Covariation between grey seal (Halichoerus grypus) abundance and natural mortality of cod (Gadus morhua) in the southern Gulf of St. Lawrence

2005 ◽  
Vol 62 (9) ◽  
pp. 1991-2000 ◽  
Author(s):  
G A Chouinard ◽  
D P Swain ◽  
M O Hammill ◽  
G A Poirier
Keyword(s):  
Gadus Morhua ◽  
Atlantic Cod ◽  
Population Analysis ◽  
Halichoerus Grypus ◽  
Natural Mortality ◽  
Grey Seal ◽  
Synthetic Populations ◽  
Time Period ◽  
Abundance Indices

More than 10 years after the collapse of Atlantic cod (Gadus morhua) fisheries in Canada, the role of increased seal populations in the decline and lack of recovery of the stocks continues to be discussed. Using removals and abundance indices from synthetic populations, we found that sequential population analysis can uncover trends in natural mortality. We used this approach to examine variation in natural mortality (M) of southern Gulf of St. Lawrence cod. M increased from the late 1970s to the mid-1990s but declined slightly recently. Results were consistent with previous work indicating that M increased in the 1980s. Changes in estimated M for this cod stock matched fluctuations in grey seal (Halichoerus grypus) abundance. The increase in grey seal abundance from the late 1970s to the mid-1990s corresponded with the increase in estimated M of cod over this time period. The correspondence between seal abundance and M of cod supports the hypothesis that seal predation may be a cause of increased M. However, the diet information available suggests that seals consume mainly juvenile cod, whereas our evidence for an increase in M is for larger cod (ages 3 years and older).

Studies on the Life-History of the Ascarid Porrocaecum decipiens in the Bras d'Or Lakes, Nova Scotia, Canada

1960 ◽  
Vol 17 (6) ◽  
pp. 763-774 ◽  
Author(s):  
D. M. Scott ◽  
W. F. Black
Keyword(s):  
Life History ◽  
Nova Scotia ◽  
Intermediate Host ◽  
Gadus Morhua ◽  
Atlantic Cod ◽  
Phoca Vitulina ◽  
Halichoerus Grypus ◽  
Grey Seal ◽  
Important Food ◽  
History Of

Larvae of the parasitic ascarid (Porrocaecum decipiens) occurred commonly in the musculature and viscera of Atlantic cod (Gadus morhua) in the Bras d'Or Lakes. They were also present in the musculature of nine other species of teleosts and probably also in the viscera of skates (Raja sp.). Most larvae were longer than 20 mm. None was shorter than 10 mm, a fact which suggested the existence of some earlier intermediate host, probably an invertebrate. More than 8,000 mysids, an important food of fishes when they first become infected, were examined for nematodes. Although 110 nematodes were found, only one certainly and four dubiously appeared to be Porrocaecum. The definitive hosts were the harbour seal (Phoca vitulina) and the grey seal (Halichoerus grypus). The distribution of seals coincided with local variations in the incidence of the parasite in cod.

Continued decline of a collapsed population of Atlantic cod (Gadus morhua) due to predation-driven Allee effects

2019 ◽  
Vol 76 (1) ◽  
pp. 168-184 ◽  
Author(s):  
Rachel D. Neuenhoff ◽  
Douglas P. Swain ◽  
Sean P. Cox ◽  
Murdoch K. McAllister ◽  
Andrew W. Trites ◽  
...  
Keyword(s):  
Gadus Morhua ◽  
Atlantic Cod ◽  
Minor Role ◽  
Grey Seal ◽  
Allee Effects ◽  
Demographic Effect ◽  
Predation Mortality ◽  
A Minor ◽  
Main Factor

Most stocks of Atlantic cod (Gadus morhua) in the Northwest Atlantic collapsed in the early 1990s, with little sign of recovery since then. In the southern Gulf of St. Lawrence (sGSL), the failed recovery is due to severe increases in the natural mortality of adult Atlantic cod. We examined the role of predation by grey seals (Halichoerus grypus) in this failed recovery by directly incorporating grey seal predation in the population model for Atlantic cod via a functional response. Estimated predation mortality of adult Atlantic cod increased sharply during the cod collapse and has continued to increase, comprising the majority of mortality since the late 1990s. While predation by grey seals appeared to play a minor role in the collapse of Atlantic cod, we found it to be the main factor preventing recovery. Our results are consistent with the hypothesis that failed recovery is due to predation-driven Allee effects, a demographic effect due to the decline in cod abundance and an emergent effect resulting from increasing grey seal abundance. Under current conditions, extirpation of sGSL Atlantic cod appears likely unless there is a large decline in the abundance of grey seals.

Natural mortality from poor condition in Atlantic cod (Gadus morhua)

2000 ◽  
Vol 57 (4) ◽  
pp. 826-836 ◽  
Author(s):  
Jean-Denis Dutil ◽  
Yvan Lambert
Keyword(s):  
Gadus Morhua ◽  
Atlantic Cod ◽  
Negative Impact ◽  
Fish Muscle ◽  
Early Summer ◽  
Wild Fish ◽  
Natural Mortality ◽  
Muscle Lactate ◽  
Lactate Dehydrogenase Activity ◽  
Poor Condition

The extent of energy depletion was assessed in Atlantic cod (Gadus morhua) in spring and early summer (1993-1995) to assess relationships between poor condition and natural mortality. Several indices of condition were compared in wild fish in the northern Gulf of St. Lawrence and in fish exposed to a prolonged period of starvation in laboratory experiments. Discriminant analyses classified only a small fraction of the wild fish as similar to cod that did not survive and a much larger fraction as similar to cod that survived starvation. This percentage increased from April to May and peaked in June 1993 and 1994. Condition factor and muscle somatic index allowed a clear distinction between live and dead fish. Muscle lactate dehydrogenase activity suggested that cod had experienced a period of negative growth early in 1993, 1994, and 1995. Fish classified as similar to starved individuals were characterized by a higher gonad to liver mass ratio than others. Reproduction may have a negative impact on survival not only in spring but also later into summer, as some individuals were found not to have recovered by late summer. This study shows that natural mortality from poor condition contributed to lower production in the early 1990s.

The Role of Circulating Catecholamines in the Ventilatory and Hypertensive Responses to Hypoxia in the Atlantic Cod (Gadus morhua)

1991 ◽  
Vol 64 (4) ◽  
pp. 1087-1109 ◽  
Author(s):  
Richard Kinkead ◽  
Regina Fritsche ◽  
Steve F. Perry ◽  
Stefan Nilsson
Keyword(s):  
Gadus Morhua ◽  
Atlantic Cod

The role of spawning stock in formation of recruitment of northeast atlantic cod Gadus morhua

2006 ◽  
Vol 46 (1) ◽  
pp. 74-82 ◽  
Author(s):  
V. M. Borisov ◽  
A. A. Elizarov ◽  
V. D. Nesterov
Keyword(s):  
Gadus Morhua ◽  
Atlantic Cod ◽  
Northeast Atlantic ◽  
Spawning Stock

scholarly journals Increased natural mortality at low abundance can generate an Allee effect in a marine fish

2014 ◽  
Vol 1 (2) ◽  
pp. 140075 ◽  
Author(s):  
Anna Kuparinen ◽  
Jeffrey A. Hutchings
Keyword(s):  
Population Dynamics ◽  
Population Growth ◽  
Density Dependence ◽  
Gadus Morhua ◽  
Allee Effect ◽  
Population Regulation ◽  
Atlantic Cod ◽  
Adult Mortality ◽  
Natural Mortality ◽  
Density Dependent

Negative density-dependent regulation of population dynamics promotes population growth at low abundance and is therefore vital for recovery following depletion. Inversely, any process that reduces the compensatory density-dependence of population growth can negatively affect recovery. Here, we show that increased adult mortality at low abundance can reverse compensatory population dynamics into its opposite—a demographic Allee effect. Northwest Atlantic cod ( Gadus morhua ) stocks collapsed dramatically in the early 1990s and have since shown little sign of recovery. Many experienced dramatic increases in natural mortality, ostensibly attributable in some populations to increased predation by seals. Our findings show that increased natural mortality of a magnitude observed for overfished cod stocks has been more than sufficient to fundamentally alter the dynamics of density-dependent population regulation. The demographic Allee effect generated by these changes can slow down or even impede the recovery of depleted populations even in the absence of fishing.

scholarly journals Systematic bias in estimates of reproductive potential of an Atlantic cod (Gadus morhua) stock: implications for stock–recruit theory and management

2006 ◽  
Vol 63 (5) ◽  
pp. 980-994 ◽  
Author(s):  
C Tara Marshall ◽  
Coby L Needle ◽  
Anders Thorsen ◽  
Olav Sigurd Kjesbu ◽  
Nathalia A Yaragina
Keyword(s):  
Egg Production ◽  
Gadus Morhua ◽  
Atlantic Cod ◽  
Reproductive Potential ◽  
Threshold Level ◽  
Systematic Bias ◽  
Stock Status ◽  
Relative Fecundity ◽  
Time Period ◽  
Spawning Stock

Stock–recruit relationships that use spawning stock biomass (SSB) to represent reproductive potential assume that the proportion of SSB composed of females and the relative fecundity (number of eggs produced per unit mass) are both constant over time. To test these two assumptions, female-only spawner biomass (FSB) and total egg production (TEP) were estimated for the Northeast Arctic stock of Atlantic cod (Gadus morhua) over a 56-year time period. The proportion of females (FSB/SSB) varied between 24% and 68%, and the variation was systematic with length such that SSB became more female-biased as the mean length of spawners increased. Relative fecundity of the stock (TEP/SSB) varied between 115 and 355 eggs·g–1 and was significantly, positively correlated with mean length of spawners. Both FSB and TEP gave a different interpretation of the recruitment response to reductions in stock size (overcompensatory) compared with that obtained using SSB (either compensatory or depensatory). There was no difference between SSB and FSB in the assessment of stock status; however, in recent years (1980–2001) TEP fell below the threshold level at which recruitment becomes impaired more frequently than did SSB. This suggests that using SSB as a measure of stock reproductive potential could lead to overly optimistic assessments of stock status.

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