Age- and Density-Dependent Bathymetric Pattern of Atlantic Cod (Gadus morhua) in the Southern Gulf of St. Lawrence

1993 ◽  
Vol 50 (6) ◽  
pp. 1255-1264 ◽  
Author(s):  
Douglas P. Swain
Keyword(s):  
Habitat Selection ◽  
Density Dependence ◽  
Shallow Water ◽  
Gadus Morhua ◽  
Atlantic Cod ◽  
Age Groups ◽  
Low Density ◽  
Positive Correlation ◽  
Using Data ◽  
Trawl Surveys

I examined the bathymetric pattern of Alantic cod (Gadus morhua) in September in the southern Gulf of St. Lawrence for age groups 3–8+ over the period 1971–91 using data from annual bottom trawl surveys. Mean age of cod tended to increase with depth in all years. The age-specific bathymetric patterns underlying this relationship differed markedly between periods of low and high abundance. When abundance was high, cod densities tended to be highest at intermediate depths, and the positive correlation between age and depth reflected a tendency for density to be highest in progressively deeper water for older fish. When abundance was low, density was either unrelated to depth or highest in shallow water, and the positive correlation between age and depth reflected a tendency for older cod to be more widely distributed than younger cod (i.e., density decreased less sharply with depth for older cod). I suggest explanations for this density dependence of cod bathymetric pattern and discuss these results in relation to the factors underlying habitat selection and bathymetric segregation by age in this population.

Seasonal variation in the habitat associations of Atlantic cod (Gadus morhua) and American plaice (Hippoglossoides platessoides) from the southern Gulf of St. Lawrence

1998 ◽  
Vol 55 (12) ◽  
pp. 2548-2561 ◽  
Author(s):  
D P Swain ◽  
G A Chouinard ◽  
R Morin ◽  
K F Drinkwater
Keyword(s):  
Seasonal Variation ◽  
Habitat Selection ◽  
Gadus Morhua ◽  
Atlantic Cod ◽  
Statistical Significance ◽  
Habitat Associations ◽  
Practical Implications ◽  
Hippoglossoides Platessoides ◽  
Trawl Surveys ◽  
Selection Of

We compared habitat associations of southern Gulf of St. Lawrence Atlantic cod (Gadus morhua) and American plaice (Hippoglossoides platessoides) between the summer feeding season on the Magdalen Shallows and the overwintering period in the Cabot Strait. Data were from bottom trawl surveys conducted in September 1993, 1994, and 1995 and January 1994, 1995, and 1996. Both species occupied much deeper, warmer water in winter than in summer. The effect of cod age on temperature distribution reversed between the two seasons, with younger cod occupying warmer water than older cod in summer and colder water in winter. Selection of both depth and temperature by cod tended to be more significant in September than in January. The reduced statistical significance of habitat selection by cod in winter was associated with a more aggregated distribution in this season. The contrast between seasons in habitat associations was particularly strong for plaice. The median habitats occupied by plaice were 58-67 m and -0.1 to 0.3°C in September and 374-426 m and 5.2-5.4°C in January. Habitat selection by plaice was significant in both seasons, but significance tended to be greater in January. Degree of aggregation in plaice distribution was similar between the two seasons. Female plaice occupied significantly warmer water than males in September but not in January. The ecological and practical implications of this striking seasonal variation in habitat associations are discussed.

scholarly journals Density-independent and -dependent habitat selection of Atlantic cod (Gadus morhua) based on geostatistical aggregation curves in the northern Gulf of St Lawrence

2010 ◽  
Vol 67 (8) ◽  
pp. 1676-1686 ◽  
Author(s):  
Hacène Tamdrari ◽  
Martin Castonguay ◽  
Jean-Claude Brêthes ◽  
Daniel Duplisea
Keyword(s):  
Habitat Selection ◽  
Gadus Morhua ◽  
Atlantic Cod ◽  
Local Density ◽  
Abiotic Factors ◽  
Age Groups ◽  
Spatial Dynamics ◽  
Entire Area ◽  
Dynamics Models ◽  
Selection Of

Abstract Tamdrari, H., Castonguay, M., Brêthes, J-C., and Duplisea, D. 2010. Density-independent and -dependent habitat selection of Atlantic cod (Gadus morhua) based on geostatistical aggregation curves in the northern Gulf of St Lawrence. – ICES Journal of Marine Science, 67: 1676–1686. Relationships were sought between local density and population abundance of Atlantic cod (Gadus morhua) in the northern Gulf of St Lawrence (Canada) over its entire area (4RS) and also within a subarea (4R) where the stock has concentrated since it collapsed during the early 1990s. Relationships were analysed using geostatistical aggregation curves computed within the two areas between years of contrasting abundance levels. The curves were interpreted in terms of four conceptual models of spatial dynamics: models D1 and D2, forced mainly by environmental heterogeneity, and models D3 and D4, in which individual behaviour is influenced by local density. Over the entire area, the cod population follows the D2 model for all years and age groups, and it is influenced by abiotic factors. Within the subarea, all four models applied, and the density-dependent basin model (D4) dominated from 2006 to 2008. The year 2006 seems to be pivotal because it coincides with the expansion of the cod population into its former area in the western Gulf (4S).

Seasonal variation in cod feeding and growth in a changing sea

2016 ◽  
Vol 73 (2) ◽  
pp. 235-245 ◽  
Author(s):  
Edda Johannesen ◽  
Geir Odd Johansen ◽  
Knut Korsbrekke
Keyword(s):  
Seasonal Variation ◽  
Gadus Morhua ◽  
Barents Sea ◽  
Atlantic Cod ◽  
Age Groups ◽  
Seasonal Difference ◽  
Compensatory Feeding ◽  
Energy Acquisition ◽  
The Difference ◽  
Using Data

Seasonal variation in feeding and growth of 3- to 9-year-old Atlantic cod (Gadus morhua) was studied using data from joint Norwegian–Russian surveys in January–March (winter survey) and September–August (ecosystem survey) in the Barents Sea. The study encompassed the warmest period on record, with large cod stock and both low (2004–2007) and high (2008–2013) abundance of Barents Sea cod’s main prey, capelin (Mallotus villosus). Feeding on capelin was most important in winter. Energy acquisition (kJ·day−1) was higher during fall, but in years with high capelin abundance the seasonal difference was smaller. There was no difference in energy acquisition in years with high and low capelin abundance, underpinning recent findings on decoupling between capelin abundance and cod demography. Compensatory feeding on alternative prey takes place in fall, but not in winter. These findings were consistent across age groups. The results found for energy acquisition was mirrored in growth; in years with low capelin abundance, there was a seasonal difference in growth, whereas in years with high capelin abundance, the difference was absent.

Influence of climatic changes on the parasites of Atlantic cod Gadus morhua off coastal Labrador, Canada

2006 ◽  
Vol 80 (2) ◽  
pp. 193-197 ◽  
Author(s):  
R.A. Khan ◽  
C.V. Chandra
Keyword(s):  
North Atlantic ◽  
Gadus Morhua ◽  
Atlantic Cod ◽  
Age Groups ◽  
Climatic Conditions ◽  
Otter Trawl ◽  
Commercial Fishery ◽  
Metazoan Parasites ◽  
Mean Values ◽  
The North

AbstractA study was conducted in 2000 and 2003, following the collapse of the commercial fishery in 1990, to compare metazoan parasites of Atlantic cod Gadus morhua, captured off coastal Labrador, with samples taken in 1980 and 1986. Fish were captured by otter trawl offshore in the North Atlantic Fish Organisation subarea 2J. Parasites were removed from the digestive tract, stained, identified and compared between the different groups. Both the prevalence and mean abundance of trematodes, larval nematodes and E. gadi were significantly lower in fish taken in 2000 and 2003 than in 1980. While mean values of trematodes and nematodes declined in 1986, those of Echinorhynchus gadi remained unchanged in 1986 and 1990. Four-year-old cod sampled in 1990 harboured significantly fewer E. gadi than older age groups. The most commonly occurring trematodes included Podocotylereflexa, Lepidapedon elongatum, Derogenes varicus and Hemiurus levinseni while the larval nematode, Anisakis sp. was predominant. Comparison of offshore samples taken in 2000 and 2003 with others taken in previous years suggests an overall decline of parasites coincident with a change in climatic conditions, the absence of a major food source, namely capelin Mallotus villosus, of cod and ultimately the decline of the Labrador population.

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.

Partial Recruitment Considerations in Setting Catch Quotas

1993 ◽  
Vol 50 (4) ◽  
pp. 734-742 ◽  
Author(s):  
A. F. Sinclair
Keyword(s):  
Gadus Morhua ◽  
Atlantic Cod ◽  
Age Groups ◽  
Combined Effects ◽  
Total Allowable Catch ◽  
Catch Rates ◽  
Fishing Fleets ◽  
The Individual ◽  
Nova Scotian ◽  
Target Fishing

An approach is presented for investigating the interactions between fishing fleets that compete by exploiting different age-groups of the same resource population. The term "partial recruitment" (PR) is used to describe the age-specific exploitation pattern experienced by a population, either from individual fishing fleets or the combined effects of several fleets. Methods are presented to calculate catch quotas for the individual fleets if the management objective is to keep fleet effort constant, or alternatively to predict catch rates by fleet if the allocation rules are based on a percentage sharing of the total allowable catch (TAC) among fleets. Simulations based on an Atlantic cod (Gadus morhua) fishery on the Nova Scotian Shelf is used to illustrate the method. The results indicate the importance of considering differences in PR among competing fishing fleets when setting catch quotas. The relative effort exerted by the fleets will affect target fishing mortalities. The fleet that concentrates on younger fish can intercept recruitment. Since the fleets exploit different age-groups, changes in fishable biomass due to recruitment variation are lagged, and fishing success will vary among fleets. Understanding the dynamics of PR may help explain why certain fleets either exceed or fall short of catch quotas.

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.

A strategy to detect fish discarding by combining onboard and onshore sampling

1997 ◽  
Vol 54 (12) ◽  
pp. 2955-2963 ◽  
Author(s):  
Jacques Allard ◽  
Ghislain A Chouinard
Keyword(s):  
Gadus Morhua ◽  
Atlantic Cod ◽  
Nonparametric Test ◽  
Reference Points ◽  
Sampling Distribution ◽  
Small Fish ◽  
Conservation Problem ◽  
Cost Efficient ◽  
Using Data ◽  
The Cost

Discarding of small fish is considered to be an important conservation problem and has become illegal in some fisheries. We present a cost-efficient strategy to help enforce regulations against discarding. A discarding indicator is defined using the change in slope between two reference points on the empirical length-frequency density of the catch. This discarding indicator is then used according to the external distribution concept: the sampling distribution of the discarding indicator, when no discarding occurred, is obtained directly from samples taken by onboard observers; the value of the discarding indicator observed by onshore observers from a boat not covered by onboard observers is then compared with this sampling distribution. This procedure offers a nonparametric test for discarding. Application of the strategy is illustrated using data from the 1991 Atlantic cod (Gadus morhua) fishery in the southern Gulf of St. Lawrence. We describe several enforcement frameworks within which the method can be applied. The cost efficiency of the strategy comes from shifting resources from high-cost onboard observation to lower cost onshore observation.

Distribution–abundance relationships for North Sea Atlantic cod (Gadus morhua): observation versus theory

2005 ◽  
Vol 62 (9) ◽  
pp. 2001-2009 ◽  
Author(s):  
Julia L Blanchard ◽  
Craig Mills ◽  
Simon Jennings ◽  
Clive J Fox ◽  
Brian D Rackham ◽  
...  
Keyword(s):  
Habitat Selection ◽  
North Sea ◽  
Gadus Morhua ◽  
Atlantic Cod ◽  
Ideal Free Distribution ◽  
Density Dependent ◽  
The North ◽  
The North Sea ◽  
Density Dependent Habitat Selection ◽  
Ideal Free

Density-dependent habitat selection has implications for fisheries management and for the recovery of depleted fish populations. According to ideal free distribution theory, populations contract into areas of highest habitat suitability as their abundance decreases. This can increase their vulnerability to fishing and predation. We detected density-dependent habitat selection by juvenile Atlantic cod (Gadus morhua) (ages 1 and 2) in the North Sea and compared the observed distribution–abundance relationships with those predicted from a model based on ideal free distribution theory and knowledge of optimal temperatures for growth, where temperature was used as a measure of suitability. As predicted by the model, in years when stock size was low, the catches were largely confined to regions with near-optimal bottom temperatures. Conversely, when population size was high, catches were spread across a larger area including regions with suboptimal temperatures. The spatial extent of optimal habitat appears to have decreased from 1977 to 2002, reflecting a gradual warming of the North Sea. The combined negative effects of increased temperature on recruitment rates and the reduced availability of optimal habitat may have increased the vulnerability of the cod population to fishing mortality.

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