Ecosystem Shifts

Introduction

The Centre of Expertise on State of the Oceans Reporting created this national report for the final stage of the Health of the Oceans Initiative (HOTO) under which it was first funded. It consists of highlights from the regional reports on Large Ocean Management Areas (LOMAs), created under the five-year (2007-2012) initiative. The initiative focused on: establishing new marine protected areas, enhancing our pollution prevention and response measures through improved surveillance, enforcement and containment, and collaboration with partners on matters in ocean and trans-boundary waters, including the Arctic and the Gulf of Maine.

Scope of the National Report

Canada's State of the Oceans Report 2012 presents highlights from regional reports on the five Large Ocean Management Areas established under the HOTO initiative, organized around themes. The availability of long term data sets and analysis varied, and in a few cases, there is either no information or very little information reported on certain themes from some of the Large Ocean Management Areas in this report (e.g., Beaufort Sea). Alternatively, there are also much more extensive theme-based reports published on the state of the Scotian Shelf area, which is particularly well studied.

Scope of Regional Reports on Large Ocean Management Areas

Regional reports are based on the analysis of data collected from five Large Ocean Management Areas designated under the initiative: the Pacific North Coast; the Beaufort Sea in the Arctic Ocean; the Gulf of St. Lawrence; the Eastern Scotian Shelf and the Placentia Bay/Grand Banks regions of the Atlantic. Regional reports vary in approach and complexity due to variations in the amount of ocean data available, and the varied fisheries and socio-economic conditions. The Centre of Expertise members who led the development and preparation of the reports were: Pacific North Coast (Bill Crawford, Jim Irvine); Beaufort Sea (Andrea Niemi); Gulf of St. Lawrence (Hugues Benoit, Jacques Gagné); Scotian Shelf (Nancy Shackell, Melanie MacLean); and Placentia Bay-Grand Banks (Nadine Templeman, Vanessa Sutton-Pande, Atef Mansour).

State of the Oceans Reporting and Ocean Science Data at DFO

During the five year time span of the initiative, the Department also collected marine data in the Arctic, as part of projects funded under the Climate Change stream of Canada's two-year International Polar Year program, notably the Canadian Archipelago Through-Flow Study and Canada's Three Oceans Project, as well as via the Department's two-year Climate Change Science Initiative. Ongoing marine data collection and analysis is also facilitated through the Atlantic Zone Monitoring Program; the Argo program and other Ocean Sciences programs at DFO.

In November 2011, the five-year Aquatic Climate Change Adaptation Services Program was launched at Fisheries and Oceans Canada (DFO). This program is intended to advance knowledge and understanding of climate change risks, impacts and opportunities in Canada's marine territories and the development of adaptation tools, relative to DFO's areas of responsibility. The Aquatic Climate Change Adaptation Services Program is focused at a large scale – Canada's marine estate in each of the Pacific, Arctic, and Atlantic marine regions, plus the Great Lakes freshwater basin.

Ecosystem Shifts

The rapid reorganization of an ecosystem from one relatively stable state to another contrasting, persistent state is called an ecosystem shift. This rapid change in ocean conditions may involve changes in species abundance, community composition and trophic (food web) reorganization. Not all organisms in an ecosystem will necessarily be involved in or influenced by a shift, which may range in scale from a few kilometres to more widespread, such as on the Scotian Shelf or even basin-wide.

There are two key causes of ecosystem shifts:

  • climate factors including climate change; and
  • anthropogenic factors such as fishing, introduced species and changes in habitat.

Ecosystem Shifts and Impacts in Canada's Oceans

A variety of natural and anthropogenic factors have led to ecosystem shifts in Canada's oceans, which have been characterized by important changes in marine food webs and the abundance of some species.

It is important to note that, to date, some shifts have been reported for only a portion of the areas under discussion.

Beaufort Sea:

Since 2002, the international Joint Ocean Ice Studies (JOIS), which involved scientists from Fisheries and Oceans Canada, has carried out an annual expedition to the Canada Basin. The expedition studies the oceanographic condition of the area, specifically the Beaufort Gyre, which is a portion of the ocean basin that circulates in a clockwise direction.

Observed changes include a dramatic reduction in the extent and age of multi-year sea ice in the Canada Basin since the mid-1990s. The decline of this thicker ice has increased the amount of fresh water in the surface layer of the Beaufort Gyre since 2003, resulting in impacts on the marine food web. How?

More fresh water combined with changing winds and ocean circulation has caused the water column to become more stratified (layered) because fresh water is less dense and doesn't mix well with saltier, deeper ocean water, forming a "cap" over the surface of the ocean. This stratification impedes mixing of the ocean layers, which is the main mechanism that transports nutrients upward into the sunlit surface layer or euphotic zone where phytoplankton grows. So as stratification increases, there is also a decline in nutrients available to nourish phytoplankton, the very foundation of the marine food web.

According to research by Fisheries and Oceans Canada, these changes have led to an increase in the smallest algae (picoplankton) in the Canada Basin, both in total amount and as a percentage of the total phytoplankton, and a decrease in larger nanoplankton during the five years in which data were collected (2004-2009). Samples taken in the late summer and early autumn of 2009 revealed a continued increase in the bacterial component of picoplankton.

Gulf of St. Lawrence:

There have been dramatic shifts to both the northern and southern Gulf of St. Lawrence ecosystems, particularly in response to fishing and to a lesser extent changes in environmental conditions. These shifts include changes in species abundance and/or biomass and food web structure and functioning.

In the 1980s, these ecosystems were dominated by large groundfish predators including Atlantic Cod (Gadus morhua), Redfish (Sebastes spp.) and White Hake (Urophycis tenuis), and small-bodied forage species such as Capelin (Mallotus villosus), Mackerel (Scomber scombrus), Herring (Clupea harengus) and Northern Shrimp (Pandalus borealis). Today, small-bodied forage species dominate the northern and southern Gulf.

Fishing of large groundfish increased during the 1980s, and continued to escalate in the early 1990s. This led to the collapse of a number of stocks, including northern and southern Gulf cod.

With the exception of northern Gulf cod, there has been very little or no fishing for large groundfish in this area since the mid-1990s. Despite this, the stocks have not recovered, leading scientists to conclude that factors other than fishing must be responsible for the lack of recovery and ongoing declines in many groundfish populations.

Decadal-scale changes in the cold intermediate layer and the deeper waters of the Gulf are thought to be a dominant climatic influence on groundfish and other bottom-dwelling marine life. At the population level, the most prominent effect has been on cod, mainly in the northern Gulf.

During periods of cold water in the northern Gulf, the physical condition of the cod declined, as indicated by a very low average ratio of body weight to length. Scientists have concluded that the poor condition of the fish caused the mortality of many cod at sea during the early to mid-1990s, precipitating fishery-induced declines and slowing the stock's recovery even after the fishing moratorium was introduced in 1994. As temperatures increased in the northern Gulf, the physical condition of the fish improved and temperature-related mortality declined.

The case of the southern Gulf cod is a study in contrast. If they did suffer from poor physical condition, the contribution of this factor to their decline is believed to be considerably less. Despite the absence of fishing and improvements in September condition since the mid-1980s, the natural mortality of southern Gulf cod remained high, causing a decline in their abundance. There is mounting evidence that predation is the cause of this mortality.

Collapsed groundfish populations throughout the Northwest Atlantic have failed to recover and predation by seals has been proposed as an important contributor. Unusually high natural mortality is particularly common among not only cod, but also other large groundfish in the southern Gulf. Evidence strongly indicates that predation by Grey Seals may be a large factor. The evidence includes:

  • generally coincident trends in the mortality rates of groundfish and increases in the seal population;
  • similar downward trends in the abundance of most species that Grey Seals prey on;
  • shifts in fish distribution away from areas frequented by seals; and
  • calculations confirming there is sufficient feeding demand by seals and an overlap with prey, both spatially and temporally, to explain elevated natural mortality in at least Atlantic Cod, White Hake and Winter Skate in the southern Gulf of St. Lawrence.

Scotian Shelf:

Due to the combined effects of human activities and changing environmental conditions, the Scotian Shelf ecosystem has undergone a major structural shift in recent decades that has affected all levels of the food web and altered the structure of marine communities. This shift involved concurrent increases in seals, small pelagic fish, bottom-dwelling macroinvertebrates and phytoplankton, and decreases in groundfish and zooplankton.

These findings are the result of Fisheries and Oceans Canada research on the Scotian Shelf, which has revealed that the removal of top predators due to intensive fishing can cause large and possibly permanent changes to ocean ecosystems, completely restructuring the food web. Research published in the early 2000s to 2005 indicated that the food web was restructured on the Scotian Shelf when overfishing of top predators such as large groundfishes led to population collapses of the these large benthic predators. Planktivorous, pelagic forage fish species and macroinvertebrates became dominant, and reached biomass levels 900% greater than those prevalent before large groundfish populations collapsed. Despite management measures, including fishing moratoriums, which were put in force in the early 1990s, the Scotian Shelf ecosystem has not reverted back to its former structure.

The prolonged duration of the altered food web, and its current recovery, was and continues to be dominated by all-consuming forage fish. The forage species that quickly escalated into dominance shortly after the large groundfish populations collapsed are now themselves in decline because they have outstripped their zooplankton food supply. However, researchers within the Department have recently found there is some evidence at the broad, system dynamics scale to indicate that this large, altered ecosystem is transient and that it is possible that it may be on a path to return to a food web dominated again by the larger groundfish. However, new cod-specific research undertaken by the Department (cod recovery potential assessment) is not completely consistent with the hypothesis that the ecosystem is currently reverting to its former structure, at least with respect to large groundfish. The spawning stock biomass of cod on the Eastern Scotian Shelf reached the lowest level observed in the 53-year record in 2003 at about 7,500 tonnes. While the stock is not considered to be recovered, it has grown rapidly to 64,000 tonnes in 2011. Despite the increase, long term projections suggest that the stock will remain below the level that would allow fishing to resume. Moreover, with respect to forage fish biomass, the 2004 cod cohort that fueled the rapid improvement in abundance of Eastern Scotian Shelf cod was produced in a year during which pelagic fish biomass appears to have been high. The divergence of views illustrates the complexity of ocean ecosystem research, and points to the need for ongoing research into these processes.

The broader, system dynamics scale perspective favours the view that the existing food web on the Scotian Shelf is being affected by a series of factors combined with a limited food supply for forage fishes. This view sees a reduction in predation accompanied by slowed increases in species abundances at both lower and higher trophic levels. This was first observed in zooplankton and subsequently in large-bodied predators, all considered consistent with a return towards the earlier ecosystem structure. In the longer term, the broader system view supposes that the current trend could lead to the inverted food web reversing, with the possibility that the collapsed fisheries could recover.

Placentia Bay-Grand Banks:

In the early 1990s, a major shift in community structure occurred along the entire shelf, which involved:

  • a decrease in the abundance of commercial and other fish species (e.g., Atlantic Cod, American Plaice, Thorny Skate, wolffishes);
  • a dramatic increase in the biomass of invertebrates (e.g., Snow Crab and Northern Shrimp);
  • the reduction in availability of Capelin and changes in their biology; and
  • a continued increase in the Harp Seal population.

The reasons for these changes are still under debate. However, potential factors include overfishing, climate change and changes in interactions among species. In contrast to observations on the Scotian Shelf, these changes did not coincide with a decrease in zooplankton or an increase in small forage species.

Stocks of many historically dominant groundfish have declined to a small percentage of their previous levels. Fishing is thought to be a major driver of these changes; however, environmental conditions in the Northwest Atlantic may also be a factor. Despite fisheries closures and other management measures, populations remain low and individuals are often smaller at maturity.

Since the collapse of groundfish stocks in the early 1990s, invertebrates have dominated fisheries catches. The increase in abundance of Northern Shrimp and Snow Crab could be due to a combination of water temperatures, which affect the early stages of life, and reduced predation from groundfish.

The population of Capelin, a key forage species with a dominant role in the Newfoundland Shelf food web, was high in the 1980s, decreased dramatically in the early 1990s and has remained low ever since. This decline was accompanied by significant changes in the species biology. For example, individual capelin continue to be smaller and changes in behaviour include later spawning times and a decrease in the extent of diurnal migrations.

In 2010, Capelin numbers were estimated to be at less than one percent of historical levels.

Among marine mammals, Harp Seals are the single most abundant species in the Newfoundland Shelf system. The Harp Seal population declined during the 1960s, reaching a minimum of less than two million in the early 1970s. By the mid-1990s, the population had tripled to about 5.5 million. Since then, the Harp Seal population has slowly increased to an estimated 8.61- 9.55 million animals in 2010.

Pacific North Coast:

Shifts in the prevalence of upwelling and downwelling winds correspond with ecosystem shifts within the Pacific North Coast Integrated Management Area (PNCIMA). Winds blow mainly from the south in winter and from the north in mid-summer, with much stronger winds in winter. Upwelling winds from the north push surface coastal waters away from shore, which are replaced by nutrient-rich deep water, providing nutrients that feed the entire food chain. In contrast, years with strong downwelling winds from the south may have delayed spring phytoplankton blooms, resulting in reduced survivals for various marine species. In general, downwelling winds have been stronger than average since an ecosystem shift in the late 1970s, except from 1988 to 1996. These southerly winds have been associated with generally increasing water temperatures and decreasing salinities.

Increases or decreases in the abundance for some aquatic populations over time are associated with oceanic ecosystem shifts. Other populations exhibit more pronounced interannual variability, perhaps related to changes occurring during a critical life history stage. For instance, Chum and especially Pink Salmon abundances generally increased after the late 1970s, while abundances of Coho Salmon decreased. The Sockeye Salmon in Smith inlet supported a valuable fishery until severe stock declines in the early to mid-1990s. Herring abundances, except those in the Prince Rupert district, have generally decreased since the late 1970s. Sardines returned to southern Vancouver Island waters in 1992 after a 45 year absence, and expanded their distribution northward into PNCIMA by 1998. The northward extent of sardine migration varies from year to year and is strongly affected by temperature. In 2009, Humboldt Squid were much more widespread and abundant in B.C. waters, including PNCIMA, than in previous years. However, in 2010 squid were virtually absent from B.C. waters.

Fisheries and Oceans Canada scientists are finding new ways to increase our understanding of marine ecosystems. For example, research on the west coast is using satellite oceanographic observations to explore the interconnections between ocean ecosystems and particular marine species: Satellites and Seabirds: What They Are Telling Us About the Marine Ecosystem

Addressing Ecosystem Shifts

It is essential that ocean activities be managed in a way that preserves the health of marine ecosystems while allowing for sustainable use.

Given the complexity of ecosystem interactions, natural science and socio-economic experts must join forces with managers to follow major ecosystem changes and assess their causes. This will help inform the development of an ecosystem approach to marine resource management in the face of climate change and aid in the development of management measures to mitigate anticipated detrimental impacts.

A variety of management actions have been implemented to protect the offshore habitats and communities in the various ecosystems including a variety of legislation and policies, and scientific research and monitoring programs.

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