for capelin migrations. Historically, warmer and saltier water was always further east in the Barents Sea during warmer periods, and colder, fresher water stayed to the north. However, scientists have recently observed that the water column north of the Barents Sea is less stratified as more of the warmer, saltier Atlantic water invades the region.

George A. Rose has suggested that the capelin’s migratory and responsive nature makes it akin to the "canary in the coalmine" that detects signals of environmental change in the Arctic Ocean [5]. Simulations of the feeding and spawning migrations must occur in an environment that is characterized by landmasses, ocean currents, and temperature variations that influence the choice of trajectory. Thus, variability in migration routes are indicative of changes in the marine environment. Figure 1 depicts fluctuations in the Icelandic capelin’s feeding and spawning migration route prior to and after 2000, while Figure 2 displays the spatial distribution’s shift towards (colder) Greenland. These changes in migration routes and spatial distribution have been linked to changes in temperature.

Figure 2. Comparison of capelin distribution between fall of 1996 and fall of 2010, based on acoustic measurements. The location of the schools’ distribution has moved towards Greenland. Figure courtesy of [2].

Our research combines analysis of data and extensive simulations to link capelin migrations and physiology to deviations in the ocean environment over the last half-century [1]. Our goal is to understand and predict capelin migrations and the organism’s interactions with the ocean environment.

To do so, we compare simulation results with data—collected over the last 50 years—to determine temperature thresholds that may lead to major disruptions in Arctic environments, as well as timescales for when such thresholds are likely to be met. On a centennial timescale, predictions of sea-level rise due to melting of the Greenland glacier are on the scale of cm [4]. We predict a similar timescale for when the freshening of the Nordic Seas (due to freshwater accumulation) reaches a critical threshold for the Atlantic meridional overturning circulation [3]. These predictions do not account for the nonlinear dynamics of the Greenland ice sheet and the dynamics of ocean currents. However, precisely because of the capelin’s responsiveness to changes in temperature and ocean currents, recent variations in its spawning routes seem to indicate that the potential sea-level rise may be larger and occur on a shorter time scale than initially predicted. Given that the capelin’s life history is tuned to the dynamics of the Arctic environment, it is perhaps our most reliable "canary in the coalmine" for the timing of global change.

References
[1] Barbaro, A., Birnir, B., & Subbey, S. (2019). The effects of changing arctic marine environment on migration patterns of the capelin: an overview. Submitted.
[2] Carscadden, J.E., Gjøsæter, H., & Vilhjálmsson, H. (2013). A comparison of recent changes in distribution of capelin (mallotus villosus) in the Barents Sea, around Iceland and in the northwest Atlantic. Prog. Ocean., 114, 64-83.
[3] Curry, R., & Mauritzen, C. (2005). Dilution of the northern North Atlantic Ocean in recent decades. Science, 308(5729), 1772-1774.
[4] Price, S.F., Payne, A.J., Howat, I.M., & Smith, B.E. (2011). Committed sea-level rise for the next century from Greenland ice sheet dynamics during the past decade. PNAS, 108(22), 8978-8983
[5] Rose, G.A. (2005). Capelin (mallotus villosus) distribution and climate: a sea canary for marine ecosystem change. ICES J. Marine Sci., 62(7), 1524-1530. 

	Alethea Barbaro is an associate professor in the Department of Mathematics, Applied Mathematics, and Statistics at Case Western Reserve University. Her research focuses on mathematical models of the movement of social organisms, and she works with agent-based models as well as kinetic and macroscopic descriptions of these models. Barbaro’s current interdisciplinary collaborations include an international project to model the migrations and life cycles of the capelin (an Arctic species of fish) in a changing environment, and an experimental project examining human evacuation dynamics.
	Björn Birnir is a professor of mathematics and director of the Center for Complex and Nonlinear Science at the University of California, Santa Barbara. His research centers on nonlinear and stochastic modeling for complex dynamical systems, including mathematical seismology and geomorphology, erosion and landslides, statistical turbulence, nonlinear phenomena in quantum systems, migration of schools of fish, swarms of bacteria, angiogenesis, and global change.
	Sam Subbey is a senior research fellow at the Institute of Marine Research in Bergen, Norway. His research interests cover inverse and ill-posed problems in porous media physics and bio-mathematics. Subbey’s focus is on application of regularization techniques, mathematical modeling, parameter estimation, and uncertainty quantification. His current research focuses on predictability and controllability of marine population dynamics when the underlying drivers are vaguely understood and system observations have modest accuracy.