Structure and functioning of the eastern Scotian Shelf ecosystem before and after the collapse of groundfish stocks in the early 1990s

2005 ◽  
Vol 62 (7) ◽  
pp. 1453-1473 ◽  
Author(s):  
Alida Bundy
Keyword(s):  
Gadus Morhua ◽  
Atlantic Cod ◽  
Trophic Levels ◽  
Total Biomass ◽  
Scotian Shelf ◽  
Mass Balance Models ◽  
Key Species ◽  
Before And After ◽  
Species Groups ◽  
System Properties

The fishery-induced collapse of the Atlantic cod (Gadus morhua) stock on the eastern Scotian Shelf has altered the species composition of this ecosystem. Ecopath mass-balance models of the ecosystem before and after the collapse were developed to explore how the structure, function, and key species of the ecosystem had changed. For the first time, an analysis of uncertainty was conducted to examine the effects of the uncertainty on model estimates. A comparison of the two Ecopath models indicated that although total productivity and total biomass of the ecosystem remained similar, there were changes in predator structure, trophic structure, and energy flow, many of which were robust to uncertainty. Biomass has significantly increased at trophic levels 3 and 4, and the composition of these trophic levels has changed as a result of the mean increase in trophic level of many species-groups. Piscivory has increased, presumably because of the high abundance of small pelagic fish, and the ratio of pelagic feeders to demersal feeders has increased from 0.3 to 3.0. Thus, the ecosystem has changed from a demersal-feeder-dominated system to a pelagic-feeder-dominated system. Although uncertainty remains concerning some model estimates, the ecosystem has been profoundly altered and exhibits classic symptoms of "fishing down the food web". However, overall system properties were generally conserved.

Can Atlantic cod (Gadus morhua) recover? Exploring trophic explanations for the non-recovery of the cod stock on the eastern Scotian Shelf, Canada

2005 ◽  
Vol 62 (7) ◽  
pp. 1474-1489 ◽  
Author(s):  
Alida Bundy ◽  
L Paul Fanning
Keyword(s):  
Gadus Morhua ◽  
Atlantic Cod ◽  
Snow Crab ◽  
Forage Fish ◽  
Scotian Shelf ◽  
The Poor ◽  
Mass Balance Models ◽  
Average Condition ◽  
High Predation ◽  
High Predation Pressure

The Atlantic cod (Gadus morhua) stock on the eastern Scotian Shelf collapsed in 1993. Over a decade later, in spite of a fisheries moratorium on cod fishing, this stock is at an all-time low. In parallel with the collapse of the cod stock, the abundance of large cod prey, including forage fish, shrimp, and snow crab, has greatly increased. The key question, which we explore using trophic mass-balance models, is what processes are preventing cod from recovering on the eastern Scotian Shelf? Cod were split into large and small cod. Modelling results indicate high predation pressure on small cod. In addition, small cod compete with the abundant forage fish for decreasing prey, and are in below-average condition. Large cod incur high, but unidentified, mortality that we suggest is derived from the poor condition of small cod carried through to adulthood. As a consequence of the removal of cod by fishing and an ensusing trophic cascade, eastern Scotian Shelf cod are trapped in a vicious circle: their abundance is being kept low by predation, causing an abundance so low that cod cannot compete for prey with their exceptionally abundant competitors. Furthermore, these competitors may also prey on younger stages of cod.

Demersal fish biomass size spectra on the Scotian Shelf, Canada: species replacement at the shelfwide scale

1997 ◽  
Vol 54 (8) ◽  
pp. 1725-1735 ◽  
Author(s):  
D E Duplisea ◽  
S R Kerr ◽  
L M Dickie
Keyword(s):  
Time Series ◽  
Body Weight ◽  
Energy Input ◽  
Demersal Fish ◽  
Species Group ◽  
Total Biomass ◽  
Species Replacement ◽  
Scotian Shelf ◽  
Size Spectra ◽  
Species Groups

Three parameters were derived from annual biomass size spectra constructed for a 22-year time series of demersal fish data for the entire Scotian Shelf: (i) total sampled biomass density, (ii) modal body weight, and (iii) the shape of the various biomass size spectra. Temporal variability of these parameters was compared among six species groups and between the species groups and their total. Species-group biomass varied more over the time series than did the total biomass, indicating the presence of a system factor. Conversely, modal body weight was as variable, and sometimes more variable, for the total than it was within species groups, indicating variability mainly at the species level. The shape of the biomass size spectrum showed annual differences in variability owing mainly to contributions of species groups. This variability was higher than variability in the combined species totals. We conclude that total biomass is limited at the combined species (systemwide) level because it depends on energy input to the system. Variability is greater at the species group (subsystem) level because the multiage species-group biomasses do not equally reflect a given energy limitation. Neither modal body weight nor the shape of the biomass spectrum is sensitive to the degree of taxonomic aggregation, suggesting that both measures are independent of energy input (primary production). We found no evidence of consistent species replacement trends on the shelfwide scale and conclude that diffuse replacement best describes species succession on the Scotian Shelf as a whole.

An individual-based analysis of the variability of eggs and their newly hatched larvae of Atlantic cod (Gadus morhua) on the Scotian Shelf

1995 ◽  
Vol 52 (5) ◽  
pp. 1083-1093 ◽  
Author(s):  
Thomas J. Miller ◽  
Tomasz Herra ◽  
William C. Leggett
Keyword(s):  
Gadus Morhua ◽  
Atlantic Cod ◽  
Seasonal Pattern ◽  
Size Variation ◽  
Seasonal Temperature ◽  
Scotian Shelf ◽  
Larval Size ◽  
Newly Hatched Larvae ◽  
Shelf Region ◽  
Egg Diameter

We assessed the seasonal pattern of size variation in cod eggs on the Scotian Shelf region of the Northwest Atlantic during the period March 1991–May 1993. Cod eggs were present from October to May during the surveys. Spawning was not strongly bimodal. There was a dominant autumn peak, in contrast to the historically dominant spring spawning. Egg diameter varied seasonally. Seasonal temperature patterns explained 52% of the variation in egg diameter. By incubating the eggs on-board ship, we also assessed the seasonality of the standard length (SL) of larvae that hatched from these eggs. Larval SL also varied seasonally. Egg diameter and SL were significantly correlated, but the correlation was weak (r2 = 0.3). However, the strength of correlation was consistent with laboratory estimates based on individual data. The results suggest that previous estimates of the egg size – larval size correlations are inflated. Temperature exerted a significant effect on both egg diameter and larval size, and is hypothesized to be the agent responsible for the observed seasonal variation.

Towards a balanced presentation and objective interpretation of acoustic and trawl survey data, with specific reference to the eastern Scotian Shelf

2016 ◽  
Vol 73 (12) ◽  
pp. 1914-1921 ◽  
Author(s):  
J. Michael Jech ◽  
Ian H. McQuinn
Keyword(s):  
Atlantic Cod ◽  
Management Strategies ◽  
Pelagic Fish ◽  
Trophic Levels ◽  
Fish Abundance ◽  
Forage Fish ◽  
Specific Reference ◽  
Trawl Survey ◽  
Scotian Shelf ◽  
Eastern Canada

A debate has developed over the ecosystem consequences following the collapse of Atlantic cod throughout the coastal waters of eastern Canada. The explosive increase in pelagic fish abundance in scientific bottom-trawl catches on the eastern Scotian Shelf has been interpreted as being due to either (i) a “pelagic outburst” of forage fish abundance resulting from predator release or conversely (ii) a change in pelagic fish vertical distribution leading to a “suprabenthic habitat occupation” thereby increasing their availability to bottom trawls. These two interpretations have diametrically opposing ecological consequences and suggest different management strategies for these important forage fish species. We argue that an objective evaluation of the available evidence supports the hypothesis that the abundance of forage fish has not increased in response to the demise of cod and other top predators, and the reliance on a single sampling gear with low catchability has biased and will continue to bias the interpretation of demographic trends of pelagic fish populations. We advocate that multiple sampling technologies providing alternative perspectives are needed for the monitoring and management of the various trophic levels if we are to achieve a balanced and objective understanding of marine ecosystems.

Forage fish and the factors governing recovery of Atlantic cod (Gadus morhua) on the eastern Scotian Shelf

2012 ◽  
Vol 69 (6) ◽  
pp. 997-1001 ◽  
Author(s):  
Douglas P. Swain ◽  
Robert K. Mohn
Keyword(s):  
Gadus Morhua ◽  
Atlantic Cod ◽  
Recruitment Rate ◽  
Forage Fish ◽  
Fish Biomass ◽  
Early Life Stages ◽  
Scotian Shelf ◽  
Recent Improvement ◽  
Forage Fishes ◽  
Main Factor

The stock of Atlantic cod ( Gadus morhua ) on the eastern Scotian Shelf (ESS) collapsed in the early 1990s and showed no sign of recovery during a 15-year fishing moratorium, but has recently increased in abundance. Both the prolonged lack of recovery and the recent improvement have been attributed to changes in the biomass of forage fishes through effects of predation and competition by these fishes on early life stages of cod. An examination of the relationships between forage fish biomass and the population dynamics of ESS cod provided no support for this hypothesis. Contrary to expectations under this hypothesis, cod recruitment rate was unrelated to forage fish biomass. The main factor delaying recovery was high natural mortality (M) of adult cod. The recent improvement in ESS cod is due to the strong 2004 year class and a decline in M. These factors cannot be attributed to an effect of forage fishes. Both the delay in recovery and recent improvement of the ESS cod stock appear to be due to factors other than interactions with forage fishes.

Depth-dependent vertebral number of Atlantic cod (Gadus morhua) on the Magdalen Shallows and Scotian Shelf: stock mixing or microgeographic trends within populations?

2000 ◽  
Vol 57 (12) ◽  
pp. 2393-2401 ◽  
Author(s):  
D P Swain ◽  
K T Frank
Keyword(s):  
Gadus Morhua ◽  
Atlantic Cod ◽  
Spatial Scales ◽  
Relative Length ◽  
Scotian Shelf ◽  
Age Classes ◽  
Vertebral Number ◽  
Size At Age ◽  
Stock Mixing ◽  
Summer Feeding

We examined spatial variation in the vertebral number of Atlantic cod (Gadus morhua) during the summer feeding season in the southern Gulf of St. Lawrence and on the Scotian Shelf. Mean vertebral number increased significantly with depth in the southern Gulf and on the northeastern Scotian Shelf but not on the southwestern Scotian Shelf. In the southern Gulf, where sampling was most extensive, mean vertebral number increased steadily as depth increased from 25 m to over 175 m. Mean vertebral number was also strongly related to relative length within age-classes, with the larger fish at age having more vertebrae. However, the association between vertebral number and depth could not be attributed to confounding between depth and size at age. These results indicate either unexpected mixing between neighbouring cod populations or unexpected structure at fine spatial scales within cod populations.

scholarly journals Shifts in spawning phenology of cod linked to rising sea temperatures

2017 ◽  
Vol 74 (6) ◽  
pp. 1561-1573 ◽  
Author(s):  
Kate McQueen ◽  
C. Tara Marshall
Keyword(s):  
Climate Change ◽  
North Sea ◽  
Gadus Morhua ◽  
Atlantic Cod ◽  
Size Structure ◽  
Gonadal Development ◽  
Trophic Levels ◽  
Irish Sea ◽  
The North ◽  
The North Sea

AbstractWarming temperatures caused by climate change have the potential to impact spawning phenology of temperate marine fish as some species have temperature-dependent gonadal development. Inter-annual variation in the timing of Atlantic cod (Gadus morhua) spawning in the northern North Sea, central North Sea and Irish Sea was estimated by calculating an annual peak roe month (PRM) from records of roe landings spanning the last three decades. A trend towards earlier PRM was found in all three regions, with estimates of shifts in PRM ranging from 0.9 to 2.4 weeks per decade. Temperatures experienced by cod during early vitellogenesis correlated negatively with PRM, suggesting that rising sea temperatures have contributed to a shift in spawning phenology. A concurrent reduction in the mean size of spawning females excluded the possibility that earlier spawning was due to a shift in size structure towards larger individuals, as large cod spawn earlier than smaller-sized individuals in the North Sea. Further research into the effects of climate change on the phenology of different trophic levels within the North Sea ecosystem should be undertaken to determine whether climate change-induced shifts in spawning phenology will result in a temporal mismatch between cod larvae and their planktonic prey.

Activity and metabolism of larval Atlantic cod (Gadus morhua) from Scotian Shelf and Newfoundland source populations

1996 ◽  
Vol 124 (4) ◽  
pp. 607-617 ◽  
Author(s):  
I. Hunt von Herbing ◽  
R. G. Boutilier
Keyword(s):  
Gadus Morhua ◽  
Atlantic Cod ◽  
Scotian Shelf ◽  
Source Populations

scholarly journals Density-dependent distribution of Atlantic cod (Gadus morhua) into deep waters on the Faroe Plateau

2009 ◽  
Vol 67 (1) ◽  
pp. 102-110 ◽  
Author(s):  
Petur Steingrund ◽  
Lise H. Ofstad
Keyword(s):  
Deep Water ◽  
Gadus Morhua ◽  
Atlantic Cod ◽  
Food Utilization ◽  
Total Biomass ◽  
Strongly Correlated ◽  
Catch Per Unit Effort ◽  
Density Dependent ◽  
Natural Production ◽  
Deep Waters

Abstract Steingrund, P., and Ofstad, L. H. 2010. Density-dependent distribution of Atlantic cod (Gadus morhua) into deep waters on the Faroe Plateau. – ICES Journal of Marine Science, 67: 102–110. The natural production of cod on the Faroe Plateau depends on the spatial overlap between cod and their prey. However, cod tend to be distributed shallower than the bulk of their prey fish, such as Norway pout and blue whiting. We used commercial and survey catch per unit effort (cpue) data to investigate the distribution of cod in the deep water (150–400 m) on the Faroe Plateau. Cod often move to deep water when they are 4 or 5 years old. The extent of this habitat shift was positively correlated with the total biomass of 3- or 4-year-old cod on the Faroe Plateau, whereas prey densities in the shallow and deep areas, respectively, seemed to have much less influence. Hence, year classes that were a year older than strong year classes were distributed deeper than normal, and vice versa. The cpue in deep water for all ages pooled was strongly correlated with the age 3–6 biomass of cod on the entire Faroe Plateau. These findings are discussed in relation to food utilization and management.

scholarly journals Long-term shift in coastal fish communities before and after the collapse of Atlantic cod (Gadus morhua)

2015 ◽  
Vol 73 (5) ◽  
pp. 1415-1426 ◽  
Author(s):  
J. Scott P. McCain ◽  
Deneen J. Cull ◽  
David C. Schneider ◽  
Heike K. Lotze
Keyword(s):  
Fish Community ◽  
Large Scale ◽  
Gadus Morhua ◽  
Atlantic Cod ◽  
Substantial Reduction ◽  
Fish Communities ◽  
Coastal Habitat ◽  
Coastal Fish ◽  
Before And After

Abstract The collapse of the Northwest Atlantic cod (Gadus morhua) stocks in the early 1990s led to widespread ecological changes offshore. Changes in coastal fish communities are less known, largely due to the lack of historical records and long-term, standardized research surveys in coastal ecosystems. We aimed to overcome this with a unique dataset known as the Fleming survey. From 1959 to 1964, a systematic beach-seine survey was conducted to examine juvenile cod abundance in 84 bays (42 of which were consistently sampled and therefore analysed) along the east coast of Newfoundland. In addition to cod, all other fish collected in the seines were recorded. These surveys were repeated from 1992 to 1996 after the cod collapse, and document a substantial reduction in a dominant inshore species—juvenile Atlantic cod. We show that total fish abundance declined significantly with the decrease of cod, whereas Shannon diversity and species evenness significantly increased. Species richness increased in some regions but decreased in others. We also found significant changes in the composition of the fish community likely due to a combination of release from predation from fewer large cod feeding in inshore areas as well as a release from competition from fewer juvenile cod occupying the coastal habitat. Region and the presence of vegetated habitats also significantly influenced the fish community. This study shows a strong reorganization of coastal fish communities after a large-scale fisheries collapse with implications for ecosystem-based and cross-ecosystem management.

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