Overfishing of large-bodied benthic fishes and their subsequent population collapses on the Scotian Shelf of Canada's east coast and elsewhere resulted in restructuring of entire food webs now ...dominated by planktivorous, forage fish species and macroinvertebrates. Despite the imposition of strict management measures in force since the early 1990s, the Scotian Shelf ecosystem has not reverted back to its former structure. Here we provide evidence of the transient nature of this ecosystem and its current return path towards benthic fish species domination. The prolonged duration of the altered food web, and its current recovery, was and is being governed by the oscillatory, runaway consumption dynamics of the forage fish complex. These erupting forage species, which reached biomass levels 900% greater than those prevalent during the pre-collapse years of large benthic predators, are now in decline, having outstripped their zooplankton food supply. This dampening, and the associated reduction in the intensity of predation, was accompanied by lagged increases in species abundances at both lower and higher trophic levels, first witnessed in zooplankton and then in large-bodied predators, all consistent with a return towards the earlier ecosystem structure. We conclude that the reversibility of perturbed ecosystems can occur and that this bodes well for other collapsed fisheries.
Trophic Cascades in a Formerly Cod-Dominated Ecosystem Frank, Kenneth T; Petrie, Brian; Choi, Jae S ...
Science (American Association for the Advancement of Science),
06/2005, Letnik:
308, Številka:
5728
Journal Article
Recenzirano
Removal of top predators from ecosystems can result in cascading effects through the trophic levels below, completely restructuring the food web. Cascades have been observed in small-scale or simple ...food webs, but not in large, complex, open-ocean ecosystems. Using data spanning many decades from a once cod-dominated northwest Atlantic ecosystem, we demonstrate a trophic cascade in a large marine ecosystem. Several cod stocks in other geographic areas have also collapsed without recovery, suggesting the existence of trophic cascades in these systems.
Traditionally, marine ecosystem structure was thought to be determined by phytoplankton dynamics. However, an integrated view on the relative roles of top-down (consumer-driven) and bottom-up ...(resource-driven) forcing in large-scale, exploited marine ecosystems is emerging. Long time series of scientific survey data, underpinning the management of commercially exploited species such as cod, are being used to diagnose mechanisms that could affect the composition and relative abundance of species in marine food webs. By assembling published data from studies in exploited North Atlantic ecosystems, we found pronounced geographical variation in top-down and bottom-up trophic forcing. The data suggest that ecosystem susceptibility to top-down control and their resiliency to exploitation are related to species richness and oceanic temperature conditions. Such knowledge could be used to produce ecosystem guidelines to regulate and manage fisheries in a sustainable fashion.
Globally, overfishing large-bodied groundfish populations has resulted in substantial increases in their prey populations. Where it has been examined, the effects of overfishing have cascaded down ...the food chain. In an intensively fished area on the western Scotian Shelf, Northwest Atlantic, the biomass of prey species increased exponentially (doubling time of 11 years) even though the aggregate biomass of their predators remained stable over 38 years. Concomitant reductions in herbivorous zooplankton and increases in phytoplankton were also evident. This anomalous trophic pattern led us to examine how declines in predator body size (approx. 60% in body mass since the early 1970s) and climatic regime influenced lower trophic levels. The increase in prey biomass was associated primarily with declines in predator body size and secondarily to an increase in stratification. Sea surface temperature and predator biomass had no influence. A regression model explained 65 per cent of prey biomass variability. Trait-mediated effects, namely a reduction in predator size, resulted in a weakening of top predation pressure. Increased stratification may have enhanced growing conditions for prey fish. Size-selective harvesting under changing climatic conditions initiated a trophic restructuring of the food chain, the effects of which may have influenced three trophic levels.
The dependence of long-term fishery yields on primary productivity, largely based on cross-system comparisons and without reference to the potential dynamic character of this relationship, has long ...been considered strong evidence for bottom-up control in marine systems. We examined time series of intensive empirical observations from nine heavily exploited regions in the western North Atlantic and find evidence of spatial variance of trophic control. Top-down control dominated in northern areas, the dynamics evolved from bottom-up to top-down in an intermediate region, and bottom-up control governed the southern areas. A simplified, trophic control diagram was developed accounting for top-down and bottom-up forcing within a larger region whose base state dynamics are bottom-up and can accommodate time-varying dynamics. Species diversity and ocean temperature co-varied, being relatively high in southern areas and lower in the north, mirroring the shifting pattern of trophic control. A combination of compensatory population dynamics and accelerated demographic rates in southern areas seems to account for the greater stability of the predator species complex in this region.
Ocean acidification is predicted to occur first in polar oceans. We investigated the saturation state of waters with respect to calcite (Ωcal) and aragonite (Ωarg) in six sections along an Arctic ...outflow pathway through the Canadian Arctic Archipelago (CAA) and into the northwestern Atlantic using dissolved inorganic carbon and total alkalinity measurements from 2003 to 2005. The study area, a key region connecting the Arctic and the North Atlantic, includes Smith Sound, Barrow Strait, Baffin Bay, Davis Strait, Hudson Strait, and the Labrador Sea. The average Ωarg in the Arctic outflow was 1.18 ± 0.17 in Barrow Strait and 1.31 ± 0.14 in Smith Sound, with areas where Ωarg < 1. The Arctic outflow through the CAA has a high content of Pacific waters, which have a low saturation state. These waters can be traced along the western Baffin Bay to Davis Strait. South of Davis Strait, this outflow is modified by mixing with slope and offshore waters of Atlantic origin and with the outflow from Hudson Strait. Despite the mixing, low saturation state water can still be identified on the southern Labrador Shelf. The aragonite saturation horizon is found at ∼150 m in Barrow Strait; at 200 m in Baffin Bay, Davis Strait, and Hudson Strait; and at 2300 m in the Labrador Sea. This study provides baseline data of the saturation states for the waters of the CAA and the northwest Atlantic. It also illustrates the downstream evolution of low saturation state Arctic outflow in the northwest Atlantic.
Since 1960, landings of Atlantic herring have been the greatest of any marine species in Canada, surpassing Atlantic cod and accounting for 24% of the total seafood harvested in Atlantic Canada. The ...Scotian Shelf‐Bay of Fundy herring fisheries (NAFO Division 4VWX) is among Canada's oldest and drives this productivity, accounting for up to 75% of the total herring catch in some years. The stocks’ productivity and overall health have declined since 1965. Despite management measures to promote recovery implemented since 2003, biomass remains low and is declining. The factors that drive the productivity of 4VWX herring are primarily unresolved, likely impeding the effectiveness of management actions on this stock. We evaluated potential drivers of herring variability by analyzing 52 time‐series that describe the temporal and spatial evolution of the 4VWX herring population and the physical, ecological, and anthropogenic factors that could affect them using structural equation models. Variation in herring biomass was best accounted for by the exploitation rate's negative effect and the geographic distribution of fishing and recruitment. Thermal phenology and temperature adversely and egg predation positively impacted the early life stage mortality rate and, ultimately, adult biomass. These findings are broadly relevant to fisheries management, but particularly for 4VWX herring, where the current management approach does not consider their early life stage dynamics or assess them within the ecosystem or climate change contexts.
The productivity and overall health of Scotian Shelf‐Bay of Fundy herring (NAFO Division 4VWX) have declined since 1965 and have failed to improve despite recovery measures in place since 2003. We evaluated potential drivers of herring decline by analyzing 52‐time series that describe the temporal and spatial evolution of the 4VWX herring population and the physical, ecological, and anthropogenic factors that could affect them using structural equation models. Variation in herring biomass was best explained by the adverse effect of exploitation rate and positive impact of the geographic distribution of fishing and recruitment, while thermal phenology, egg predation, and temperature impacted the early life stage mortality rate and, ultimately, adults’ biomass.
The oceanographic, meteorological and sea-ice conditions in Baffin Bay are studied using historical hydrographic, satellite and meteorological data, and a set of current meter data from a mooring ...program of the Bedford Institute of Oceanography. Baffin Bay is partially covered by sea-ice all year except August and September. The interannual variation of the ice extent is shown to be correlated with winter air temperature. Available hydrographic data were used to study the water masses and the horizontal and vertical distribution of temperature/salinity. Three water masses can be identified-Arctic Water in the upper 100-300 m of all regions except the southeast, West Greenland Intermediate Water at 300-800 m in most of the interior of Baffin Bay, and Deep Baffin Bay Water in all regions below 1200 m. The temperature and salinity in Baffin Bay have limited seasonal variability except in the upper 300 m of eastern Davis Strait, northern Baffin Bay and the mouth of Lancaster Sound. Summer data have a temperature minimum at not, vert, similar100 m, which suggests winter convection does not penetrate deeper than this depth. Current meter data and results of a circulation model indicate that the mean circulation is cyclonic. The seasonal variation of the currents is complex. Overall, summer and fall tend to have stronger currents than winter and spring at all depths. Among the different regions, the largest seasonal variation occurs at the mouth of Lancaster Sound and the Baffin Island slope. Model generated velocity fields show a basic agreement with the observed currents, and indicate strong topographic control in the vicinity of Davis Strait and on the Greenland shelves. The model also produces a southward counter current on the Greenland slope, which may explain the observed high horizontal shears over the Greenland slope. Estimates of the volume and fresh water transports through Lancaster, Jones and Smith Sounds are reviewed. Transports through Davis Strait are computed from the current meter data. The balance of freshwater budget and sensitivity of the thermohaline circulation to freshwater transport are discussed.
The Sable Gully, a broad, shelf break submarine canyon approximately 40km east of Sable Island on the eastern Scotian Shelf, separates Banquereau and Sable Island Banks. Unique among canyons on the ...eastern Canadian continental shelf because of its depth, steep slopes and extension far onto the shelf, its ecological significance and increasing human pressures led to its designation in 2004 under Canada's Oceans Act as the first Marine Protected Area (MPA) in the Atlantic Region. To improve the state of knowledge of the Gully MPA, a multi-disciplinary field program was carried out in 2006–07; the physical oceanographic component consisted of the deployment (April 2006) and recovery (August 2007) of four current meter moorings and CTD surveys. Analysis of this 16-month mooring deployment demonstrates that the mean circulation above the canyon rim (~200m) is characterized by a southwestward flow that appears unaffected by the canyon topography. There is also some indication of the existence of an eddy at rim depth. Below 500m, the circulation is dominated by an upcanyon flow (of order 0.02ms−1) at the mooring array (halfway between the canyon head and mouth). The mean, 200m-bottom transport towards the head of the Gully was estimated as 35,500m3s−1, implying an upwelling velocity of 1.7×10−4ms−1 (14md−1) over the area. Results also show bottom-intensified tidal flows and non-linear constituents due to the interaction of K1, O1, M2 and S2 components along the thalweg of the canyon; the strong overtides and compound tides observed in the Gully make it unique among canyons. Further analyses provide evidence of enhanced mixing in the Gully (Kv~180×10−4m2s−1), which is approximately 20 times that observed on the adjoining Scotian Shelf. Total variance of the currents in the Gully is about 2.5 times greater than that observed on the nearby continental slope with an equivalent water depth.
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