Research Reveals that Sea Ice and the Timing of Key Ecosystem Events Influences the Capelin Stock off Canada's East Coast
Research supported by the Newfoundland Ecosystem Research Initiative (ERI) of Fisheries and Oceans Canada has revealed new insights into the factors that regulate the abundance of Capelin (Mallotus villosus), a keystone forage species in the Northwest Atlantic marine ecosystem. This small, slender forage fish is a critical link between smaller species, such as phytoplankton and zooplankton (tiny invertebrates including copepods), and the larger species at the top of the marine food web such as Atlantic Cod, Greenland Halibut, Harp Seals, seabirds and whales.
The Newfoundland ERI (2007-2012), part of the national ERI, was a five-year research program to improve and expand the monitoring and collection of data to develop a holistic view of the region's ecosystem. This collaborative, multidisciplinary project involved scientists from all research divisions in the Department's Newfoundland and Labrador Region and Memorial University.
"To understand the dynamics of this ecosystem, we need to identify the factors that influence the abundance of Capelin since they're such an ecologically important species," says Fisheries and Oceans Canada aquatic sciences biologist Alejandro Buren, of the Northwest Atlantic Fisheries Centre, who has been studying the forage fish since 2005. "For top predators, Capelin is like a nutritious homemade meal because they're high in fat and energy rich. There is other food available, but it's more like a having a cheeseburger."
The Capelin stock off the Newfoundland and Labrador Shelf suffered a major decline in 1991, from which it has not yet recovered. Their biomass index (estimated using an index representing a portion of the stock) plunged from an estimated six-million tonnes in 1990 to about 150,000 tonnes in 1991. In addition, spawning was delayed by up to four weeks and there was a decrease in their size, age at maturity, and body condition.
"Capelin go through population cycles in other ecosystems, including the Barents Sea. The difference on the Newfoundland and Labrador Shelf is the stock has not recovered since the early 1990s," says Buren. In 2007, he began to investigate the main mechanisms that regulate the Capelin population in this ecosystem. The research is a key part of his PhD thesis at Memorial University of Newfoundland—under the supervision of Professor Bill Montevecchi and Fisheries and Oceans Canada research scientist Dr. Mariano Koen-Alonso—which explores the connections between Harp Seals, Capelin and Atlantic Cod.
"Since consumption by top predators doesn't change instantly, the predator-prey relationship cannot account for the sudden decline of this forage fish," says Buren, which leaves the question: what does?
Restructuring of the Newfoundland and Labrador Shelf ecosystem
The dramatic drop in Capelin abundance was one of a series of radical changes in the structure of the marine ecosystem of the Newfoundland and Labrador shelf during the early 1990s, which included:
- the collapse of Atlantic Cod stocks, the dominant groundfish and a major predator in the ecosystem;
- an overall decline in the demersal fish community—species that live on or near the sea floor;
- shifts in the populations, diets, and life cycle events (i.e. breeding) of seabirds;
- ongoing increases in the Harp Seal population; and
- increases in shellfish populations including crab and Northern Shrimp.
Three different computer models were used to investigate the connections between various components of the ecosystem, including the link between:
- the maximum extent of sea ice and Capelin spawning;
- the timing of ice retreat and the abundance of the copepod Calanus finmarchicus, the most important food for Capelin in the Northwest Atlantic; and
- the timing of sea ice retreat and Capelin biomass.
Key ecosystem connections
Capelin feed in the late summer and fall, building up their body fat reserves to a maximum by the end of the year. As they do not feed in the winter, their fat reserves reach a low by late spring. They must reallocate fat to support reproduction. By the time Capelin move toward warming surface waters, they need immediate access to food to support reproduction. Buren's study hypothesized that if there isn't enough food for Capelin during this critical period, it would increase mortality either due to starvation, higher susceptibility to predation, and/or reduced ability to compete.
Based on analysis of more than 20 years of data on Capelin and sea ice in waters off Newfoundland, Buren's study found a strong link between the maximum extent of sea ice and the time of peak Capelin spawning.
"The more ice there is during the winter, the later Capelin tend to spawn in the spring. We believe this is related to how ice affects populations of their key prey, Calanus," says Buren.
Prior research at the Bedford Institute of Oceanography revealed that freshwater runoff from melting of the ice pack in spring causes the water column to stratify into layers of different densities, reducing vertical mixing. The top layer becomes rich in nutrients, triggering the spring phytoplankton bloom.
Buren found that the synchronization of the bloom with other key ecosystem events has an influence on Capelin. Spring is also when Calanus emerge from deep water to feed on the phytoplankton bloom and reproduce. The timing of their emergence varies widely on the Newfoundland and Labrador Shelf; if it coincides with the peak of the spring bloom, more adult Calanus and their offspring will survive. If the bloom occurs too early in the season, the copepod will likely emerge too late to take full advantage of it, reducing their survival rate and population. Consequently, there also won't be enough food for Capelin and their biomass will suffer.
The study also revealed connections between environmental drivers and the ecosystem changes that occurred on the Newfoundland and Labrador Shelf during the early 1990s. Based on this finding, the restructuring of the ecosystem can officially be called a regime shift, which involves rapid, widespread and persistent changes in the structure of marine ecosystems from one state to another.
"The evidence suggests that the convergence of two phenomena caused the regime shift in the early 1990s. On the one hand, the system has been exploited for centuries, reducing its capacity to respond to perturbation and quickly recover. This makes the system susceptible to reconfiguration if exposed to a punctuated dramatic event," says Buren. "Such an event occurred in 1991 when, as a result of climate change, a pulse of cold, fresh water from melting sea ice reached the Newfoundland and Labrador Shelf, creating unusual climatic conditions in the area including the coldest water temperatures in the last 50 years. On the other hand, the conditions in cyclic climatic patterns resembled those that were linked to a regime shift in the North Atlantic in the early 1920s. This potentially affected the configuration of the ecosystem, too."
The findings highlight that, in addition to fisheries catches, climate change, environmental factors and species interaction are significant drivers in marine ecosystems, and understanding the relationships between these factors is essential for the development of effective ecosystem-based management approaches.
"Until now, we haven't had a clear view of what environmental factors cause Capelin biomass to fluctuate," says Buren. "Understanding this key component of the ecosystem is necessary to provide sound science advice for the development ecosystem-based management approaches, including Capelin management practices and the development of strategies to promote the recovery of species higher in the food web such as Atlantic Cod."
A key unknown is the potential impact of climate change on future interactions between seasonal sea ice dynamics and other components of the ecosystem.
"Timing is everything in the ecosystem, and its components need to be aligned for it to work well. Climate change increases climate variability and could also cause a non-linear response—a sudden shift from one state to another," says Buren. "Potential changes in maximum sea ice extent and the timing of ice retreat due to warming could affect patterns of synchrony with other components of the ecosystem, including Calanus and Capelin. If that synchrony is broken, we don't know what the impacts will be."
For more information about this subject, the research paper is available online: Bottom-Up Regulation of Capelin, A Keystone Forage Species.
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