scholarly journals Effect of Allocation and Stratification Strategies on Precision of Survey Abundance Estimates for Atlantic Cod (Gadus morhua) on the Eastern Scotian Shelf

1987 ◽  
Vol 7 ◽  
pp. 137-144 ◽  
Author(s):  
S Gavaris ◽  
S J Smith
Keyword(s):  
Gadus Morhua ◽  
Atlantic Cod ◽  
Scotian Shelf ◽  
Abundance Estimates

scholarly journals Temporal trends in probabilistic maturation reaction norms and growth of Atlantic cod (Gadus morhua) on the Icelandic shelf

2009 ◽  
Vol 66 (10) ◽  
pp. 1719-1733 ◽  
Author(s):  
Heidi Pardoe ◽  
Anssi Vainikka ◽  
Gudmundur Thórdarson ◽  
Gudrun Marteinsdóttir ◽  
Mikko Heino
Keyword(s):  
Gadus Morhua ◽  
Atlantic Cod ◽  
Temporal Trends ◽  
Age Groups ◽  
Reaction Norm ◽  
Genetic Change ◽  
Regional Survey ◽  
Fish Stocks ◽  
Abundance Estimates ◽  
Intensive Fishing

Decreasing temporal trends in probabilistic maturation reaction norm (PMRN) midpoints, symptomatic of earlier maturation despite environmentally induced variation in growth, have been observed in many exploited fish stocks. Here, we studied the growth and maturation trends of female and male Icelandic cod ( Gadus morhua ) by estimating PMRN midpoints for cohorts 1964–1999 and found evidence that a shift towards maturation at smaller sizes and younger ages has occurred independently of changes in growth, condition, and temperature. Weighting the data with regional survey abundance estimates to account for spatial heterogeneity in maturity status and sampling intensity did not qualitatively affect the temporal trends. Length-at-age also decreased through the study period, which, through simulations, could be attributed to the energetic costs of earlier maturity at maturing age groups but not at younger ages. These findings support the hypothesis that such changes in maturation schedules are not caused by environmental factors alone but could also reflect a genetic change, potentially in response to intensive fishing.

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.

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.

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.

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.

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.

Distribution of Early-Stage Atlantic Cod (Gadus morhua), Haddock (Melanogrammus aeglefinus), and Witch Flounder (Glyptocephalus cynoglossus) Eggs on the Scotian Shelf: A Reappraisal of Evidence on the Coupling of Cod Spawning and Plankton Production

1992 ◽  
Vol 49 (2) ◽  
pp. 238-251 ◽  
Author(s):  
Keith Brander ◽  
Peter C. F. Hurley
Keyword(s):  
Gadus Morhua ◽  
Atlantic Cod ◽  
Early Stage ◽  
Calanus Finmarchicus ◽  
Melanogrammus Aeglefinus ◽  
Scotian Shelf ◽  
Plankton Production ◽  
Peak Abundance ◽  
Haddock Melanogrammus Aeglefinus ◽  
Timing Of Spawning

In spring, Atlantic cod (Gadus morhua) spawned progressively later from southwest to northeast along the Scotian Shelf and matched the variation in peak abundance of Calanus finmarchicus. In fall, cod spawned in some, but not all, areas where Calanus were abundant. This is consistent with the hypothesis that cod spawning is coupled to copepod production, which is a part of the "match–mismatch" hypothesis proposed by D. H. Cushing. The timing of spawning was estimated from the distributions of early- and late-stage eggs of cod, haddock (Melanogrammus aeglefinus), and witch flounder (Glyptocephalus cynoglossus) collected during Scotian Shelf Ichthyoplankton Survey (SSIP) cruises in 1979–81.

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