Northern Grand Bank Research Articles

Northern cod re-establish historical migration patterns linked to capelin: insights from pop-up satellite archival tags

In May 2012–2015, 90 large (85–117 cm) Northern cod ( Gadus morhua) with pop-up satellite archival tags (PSATs) set to release 1 year later were released from offshore spawning aggregations on the northeast Newfoundland shelf. Forty-nine (54%) transmitted or were recovered; 82% migrated inshore and 15% remained offshore (balance predated). Geo-locations were based on release, capture, and surfacing positions, and at tag-recorded depths <50 m, on Kalman-predicted light-based longitudes matched with coastal bathymetry. Migrating fish moved southwestward on average 5° longitude (350 km) and 2° latitude (222 km) to the northeast coast of Newfoundland, southern Labrador, and northern Grand Bank, as historically from similar release points. Migration onset through the Labrador Current (<0 °C) and arrival inshore were correlated with capelin ( Mallotus villosus) peak spawning, inshore duration with capelin, and cod biomass. Both species were delayed a month from historical norms. The probability of a cod remaining inshore after 1 December was 50%, which could impact time-dependent surveys. Fishing exploitation was 6% and predation 5%. Northern cod have re-established historical spatial migration patterns, with capelin a key influence on timing and duration inshore.

Congruence of stock production and assessment areas? An historical perspective on Canada’s iconic Northern cod

Fisheries management requires spatially congruent production and assessment areas. Canada’s Northern cod (Gadus morhua), initially considered a stock complex distributed from northern Labrador to the northern Grand Bank, had its northern boundary reduced to southern Labrador in the early 1970s. Spatial incongruence has resulted in spawning stock biomass (SSB) and recruitment (R) between historical and recent eras. To investigate temporal changes, four stanzas of SSB and R were derived from statistical Perron breaks. In stanza 1, the 1960s, spawning off northern Labrador coincided with higher SSB and R than in following stanzas from the 1970s onward. SSB–R relationships that include 1960s data do not represent potential production from a more southerly distributed stock. Loglinear models of R (density-dependent models did not improve fit) indicated SSB had greatest effect and, with indices of climate and south–north distribution, explained 86% of variance. Lack of density dependence suggests long-standing recruitment overfishing, making reference points problematic. SSB growth is suggested as an alternative management target. Rebuilding the Northern cod to historical abundance requires a full “portfolio” of spawning from northern Labrador to the Grand Bank (a remanaged 2+3KL stock). With a contracted range, lower production should be anticipated.

Burial and exhumation history of the Labrador-Newfoundland margin: first observations

The continental shelf of Labrador and Newfoundland has a long history of hydrocarbon exploration, and the accumulated oil production from the northern Grand Banks exceeds one billion barrels (Fig.1). The Canada-Newfoundland & Labrador Offshore Petroleum Board (www.cnlopb.ca) awarded several new licenses on the northern Grand Banks in 2015 and announced licensing rounds for the Labrador Sea region in the coming years.

An ensemble forecast model of iceberg drift

A study of the application of ensemble techniques to iceberg forecasting has been performed using a numerical iceberg drift model. A simple ‘Monte Carlo’ approach was used in which variations for each key environmental parameter and iceberg property were sampled randomly to generate 250 ensemble members. The range of variations was estimated to represent the 95% confidence level in the parameter's value. A set of 216 iceberg tracks from the northern Grand Banks region, collected between 2002 and 2007, was used to assess the ensemble performance. While the ensemble mean drift forecast did not improve over the deterministic forecast, the ensemble model is shown to be consistent and the statistical properties of the ensemble provide useful information on the uncertainty inherent in the forecasts.

Combining microsatellites and geometric morphometrics for the study of redfish (Sebastes spp.) population structure in the Northwest Atlantic

An approach combining genetics and geometric morphometrics has been used to define the population structure of two redfish species, Sebastes mentella, and S. fasciatus, in the northwest Atlantic. The genotype at 13 microsatellite loci, and body shape—defined by 10 anatomical landmarks—were determined on 1091 specimens representing 19 S. fasciatus and 16 S. mentella fishing aggregations from the northwest Atlantic. Two of the samples came from two fjords, the Saguenay and Bonne Bay. A Mantel test performed on the matrices of pairwise genetic and morphometric distances between samples revealed an overall strong concordance between the two data sets, within species. Altogether, the combination of genetic and morphometric data indicated weak but consistent population structure in the Northwest Atlantic. For S. mentella, the results showed that (i) the Gulf of St. Lawrence-Laurentian Channel (GSL-LCH) area comprises a single biological population, with a signature of introgressive hybridization with S. fasciatus, (ii) the Saguenay Fjord harbours a sink population from the GSL-LCH area, and (iii) there are two populations segregated by depth in the Labrador Sea, which are likely connected with the deep-pelagic and shallow-pelagic populations documented for the Irminger Sea. For S. fasciatus, the combined analysis revealed the presence of a highly differentiated population in the Bonne Bay Fjord. The analysis also detected a weak structure at a large geographical scale, between three broad geographical regions, (i) the GSL-LCH area, where there is a signature of introgressive hybridization with S. mentella, (ii) the Gulf of Maine, and (iii) the continental slope from the northern Grand Banks to the mouth of the Laurentian Channel. Population structure of S. fasciatus at a smaller spatial scale is suggested by genetic (in the GSL-LCH area) and morphometric (in the Gulf of Maine) heterogeneity, but warrant further investigations. The implications of the observed population structure for fishery management are discussed.

Bottom-up limits to Newfoundland capelin (Mallotus villosus) rebuilding: the euphausiid hypothesis

Abstract Capelin (Mallotus villosus) is the key forage fish species in the Newfoundland and Labrador Shelf ecosystem. Capelin stocks collapsed in the early 1990s, concurrent with declines in “northern” Atlantic cod, Gadus morhua. Neither has fully recovered yet. Changes in growth, condition, and behaviour accompanied capelin declines on the northern Grand Banks (NGB), and remain two decades later. Feeding, growth, and condition of NGB capelin were all lower when compared with capelin from the eastern Scotian Shelf (ESS), where abundance increased following predator declines. For age 2–5 capelin of both sexes, all but one of five comparable age–sex groups were significantly larger on the ESS (e.g. age 3 females average 169 mm on the ESS and 151 mm on the NGB). Neither temperature nor density-dependence explain these differences. However, dietary differences were prominent. ESS capelin had higher total fullness indices (TFIs) than NGB fish at all sizes [mean TFIESS = 1.43 (± 1.14), mean TFINGB = 0.48 (± 0.70)]. Euphausiids (especially Thysanoessa spp.) were a main dietary component on the ESS but not on the NGB. Stable isotope analyses (δ15N and δ13C) for NGB capelin also indicated few dietary euphausiids. Trophic fractionation of δ 15N was 4.740/00, suggesting NGB capelin were food limited. Capelin recovery on the Newfoundland and Labrador Shelf appears limited by bottom-up forcing, in particular lack of euphausiid prey.

Elastic thickness estimates at northeast passive margin of North America and its implications

Global estimates of the elastic thickness (Te) of the structure of passive continental margins show wide and varying results owing to the use of different methodologies. Earlier estimates of the elastic thickness of the North Atlantic passive continental margins that used flexural modelling yielded a Te value of ∼20–100 km. Here, we compare these estimates with the Te value obtained using orthonormalized Hermite multitaper recovered isostatic coherence functions. We discuss how Te is correlated with heat flow distribution and depth of necking. The E–W segment in the southern study region comprising Nova Scotia and the Southern Grand Banks show low Te values, while the zones comprising the NE–SW zones, viz., Western Greenland, Labrador, Orphan Basin and the Northern Grand Bank show comparatively high Te values. As expected, Te broadly reflects the depth of the 200–400 ◦ C isotherm below the weak surface sediment layer at the time of loading, and at the margins most of the loading occurred during rifting. We infer that these low Te measurements indicate Te frozen into the lithosphere. This could be due to the passive nature of the margin when the loads were emplaced during the continental break-up process at high temperature gradients.

Egret-Hibernia(!), a significant petroleum system, northern Grand Banks area, offshore eastern Canada

Egret-Hibernia(!) is a well-explored petroleum system (3.25 billion barrels oil equivalent [BOE]) located in the Jeanne d'Arc Basin on the Labrador–Newfoundland shelf. Rifting and sediment fill began in the Late Triassic. Egret source rock was deposited in the Late Jurassic at about 153 Ma. After this time, alternating reservoir rock and seal rock were deposited with some syndepositional faulting. By the end of the Early Cretaceous, faults and folds had formed numerous structural traps. For the next 100 m.y., overburden rock thermally matured the source rock when it reached almost 4 km (2.5 mi) burial depth. For 2 km (1.25 mi) below this depth, oil and gas were expelled, until the source was depleted. The expelled petroleum migrated updip to nearby faulted, anticlinal traps, where much of it migrated across faults and upsection to the Hibernia Formation (44% recoverable oil) and Avalon Formation (28%). Accumulation size decreased, and gas content increased from west to east, independent of trap size. These changes correspond to a decrease in source rock richness and quality from west to east.Almost all (96%) of the discovered petroleum resides in the Lower Cretaceous or older reservoir rock units. All accumulations found to date are normally pressured in structural traps. Fifty-two exploration wells found eighteen discoveries. Their size ranges from 1.2 to 0.01 billion BOE. Most discoveries were made between 1979 and 1991. The discovery cycle began with larger accumulations and progressed to smaller accumulations. The estimated sizes of the larger accumulations have grown since 1990. Estimated mean value for undiscovered hydrocarbons is 3.8 billion BOE, thereby raising the ultimate size of Egret-Hibernia(!) to 6.19 billion BOE.

Comparing productivity of North Atlantic cod (Gadus morhua) stocks and limits to growth production

AbstractData from stock assessments were used to compare stock biomass, annual growth (Gs) and surplus production per capita (TSPc) and per unit biomass (TSPb) among 15 cod (Gadus morhua) stocks in the North Atlantic. TSPc ranged from 99 to 1012 g per fish among stocks and averaged 448 g per fish. TSPb ranged from 140 to 469 g kg−1 among stocks and averaged 294 g kg−1. Gs varied considerably with low growth production associated with low surplus production. On average, cod produced 724 g per fish in growth annually with cod in the least productive stock producing 7.2 times less than in the most productive stock. The stocks divided into four clusters reflecting four levels of production. Celtic Sea, Irish Sea and West Scotland cod showed the highest levels of production whereas Eastern Scotian Shelf, Northeast Arctic, Northern Grand Bank, Northern and Southern Gulf of St Lawrence showed the lowest levels. Surface and bottom salinity and temperature differed significantly among clusters in a canonical discriminant analysis. Temperature and salinity correlated with the first and second canonical variates, respectively. The most productive stocks were associated with higher bottom salinity and temperature. None of the stocks, including stocks with a fast growth rate and living at higher temperatures, had a specific growth rate (SGR) close to the maximum rate observed in laboratory experiments. The difference between observed and maximum SGR decreased at temperatures above 6°C. Very cold temperatures resulted in smaller cod also achieving SGR values closer to the maximum. Temperature is a major determinant of stock production.

Juvenile capelin (Mallotus villosus) off Newfoundland and Labrador in the 1990s

Anderson, J. T., Dalley, E. L., and O'Driscoll, R. L. 2002. Juvenile capelin (Mallotus villosus )o ff Newfoundland and Labrador in the 1990s. - ICES Journal of Marine Science, 59: 917-928. Juvenile capelin, Mallotus villosus (Muller), were distributed primarily on the northern Grand Banks and secondarily along the northeast coast of Newfoundland during late summer, 1994-1999. In some years, distributions extended to coastal Labrador. Capelin were seldom observed over deep water of the northeast Newfoundland Shelf or in shallow water of the southern Grand Banks. There were large areas encompassing wide ranges of capelin densities measured by the IYGPT trawl where the acoustic system did not detect capelin. Mean growth rate of capelin from 0- to 2-group was represented by length (mm)=90.9 age (years) 0.57 (r 2 =99%). The 1-group capelin formed discrete schools that varied significantly in fish length from 65 to 110 mm (p<0.05). Spatially, the different size groups occurred at scales of <55 km and possibly <3 km in some cases. The geographic distribution of age groups was not a simple function of cohort abundance. Capelin were distributed primarily in the middle ranges of temperature (5-8C) and zooplankton biomass (2-4 g dry weight m 2 ) sampled during the surveys. It is possible that interspecific competition limits the distributions of juvenile capelin, where juvenile Arctic cod (Boreogadus saida )a nd sandlance (Ammodytes sp.) were abundant to the north and south respectively. 2002 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd.

A modeling study of upper ocean winter processes in the Labrador Sea

A coupled ice‐mixed layer model is developed to study winter processes in the Labrador Sea. The seasonal change of mixed‐layer properties, air‐sea fluxes, and ice coverage is examined. The effects of severe atmospheric conditions and salinity change on the upper ocean are investigated. The model is integrated from November to June to cover the entire winter period. Objectively analyzed temperature and salinity fields for November are used as the initial conditions of the model ocean. Meteorological parameters derived from the NCEP reanalysis are used as forcings. The results show that heat loss of the ocean reaches a maximum at the end of January. High heat loss, 250 to 400 W m−2, occurs in the northern Labrador Sea between 56°N and 63°N in ice‐free waters near the ice edge. Pack ice significantly reduces the heat loss with a typical value of 50 W m−2. In the northern Labrador Sea, surface cooling causes the mixed layer to deepen continuously through winter and reaches a maximum of 150–450 m in late March. A decrease of surface salinity by 1% (for example, from 34.35 to 34) results in a decrease of mixed‐layer depth by 17–55%. The high stratification associated with the low surface salinity has little effect on ice coverage, since the reduced level of vertical mixing cannot lower the surface temperature sufficiently to increase the ice area. This suggests that freshening of the Labrador Sea during the Great Salinity Anomaly in the late 1960s to early 1970s did not promote ice production. An increase of wind speed by 40% and a decrease of air temperature up to 4°C, representing the conditions of cold winters, doubles the mixed‐layer depth to a maximum of 900 m and extends the southern ice limit by 200 km on the northern Grand Banks. This suggests that interannual variation of convection and ice extent are mainly controlled by local meteorological conditions.

Influence of growth and survival costs of reproduction on Atlantic cod, Gadus morhua, population growth rate

A stochastic, age-structured life history model was used to examine how age at maturity (theta), pre- (Zimm) and postreproductive (Zmat) mortality, and postreproductive growth rate can affect maximum reproductive rates of fish at low population size. Simulations suggest that annual (r) and per-generation (R0) metrics of population growth for Newfoundland's northern Grand Bank Atlantic cod, Gadus morhua, are primarily influenced by changes to mortality prior to and following reproduction. At observed weights at age and Zmat = 0.2, r ranged between 0.135 and 0.164 for cod maturing at between 4 and 7 years. Incremental increases in either Zimm or Zmat of 0.1 were associated with 0.03-0.05 reductions in r. To effect similar reductions, individual growth rate would have to decline by approximately one half. At observed weights at age, increases in Zmat from 0.20 to 0.45 increased the probability of negative per-generation growth from 3 to 26% for cod maturing at 4 years and from 6 to 46% for cod maturing at 7 years. Thus, even in the absence of fishing mortality, little or no population growth by Atlantic cod may not be unexpected in the presence of environmental stochasticity, particularly when accompanied by increases in mortality and declining individual growth.

The effect of stock abundance on range contraction of yellowtail flounder ( Pleuronectes ferruginea) on the Grand Bank of Newfoundland in the Northwest Atlantic from 1975 to 1995

Research vessel surveys showed that yellowtail flounder ( Pleuronectes ferruginea) on the Grand Bank, off New-foundland in the Northwest Atlantic, declined in abundance between the 1970s and the mid 1990s. The northern limit of distribution decreased substantially in the late 1980s and early 1990s, coincident with large declines in population abundance and a decrease in bottom water temperatures. This range contraction continued into the mid 1990s, despite a stabilization of the population size and a reversal of the cooling trend. In 1995, this species was rarely found on the northern Grand Bank. The area west of the Southeast Shoal on the southern Grand Bank had relatively high densities of yellowtail flounder throughout the period studied. The area occupied by the stock was positively correlated with stock abundance from surveys, but not with bottom temperatures from these same surveys. We conclude that the contraction in the area of distribution for this stock to the preferred habitat around the Southeast Shoal is primarily a function of low stock size, which resulted from increased fishing activity in the mid to late 1980s.

Bioenergetic analysis of the effects of temperature and prey availability on growth and condition of northern cod (Gadus morhua)

Atlantic cod (Gadus morhua) stocks of the Northwest Atlantic have experienced steady decreases in growth rate since 1989 and in physiological condition since 1988. The declines have been attributed to various factors, including size- selective fishing pressure, anomalously low temperature fields, and a decline in available prey. The precise balance of these factors is unknown. We applied simple correlation models to determine that bottom water temperature and capelin ( Mallotus villosus) biomass together explain 52% of the interannual variance in mean condition of 2J3K cod and 23% of the interannual variance in 2J3KL mean cod growth rates. Bioenergetic models can potentially improve our understanding of the mechanisms behind these relationships because they allocate energy as a function of environmental parameters such as water temperature and prey consumption. We developed a bioenergetic model for Atlantic cod to determine whether temperature could explain the observed decreases in growth rate of northern cod stocks. Our results suggest that the low growth rates of 2J3K northern cod observed in recent years are not due to physiological temperature limitation, while the growth rates of cod inhabiting cooler waters on the northern Grand Banks (3L) may, at least in certain years, be temperature limited.

Skewed sex ratios in spawning shoals of Atlantic cod (Gadus morhua)

Research vessel survey data from the northern Grand Bank and southern Scotian Shelf/Bay of Fundy were analysed for evidence of the occurrence of shoals of Atlantic cod (Gadus morhua) with unequal sex ratios during the spawning season. Cod were found to form both male dominated and female dominated shoals during spawning. In both areas, female dominated shoals were found in significantly deeper waters. A higher proportion of both males and females were in spawning condition in the male dominated fishing sets and a higher proportion of females had completed spawning in the female dominated fishing sets. Males may arrive at the spawning area first, with females moving into the area when ready to spawn and then returning to deeper, warmer water once they have completed spawning. ? 1996 International Council for the Exploration of the Sea

Genetic differentiation between inshore and offshore Atlantic cod (Gadus morhua) off Newfoundland: microsatellite DNA variation and antifreeze level

Microsatellite DNA provided evidence that Atlantic cod overwintering in inshore Newfoundland are genetically distinguishable from cod overwintering offshore. We compared variation in five loci in samples from inshore locations around Trinity Bay, Newfoundland, and from an offshore region on the northern Grand Bank (North Cape). Cod collected inshore were divided into two groups on the basis of antifreeze level in blood: those with high antifreeze levels presumed to have overwintered in cold ( 2°C) waters. Fish overwintering inshore differed from offshore fish in allele sharing and in (δμ) 2 distances, while fish with low antifreeze levels did not. Subpopulation structure ( Rst) was detected when offshore cod were compared with inshore cod with high levels of antifreeze but not when compared with those with low levels of antifreeze. These results suggest that cod overwintering inshore are genetically distinct from offshore cod. Inshore and offshore cod from the areas studied remain genetically distinct despite the fact that individuals from the two populations intermingle inshore during the summer and fall feeding migration. Thus, we found evidence of population structure at a finer geographical scale than has been shown to date for this species.

Genetic differentiation between inshore and offshore Atlantic cod (&lt;I&gt;Gadus morhua&lt;/I&gt;) off Newfoundland: microsatellite DNA variation and antifreeze level

Microsatellite DNA provided evidence that Atlantic cod overwintering in inshore Newfoundland are genetically distinguishable from cod overwintering offshore. We compared variation in five loci in samples from inshore locations around Trinity Bay, Newfoundland, and from an offshore region on the northern Grand Bank (North Cape). Cod collected inshore were divided into two groups on the basis of antifreeze level in blood: those with high antifreeze levels presumed to have overwintered in cold ( 2°C) waters. Fish overwintering inshore differed from offshore fish in allele sharing and in (δμ) 2 distances, while fish with low antifreeze levels did not. Subpopulation structure ( Rst) was detected when offshore cod were compared with inshore cod with high levels of antifreeze but not when compared with those with low levels of antifreeze. These results suggest that cod overwintering inshore are genetically distinct from offshore cod. Inshore and offshore cod from the areas studied remain genetically distinct despite the fact that individuals from the two populations intermingle inshore during the summer and fall feeding migration. Thus, we found evidence of population structure at a finer geographical scale than has been shown to date for this species.

Winter distribution and movements of northern Atlantic cod (Gadus morhua) along the Newfoundland—Labrador continental shelf edge derived from observations on commercial trawlers

Catch‐effort data recorded by observers aboard commercial trawlers fishing the Newfoundland‐Labrador continental shelf during the winters of 1980–1991 were used to define the distribution and movements of pre‐spawning and spanning Atlantic cod, Gadus morhua. Although cod were widely distributed over the outer continental shelf in the months of January to April, commercial concentrations were consistently located in three areas along the shelf edge. These three areas, constituting only 16% of the 190000 km2 fishing ground for northern cod, were centred north‐east of Belle Isle Bank, north‐east of Funk Island Bank, and between Funk Island Bank and the northern Grand Bank Tow positions with catch rates 5000 kg h‐1 were plotted on a weekly basis to define movements of cod schools during the winters of 1986–1988. It was assumed that change in the collective position of trawlers taking large quantities of cod reflected shoal movements. Analyses indicate that cod moved southeast along the shelf edge in January and February of each year, crossing from NAFO Division 2J into 3K Each March, cod in Division 3K apparently moved northwest against prevailing ocean currents into the Hamilton Bank region of 2J. These movements do not appear to be a direct response to ocean temperature or pack ice conditions, although cod moving along the shelf break were generally in bottom waters of 24°C. We suggest these patterns reflect the migratory behaviour of pre‐spawning and spawning cod.

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

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.

Vertical structure of currents on the northern Grand Bank—a view from a bottom mounted Acoustic Doppler Current Profiler

Data obtained from a bottom mounted Acoustic Doppler Current Profiler (ADCP) deployed on the northern Grand Bank for 70 days from mid-March to early July of 1990 were used to study the vertical structure of the currents. Good data were obtained from 70% of the water column sampled by the instrument. Only data near the sea surface were contaminated by the sidelobes of the acoustic signals, and data close to the sea bottom had large errors, possibly caused by the extended transducer ringing due to a protective cover. To determine the vertical current structure in different frequency bands, a rotary spectral analysis and empirical orthogonal function analysis were performed. The results show that the tidal and low-frequency (2–12 days) currents are predominantly barotropic with no significant change in the vertical structure over mooring period. The low-frequency currents had a stronger clockwise component than counter-clockwise component. The asymmetry of the rotary motion is attributed to more frequent storms passage to the north of the ADCP site than to the south of the site. The inertial motion has no barotropic mode. The current structure is a two-layer flow with currents of similar magnitudes but opposite directions in the two layers. Such a structure suggests direct forcing by moving storms. The time change of the structure over the mooring period is consistent with this hypothesis. Implications of the observed current structure for numerical modeling of currents on the Grand Bank are discussed.