ISSN:0706-652X    

DOI:10.1139/f89-274

出版商:NRC Research Press    

年代:1989

数据来源: NRC

摘要:

Strong annual signals are found in temperature, salinity, and current measurements at two sites off Cape Sable, Nova Scotia. Differences in means and annual cycles indicate that the along-isobath current is not controlled by wind stress at these time scales. The phase of the primary peak of freshwater runoff into the Gulf of St. Lawrence (RIVSUM) is consistent with the advection of a salinity minimum to Cape Sable at an average speed of 6 km∙d−1, but the secondary peak is not observed and may be masked by the seasonal inflow of slope water into the Gulf of Maine via Northeast Channel. A diffusive model, forced by air–sea interaction, provides a reasonable fit to the annual temperature cycles with vertical diffusivities of 15–50 × 10−4 m2∙s−1. Correlations between residual longshore wind stress and along-isobath current, deep salinity, and temperature are consistent with wind-driven upwelling. Furthermore, warm-core rings in the slope water sometimes enhance onshore heat and salt fluxes to create positive temperature and salinity anomalies off Cape Sable, especially in late summer. February ocean temperature anomalies to 50 m are related to winter-average cross-shore wind (and related heat flux residuals), but do not persist on seasonal time sca

ISSN:0706-652X    

DOI:10.1139/f89-275

出版商:NRC Research Press    

年代:1989

数据来源: NRC

摘要:

A two-dimensional (horizontal) surface layer particle dispersion model for the Southwest Nova Scotia region is described and evaluated. The model is based on a barotropicM2tidal model and wind-driven response model. It includes a constant residual current field plus time varyingM2tidal and wind-driven currents. Tidal currents are shown to cause particle trajectories to vary with the time of particle release. Constant winds are shown to have an important influence on particle trajectories, although periodic winds have little effect. Comparisons between simulated particle trajectories and satellite-tracked drogue trajectories show the model can qualitatively reproduce drogue trajectories on the western, but not on the eastern portion of Browns Bank. However, the model consistently underestimates the observed displacements. These discrepancies are probably due to (1) unmodeiled baroclinic components of the circulation and/or (2) the inability of the model to resolve strong topographic gradients and the resulting rectified currents over the steep flanks of the Bank. The use of a previous empirically-defined circulation does not significantly improve the model's performance.

ISSN:0706-652X    

DOI:10.1139/f89-276

出版商:NRC Research Press    

年代:1989

数据来源: NRC

摘要:

Consistent alongshore and/or onshore–offshore gradients in temperature, salinity, and inorganic nutrient were observed off southwest Nova Scotia in June/July, 1981–85. The distributions were established primarily by mixing of relatively warm, high-salinity, nutrient-rich Slope Water with the cold, low-salinity, nutrient-poor water of the Nova Scotia Current. Tidal mixing and frontal dynamics were of secondary importance. Consistent with the distribution of nutrient and the earlier onset of stratification offshore, chlorophyll a concentrations were generally highest offshore and lowest inshore, and the distributions of Zooplankton displacement volume and of dominant copepod species were similar. Salinity, nutrient, and chlorophyll a concentrations and the abundance of zoo-plankton were all generally higher throughout the spring of 1985 than in 1983–84. In 1985, there was greater upwelling and Slope Water influence and decreased flow of the shelf current. This was associated with a reversal of the North Atlantic atmospheric pressure anomaly field, which led to predominantly offshore and southwest alongshore winds in winter and spring, 1985. Correlations of recruitment to northwest Atlantic cod (Gadus morhua) stocks with wind and North Atlantic atmospheric pressure anomalies (Koslow et al. 1987) may be based upon the wind's influence on nutrient availability and plankton abundance.

ISSN:0706-652X    

DOI:10.1139/f89-277

出版商:NRC Research Press    

年代:1989

数据来源: NRC

摘要:

The depth distribution of four development stages of haddock (Melanogrammus aeglefinus) eggs was measured during May 1984 in a continuously stratified water column over Browns Bank. The egg profile varied with development. Early stage eggs were most concentrated near the sea surface whereas late stage eggs were distributed uniformly over depth or had a subsurface maximum. In order to explain the shape of these profiles a one-dimensional (vertical) advection–diffusion model including density stratification was used. Model egg profiles compared favourably with observed profiles and indicate that a large proportion (>30%) of haddock eggs are below the Ekman depth (δe. This proportion increases as the eggs develop. For example more than 50% of the stage III and IV eggs are below δe. Surface drift bottles and other indicators of the very near surface wind-driven transport would appear to be poor indicators of egg transport in haddock and possibly several other species, such as Atlantic cod (Gadus morhua) and mackerel (Scomber scombrus).

ISSN:0706-652X    

DOI:10.1139/f89-278

出版商:NRC Research Press    

年代:1989

数据来源: NRC

摘要:

Literature data was used to examine the hypothesis that variation in the spawning time, defined as the peak in egg abundance, of Northwest Atlantic haddock (Melanogrammus aeglefinus) stocks is explained by water temperature and to derive calibration curves relating incubation temperature to haddock egg stage duration. Spawning times occurred, on average, in March on Georges Bank, late April/early May on Browns Bank, and in June/July on the Grand Banks. Temperatures at peak spawning overlapped considerably but differed in terms of phase in the annual temperature cycle. Inter-annual variation in spawning time of Georges Bank haddock varied by 3 mo and was significantly correlated with water temperature. Browns Bank haddock spawning varied by only 1 mo and was not correlated with water temperature. Egg stage duration varied with incubation temperature. A power curve best described the relationship and was used to estimate the historical, annual variation in haddock egg stage duration which ranged from 10 to 20 d (mean = 16 d) on Georges Bank and from 10 to 30 d (mean = 18 d) on Browns Bank during 1946–80. Collectively, our analysis calls into question the generality of the assumption of constant spawning times for marine fish species and provides essential information for field measurement of haddock egg production rates, mortality, advection, and dispersal.

ISSN:0706-652X    

DOI:10.1139/f89-279

出版商:NRC Research Press    

年代:1989

数据来源: NRC

摘要:

Relationships between the density (sigma-t) structure of the water column and the depth distribution of haddock (Melanogrammus aeglefinus) eggs and larvae in May of 1985 and 1986 were investigated. During the early embryonic period the egg concentration decreased exponentially with depth; this contrasted sharply with stage IV eggs which showed a subsurface maximum, the location of which was determined by their specific gravity. Depth of the centre-of-mass (Zcrn) of stage IV eggs ranged from 13 to 47 m over the survey area whereas the sigma-tassociated with the Zcmdepth varied only slightly (CV = 0.4%) about a mean of 25.58. In stratified offshore waters, haddock eggs were aggregated at middepth but exhibited a progressively more uniform vertical distribution towards the weakly stratified nearshore. Where low-density water (sigma-t < 25.5) occurred near-shore eggs were concentrated at the deepest sampling stratum, suggesting that peak egg concentrations were below this level or possibly on the bottom. The depth distribution of haddock larvae was identical to that of late-stage eggs and was therefore established by buoyancy changes occurring during the embryonic period. Day and night Zcmwas similar, averaging 28 and 31 m, respectively, and stability of the depth distribution of larvae appears to persist well into the juvenile period.

ISSN:0706-652X    

DOI:10.1139/f89-280

出版商:NRC Research Press    

年代:1989

数据来源: NRC

摘要:

This study was designed to assess the dispersal of haddock (Melanogrammus aeglefinus) larvae from their Browns Bank spawning site in terms of either drift or retention mechanisms. Larvae were collected in comprehensive surveys of southwestern Nova Scotia conducted at monthly intervals through the winter–spring of 1985. To avoid any confounding of larval displacement patterns by the numerically dominant young larvae, all spatial analyses were structured by age through otolith microstructure techniques. The results of a trend surface analysis indicated that both drift and retention processes operated in concert to split and disperse the larval population. Retention was probably physically based, through the action of a "leaky" gyre around Browns Bank. Although similar processes are believed to have influenced the distribution of Browns Bank cod (Gadus morhua) larvae, the interspecific difference in spawning time, coupled with the apparent instability of the gyre, produced a different balance between transported and retained larvae. A drift–retention dichotomy for Browns Bank ichthyoplankton has implications for both recruitment and stock structure studies.

ISSN:0706-652X    

DOI:10.1139/f89-281

出版商:NRC Research Press    

年代:1989

数据来源: NRC

摘要:

Synoptic ichthyoplankton surveys conducted at monthly intervals during the winter–spring of 1983–85 were used to determine the location, timing, and magnitude of spawning by haddock (Melanogrammus aeglefinus) and Atlantic cod (Gadus morhua) off southwest Nova Scotia. There was a marked similarity in the spawning locations of the two species: primary spawning occurred on Browns Bank, although lower levels of spawning were observed on adjacent banks and in the inshore region. Cod egg abundance peaked in April in all years, while that of haddock varied between April–June.

ISSN:0706-652X    

DOI:10.1139/f89-282

出版商:NRC Research Press    

年代:1989

数据来源: NRC

摘要:

Spatial variation in distribution, age, and recent growth of postlarval cod (Gadus morhua) were examined off southwestern Nova Scotia during the early summer of 1985 and 1986. Ages ranged between 40–120 d posthatch throughout the sampling area, from the spawning grounds on Browns Bank, to nearshore (<55 m depth) and offshore regions 150 km north. The hatch-date distributions during 1986 between cruises 3 wk apart were significantly different in the nearshore, contrary to the age structure on the Bank which appeared more stable. These observations are consistent with retention of cod in the Browns Bank gyre, coupled with episodic leakage and northerly advection in the residual current. Recent growth determined from otolith increment widths was significantly less for those cod sampled at nearshore stations than for cod offshore and on the Bank. Recent growth was significantly correlated with Zooplankton biomass in a size range suitable for postlarval cod, while sea temperature was correlated in only one cruise. Nearshore areas had on average 25% of the Zooplankton biomass found on the Bank. Recent growth indices of the third and fourth week precapture were not significantly different between the nearshore and offshore, implying that the cod had shared a common environment, and common origin such as Browns Bank.

ISSN:0706-652X    

DOI:10.1139/f89-283

出版商:NRC Research Press    

年代:1989

数据来源: NRC