Offshore spawning and year-class strength of northern cod (2J3KL) during the fishing moratorium, 1994-1996

2001 ◽  
Vol 58 (7) ◽  
pp. 1386-1394 ◽  
Author(s):  
John T Anderson ◽  
George A Rose
Keyword(s):  
Gadus Morhua ◽  
Juvenile Fish ◽  
Late Summer ◽  
Rapid Decline ◽  
Natural Mortality ◽  
Potential Fecundity ◽  
Larval Stages ◽  
Northern Cod ◽  
Class Strength ◽  
Pelagic Juvenile

Two new surveys have been conducted in recent years to assess the status of the northern cod (Gadus morhua) stock (2J3KL). An offshore acoustic survey in spring sampled concentrations of spawning adults and estimated potential fecundity. A pelagic juvenile fish survey carried out in late summer measured cod year-class strength prior to settlement. Spawning biomass of offshore cod decreased from 11 700 t in 1994 to 2000 t in 1995 to 900 t in 1996 with a corresponding decrease in eggs spawned. This decline was mirrored in the distribution and abundance of pelagic juvenile cod. Natural mortality estimated between the egg and pelagic juvenile stages increased from –0.068·day–1 in 1994 to –0.084·day–1 in 1995 to –0.112·day–1 in 1996. The decline in year-class strength of cod appeared to result primarily from a reduction in the number of offshore spawners compounded by an increase in natural mortality during the egg and larval stages. The decline in spawners occurred 2 years after the implementation of a fishing moratorium and cannot be linked to fishing. The rapid decline in abundance of adult fish combined with increased natural mortality during the egg and larval drift is consistent with depensation.

The early life-history dynamics of Northeast Arctic cod: levels of natural mortality and abundance during the first 3 years of life

2016 ◽  
Vol 73 (2) ◽  
pp. 246-256 ◽  
Author(s):  
Bjarte Bogstad ◽  
Natalia A. Yaragina ◽  
Richard D.M. Nash
Keyword(s):  
Life History ◽  
Early Life ◽  
Egg Production ◽  
Gadus Morhua ◽  
Early Life History ◽  
Natural Mortality ◽  
Arctic Cod ◽  
Stock Assessments ◽  
Class Strength ◽  
New Generation

Recruitment at age 3 of the Northeast Arctic cod (Gadus morhua) is highly variable. It has generally been believed that year-class strength for this stock is determined prior to settlement to the bottom after about 6 months. However, newer observations indicate that year-class strength may change considerably between settlement and recruitment at age 3. Our analyses cover the 1983–2009 year classes where comprehensive data from total egg production (TEP), surveys, and stock assessments were available for a thorough examination of these cohorts. On average, only 6 out of 1 million of a new generation at the TEP stage reaches the age of recruitment to the fishery. The between-cohort variability in abundance is greatest at the ages 0–1 stage. Although the mortality is highest during the first months of life, the year-class strength can also be affected considerably by processes taking place between the 0-group stage (∼6 months) and age 3. The mortality in this period of life seems to be strongly density-dependent, and cannibalism is an important source of mortality.

scholarly journals A state-space stock assessment model for northern cod, including under-reported catches and variable natural mortality rates

2016 ◽  
Vol 73 (2) ◽  
pp. 296-308 ◽  
Author(s):  
Noel G. Cadigan
Keyword(s):  
State Space ◽  
Gadus Morhua ◽  
Stock Assessment ◽  
Mortality Rates ◽  
Assessment Model ◽  
Data Sources ◽  
Natural Mortality ◽  
Similar Data ◽  
Subjective Data ◽  
Northern Cod

A state-space assessment model for the northern cod (Gadus morhua) stock off southern Labrador and eastern Newfoundland is developed here. The model utilizes information from offshore trawl surveys, inshore acoustic surveys, fishery catch age compositions, partial fishery landings, and tagging. This is done using an approach that avoids the use of subjective data-weighting. Estimates of fishing mortality rates (F) are usually conditional on assumptions about natural mortality rates (M) in stock assessment models. However, by integrating much of the information on northern cod, it is possible to estimate F and M separately. It is also possible to estimate a change in the offshore survey catchability by including inshore acoustic biomass estimates. The proposed model also accounts for biased total catch statistics, which is a common problem in stock assessments. The main goal of the model is to provide realistic projections of the impacts of various levels of future fishery catches on the recovery of this stock. The projections incorporate uncertainty about M and catch. This is vital information for successful future fisheries. The model has been developed for the specific data sources available for northern cod, but it could be adapted to other stocks with similar data sources.

Was an increase in natural mortality responsible for the collapse off northern cod?

1995 ◽  
Vol 52 (6) ◽  
pp. 1274-1285 ◽  
Author(s):  
Ransom A. Myers ◽  
Noel G. Cadigan
Keyword(s):  
Gadus Morhua ◽  
Atlantic Cod ◽  
Natural Mortality ◽  
Correlated Errors ◽  
Estimation Errors ◽  
Research Survey ◽  
Northern Cod ◽  
Northern Atlantic ◽  
Invalid Inference ◽  
Trawl Surveys

The collapse of the northern Atlantic cod (Gadus morhua) fishery off southern Labrador and to the northern Grand Bank of Newfoundland, once the largest cod fishery in the world, was a social and economic disaster for the region. An analysis of traditional catch-at-age data in conjunction with research surveys, which assumed that research survey estimation errors of abundance by age and year were independent, led assessment biologists to the conclusion that the collapse was caused by an increase in natural mortality in the first half of 1991. We constructed a statistical model to test this hypothesis. The results do not support the hypothesis. There is ambiguous evidence that natural mortality has increased since 1991; however, these results are found only in a model that has extraordinary patterns in the residuals. Our analysis suggests that even if natural mortality has been higher in recent years (as estimated using a model with correlated errors for research surveys), overfishing was sufficiently high to cause a collapse of this population. We also demonstrate that the usual assumption that estimation errors from research trawl surveys are independent is not valid, and can lead to invalid inference and unreasonable estimates of abundance.

Distribution shifts and overfishing the northern cod (Gadus morhua): a view from the ocean

2000 ◽  
Vol 57 (3) ◽  
pp. 644-663 ◽  
Author(s):  
G A Rose ◽  
B deYoung ◽  
D W Kulka ◽  
S V Goddard ◽  
G L Fletcher
Keyword(s):  
Gadus Morhua ◽  
Environmental Changes ◽  
Rapid Decline ◽  
Trawl Survey ◽  
The South ◽  
The North ◽  
Northern Cod ◽  
Before And After ◽  
Distribution Shifts ◽  
Size At Age

Research on northern cod (Gadus morhua) from 1983 to 1994 indicated that a southward shift in distribution in the early 1990s was real and not an artifact of sequentially fishing down local populations. In the early 1990s, seasonal fishery and survey data showed distribution changes where there was no fishery, and large tonnage and densities (450 000 t, densities fourfold higher than 1980s levels) appeared in the south concurrent with declines in the north. All fishery, acoustic, and trawl survey indices increased in the south, while the stock declined. Southern-caught cod in the early 1990s exhibited northern characteristics: (i) antifreeze production capacities above historical norms and equivalent to those of northern fish, (ii) vertebral counts above historic norms and equalling northern counts, and (iii) declines in size-at-age to levels associated with northern fish. The cause of the shift is thought to be a combination of abiotic (climate) and biotic (capelin (Mallotus villosus)) environmental changes and cumulative long-term fisheries effects on cod behavior. The shifted distributions increased vulnerability to Canadian and foreign fisheries and led to a rapid decline in abundance, both before and after the moratorium on fishing in Canadian waters in 1992. Rebuilding will occur in three steps: environmental restoration, recolonization by adults, and enhanced recruitment across the shelf.

Abundance and distribution of pelagic 0-group cod (Gadus morhua) in Newfoundland waters: inshore versus offshore

1995 ◽  
Vol 52 (1) ◽  
pp. 115-125 ◽  
Author(s):  
J. T. Anderson ◽  
E. L. Dalley ◽  
J. E. Carscadden
Keyword(s):  
Large Scale ◽  
Gadus Morhua ◽  
Offshore Area ◽  
The North ◽  
Spawning Areas ◽  
Northern Cod ◽  
Primary Habitat ◽  
Abundance And Distribution ◽  
Eggs And Larvae ◽  
Pelagic Juvenile

The Northern cod stock has previously been described as undergoing a large-scale denatant–contranatant migration during which spawning occurs primarily in the north off Labrador. Eggs and larvae are advected southward to the inshore areas along the NE coast of Newfoundland where they settle as juveniles, then there is a gradual return migration to the northern spawning areas. We examined abundance distributions of pelagic juvenile (0-group) cod in the fall between inshore and offshore areas to determine if these fish occurred predominantly inshore following pelagic drift. Comparison between areas demonstrated no differences in abundances and mean sizes. Our results do not support the concept of large-scale advection of eggs and larvae spawned offshore to inshore areas, but indicate that cod eggs spawned offshore remain offshore. Back calculations of spawning times indicate the pelagic juvenile cod sampled offshore were spawned predominantly in April, whereas fish caught inshore were spawned predominantly in May-June, as well as April. Estimates of spawning abundance indicate that the inshore areas along the NE coast of Newfoundland may represent <10% of eggs spawned annually. Historically, the offshore area appears to have been the primary habitat for pelagic juvenile cod.

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.

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