Margaret's Bay Research Articles

Temporal and spatial variability in hydrography and dissolved oxygen along southwest Nova Scotia using glider observations

Dissolved oxygen (DO) in the global ocean is on the decline, resulting in the degradation of coastal habitats. As aquaculture production occurs in these regions, proper understanding of coastal DO dynamics is important for improved farm management (e.g. site selection). The main objective of this study was to quantify along-shore and cross-shore variability in DO dynamics, as well as onshore advection of offshore waters to the bays that could contain aquaculture farms. For that purpose, a Slocum underwater glider was deployed between September 25 and October 12, 2020 to collect high-resolution data of temperature, salinity and DO along a transect between Shelburne Bay and St. Margarets Bay, Nova Scotia, Canada, with an average distance of about 15 km from shore. The observations revealed the variable nature of cross-shore water properties. Shoreward bottom currents transport offshore waters to the coast, and combined with longer residence times, mixing of these waters with those present created a mosaic of differing water properties, with warmer, fresher, and less oxygenated shoreward waters. The glider intercepted an upwelling event due to strong and persistent southwesterly winds, which cooled the upper water layers by 6 °C and increased DO by 1.4 mg L−1. This strong upwelling event detected at 10 km from the coast was also captured 30 h later within St. Margarets Bay, depicting a potential offshore-inshore interaction. Therefore, bay-wide ecosystems and aquaculture production could be affected by intrusions of offshore waters.

Numerical Study of Topographic Effects on Wind-Driven Coastal Upwelling on the Scotian Shelf

Wind-driven coastal upwelling can cause a sudden drop in sea surface temperatures (SSTs) of up to more than 8 °C on the inner Scotian Shelf (ScS) in the summer months. Three major coastal upwelling events on the ScS in the summer of 2012 are analyzed using in-situ SST observations and satellite remote sensing SST data. A spatial correlation analysis of satellite SST data shows an asymmetric distribution in the along-shore direction with smaller correlation coefficients in the downstream area than in the upstream area over the inner ScS during upwelling events. A regression analysis indicates that the wind impulse plays a major role in generating the SST cooling during the initial response stage of upwelling events. A nested-grid ocean circulation model (DalCoast-CSS) is used to examine the effect of irregular coastline and rugged bathymetry on the spatial and temporal variability of wind-driven upwelling over the inner ScS. The model has four submodels downscaling from the eastern Canadian Shelf to the central ScS. The model external forcing includes tides, winds, river discharges, and net heat flux at the sea surface. A comparison of model results with the satellite SST data reveals a satisfactory performance of the model in reproducing the development of coastal upwelling on the ScS. Model results demonstrate that the irregular coastline and rugged bathymetry play important roles in influencing the temporal and spatial evolution of the upwelling plume over the inner ScS. The irregular coastline (e.g., cape) is responsible for the relatively warm SSTs in two downstream inlets (i.e., St. Margarets Bay and Mahone Bay) and adjacent coastal waters. The rugged bathymetry (e.g., submerged bank) influences the spatial extent of filaments through the advection process.

Harbouring the enemy: kelp holdfasts protect juvenile sea urchins from predatory crabs

Predation is an important agent of post-settlement mortality of sea urchins that is mediated by the availability and suitability of spatial refuges, particularly during the vulnerable juvenile stage. In laboratory and field caging experiments, we show that holdfasts of a dominant kelp, Saccharina latissima, provide a spatial refuge for juvenile sea urchins Strongylocentrotus droebachiensis (< 20 mm, test diameter) from crabs Cancer borealis and C. irroratus, considered to be the dominant predators of sea urchins in kelp-bed ecosystems in the northwestern Atlantic. In treatments with individual crabs of either species, the presence of holdfasts reduced predation on juvenile sea urchins by ~20 to 30% compared to treatments with no refuge. Crabs consumed juve- niles (from 5 to 19 mm) in each of three 5 mm size classes in proportion to their abundance, regard- less of treatment. In kelp beds in St. Margarets Bay, Nova Scotia, Canada, the number of juvenile sea urchins per holdfast ranged from 0.3 to 0.9, with juveniles in holdfasts accounting for two- thirds of the total urchin population density at one site. Up to 4 juveniles occurred within a single holdfast, and there was a significant positive relationship between juvenile size (but not number) and holdfast volume. Small adult sea urchins were not found within holdfasts in kelp beds and rarely occupied holdfasts presented to them in laboratory cages. Our findings indicate an ontoge- netic shift in sea urchin-kelp interactions, whereby kelp facilitates recruitment of its major grazer.

Disease as a control of sea urchin populations in Nova Scotian kelp beds

In Nova Scotia, Canada, periodic outbreaks of amoebic disease (paramoebiasis) cause mass mortality of sea urchins Strongylocentrotus droebachiensis in subtidal barrens. How- ever, in kelp beds, where sea urchins are cryptic and generally less dense than in barrens, disease outbreaks are not readily observed and the importance of disease in regulating these populations is unknown. To determine whether sea urchin populations in kelp beds are controlled by disease, we analyzed population data from kelp beds at a single location (St. Margarets Bay) across a span of 44 yr (1968 to 2012) to compare changes in size structure and density in relation to the timing of disease outbreaks in adjacent sea urchin aggregations and barrens. We found that sea urchin den- sity, maximum test diameter and percentage of adults decreased following disease outbreaks and increased during intervening periods without disease, indicating that disease regulates the popu- lation in kelp beds by limiting survival to adulthood. Our results suggest that disease has replaced predation as a major agent controlling sea urchin populations in Nova Scotian kelp beds.

An outbreak of sea urchin disease associated with a recent hurricane: Support for the “killer storm hypothesis” on a local scale

The frequency of epizootics causing mass mortality of sea urchins (Strongylocentrotus droebachiensis) along the coast of Nova Scotia, Canada has increased over the past three decades. Laboratory and field studies show that outbreaks of disease are caused by the amoeba Paramoeba invadens, and are associated with hurricane activity and warm seawater temperatures. A statistical model indicates that the probability of a mass mortality event increases with the proximity of a hurricane to the coast and the maximum sustained wind speed, and is greater when post-storm seawater temperature is above a threshold for disease propagation. To assess the reliability of the model in predicting mass mortality events on an annual scale, and to examine spatial variability in mortality (in the event of a disease outbreak) on a local scale (meters to kilometers), we transplanted sea urchins into cages in kelp beds at 6 sites around St. Margarets Bay, Nova Scotia, where localized outbreaks of paramoebiasis have been observed following hurricanes in the past. On 4 September 2010 a Category 1 hurricane (Earl) made landfall 110km SSW of the experimental area. Based on the parameters of the storm, the model forecasted a 43% probability of a disease outbreak. Morbidity of caged animals was first documented on 6 September 2010, and morbidity and mortality in the cages was ~50% by the end of September and ~85% two months after the storm. Laboratory experiments indicated that the temperature-dependent transmission or induction of morbidity was consistent with paramoebiasis. Our findings provide support for the efficacy of the model to predict the occurrence of disease outbreaks, although the source population(s) of the pathogenic agent and oceanographic mechanisms affecting its introduction and spread along the coast of Nova Scotia remain poorly understood.

Physical forcing of distributions of bryozoan cyphonautes larvae in a coastal embayment

In shallow rocky subtidal ecosystems in the western North Atlantic, outbreaks of the nonindigenous encrusting epiphytic bryozoan Membranipora membranacea cause defoliation of laminarian algae, facilitating a phase transition from native kelp beds to meadows of invasive algae. We quantified spatial (m [depth] to km [sites]) and temporal (hourly to monthly) patterns in the abundance of larvae of M. membranacea, and a morphologically similar native species, Electra pilosa, in relation to physical structure (temperature, salinity, density) of the water column in St. Margarets Bay, Nova Scotia, Canada, on 8 dates from September to November 2007. During the study period, the water column ranged from strongly stratified to well mixed, and cross-shore movements of water masses (wind-driven upwelling and downwelling) were indicated by shoaling of the isoclines. When a strong pycnocline was located between the 2 sampled depths (4 and 12 m), larvae of both species were more abundant in the shallower, warmer, fresher layer. The linear relationship between strength of stratification and the relative abundance of larvae between depths was significant over 2 temporal (hourly, weekly) and 1 spatial (km) scale examined for M. membranacea, but not for E. pilosa. Highest abundance of larvae of both species in the warm fresh surface layer suggests onshore transport during wind-driven downwelling events. Dissimilar patterns in size-frequency distributions between the indigenous and nonindigenous bryozoan larvae suggest that species-specific characteristics of larvae of M. membranacea may be a contributing factor to its success as an invader in the western North Atlantic.

Modelling of a Laser yacht mast

The purpose of this work to investigate the deflection of a Laser yacht mast caused by trimming forces (in plane deflection) occurring within the Laser yacht rig and create a model that reflects this. Finite element analysis software is used for the creation and analysis of the model based on structural mechanics principles. Verification of the model is carried out through comparison with experimental results, where deflection is determined in different wind conditions, and improvements of the model are based on the disparity of the model and the experimental results. References Anon, SolidWorks---COSMOSWORKS, Solidworks, Concord, MA, 2008, CAD FEA Software. P. Benham, R. Crawford, and C. Armstrong, Mechanics of Engineering, Pearson Education Ltd., England, 1996. D. Boote and M. Caponetto, Numerical Approach to the Design of Sailing Yacht Masts, The Tenth Chesapeake Sailing Yacht Symposium, ed Anon, Society of Naval Architects and Marine Engineers, Annapolis, 1991. H. Enlund, A. Pramila, and P. Johansson, Calculated and Measured Stress Resultants in the Mast And Rigging of a Baltic 39 Type Yacht, Transactions of the Royal Institution of Naval Architects Part B: International Journal of Small Craft Technology 148 (1984) 41--49. G. Grabe, The Rig of the Research Sailing Yacht DYNA Measurements of Forces and fea, High Performance Yacht Design Conference 4--6, December, Auckland, 2002. G. Grabe, The Rig of the UCA---Finite Element Analysis, International HISW Symposium Yacht Design and Construction, Amsderdam Nov 15--16 2004. ISAF, IOC Confirms 10 Sailing Events For London 2012 Olympic Games, Centreboard Classes, International Sailing Federation, London, 2009. http://www.sailing.org/news/29214.php L. W. E. Media, Nautel Laser Masters World Championship, Margaret's Bay, 2009. http://can09.laserinternational.org/ D. D. Pearson, Tom Slingsby crowned 2008 Laser world champion, 2008. http://www.sailing.org/news/22202.php R. Shenoi, P. Conti, S. Turnock, and M. Scarponi, Mini 6.50 Mast Optimisation Using a Design of Experiment Approach and Finite Element Analysis Simulation, Transactions of the Royal Institution of Naval Architects Part B: International Journal of Small Craft Technology (B1) 148 (2006) 41--49. R. L. Tillman, The Complete Book of Laser Sailing, McGraw-Hill, International Marine/Ragged Mountain Press, Thomaston, 2000.

Channel Equalization for Single Carrier MIMO Underwater Acoustic Communications

Multiple-input multiple-output (MIMO) underwater acoustic (UWA) channels introduce both space-time interference (STI) and time-varying phase distortion for transmitted signals. In such cases, the equalized symbols produced by conventional equalizer aiming for STI cancelation suffer phase rotation and thus cannot be reliably detected. In this paper, we propose a new equalization scheme for high data rate single carrier MIMO UWA channels. Different from existing methods employing joint equalization and symbolwise phase tracking technology, the proposed scheme decouples the interference cancelation (IC) operation and the phase compensation operation, leading to a generalized equalizer structure combining an IC equalizer with a phase compensator. The decoupling of the two functionalities leads to robust signal detection, which is most desirable in practical UWA applications. MIMO linear equalizer (LE) is adopted to remove space-time interference, and a groupwise phase estimation and correction method is used to compensate the phase rotation. In addition, the layered space-time processing technology is adopted to enhance the equalization performance. The proposed equalization scheme is tested to be very robust with extensive experimental data collected at Kauai, Hawaii, in September 2005, and Saint Margaret's Bay, Nova Scotia, Canada, in May 2006.

High recruitment of the introduced bryozoan Membranipora membranacea is associated with kelp bed defoliation in Nova Scotia, Canada

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 369:139-151 (2008) - DOI: https://doi.org/10.3354/meps07669 High recruitment of the introduced bryozoan Membranipora membranacea is associated with kelp bed defoliation in Nova Scotia, Canada Megan Saunders*, Anna Metaxas Department of Oceanography, Dalhousie University, 1355 Oxford Street, Halifax, Nova Scotia B3H 4J1, Canada *Email: msaunders@dal.ca ABSTRACT: For many organisms, the magnitude of recruitment of settlers into the juvenile population is a key life-history parameter, which has pronounced effects on population dynamics. However, for benthic colonial organisms, abundance may also be highly influenced by availability of space, growth rate, senescence, and mortality. On the southern shore of Nova Scotia, Canada, outbreaks of the introduced epiphytic bryozoan Membranipora membranacea on kelps have occurred periodically since 1992. We monitored the abundance, size distribution, and percent cover of M. membranacea colonies on blades of the kelp Saccharina longicruris at 2 sites in St. Margarets Bay, Nova Scotia, from July 2005 to November 2006. Following a warm winter, juvenile and adult colonies occurred earlier and were an order of magnitude more abundant in 2006 compared to 2005. The percent cover of M. membranacea on kelp was higher in 2006 compared to 2005 in summer and early autumn, but there was no difference between both years in late autumn. Colonies were larger in summer 2006 than in 2005, but were smaller in late autumn in 2006, suggesting that large colonies had been lost due to kelp blade breakage. By November 2006, there was a 70% decrease in the canopy cover of kelp on the benthos, despite a similar frequency and magnitude of storms compared to the previous year. We propose that outbreaks of M. membranacea in the northwest Atlantic are the result of periods of early and high settlement and recruitment. By causing kelp defoliation, these outbreaks can have a pronounced ecological effect on kelp beds. KEY WORDS: Recruitment · Population dynamics · Introduced species · Benthic colonial organisms · Kelp beds · Membranipora membranacea Full text in pdf format PreviousNextCite this article as: Saunders M, Metaxas A (2008) High recruitment of the introduced bryozoan Membranipora membranacea is associated with kelp bed defoliation in Nova Scotia, Canada. Mar Ecol Prog Ser 369:139-151. https://doi.org/10.3354/meps07669 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 369. Online publication date: October 13, 2008 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2008 Inter-Research.

Efficiency of bait usage in the Nova Scotia lobster fishery: a first look

In studies of commercial fishing, little attention has been paid to the use of bait. We examined lobster fishery bait-to-catch ratios in St. Margaret's Bay and Mahone Bay, Nova Scotia. Data were obtained from the Fishermen and Scientists Research Society's Lobster Recruitment Project. Local ecological knowledge of the resource users from within the study area was also gathered using semi-structured interviews. The estimated amount of bait required to catch each lobster, weighing about 480 g, ranged from 185 g (November) to 1455 g (April) for average months during the 2002/3, 2003/4 and 2004/5 fishing seasons. We estimated the overall ratio of bait-to-catch to be about 1.9 units of bait per unit of catch. The eco-efficiency and food-supply implications of consuming more bait than product in a fishery are outlined.

Things I Learned on St. Margaret's Bay, and: Bodies of Water

Things I Learned on St. Margaret's Bay, and: Bodies of Water

The Amaknak Bridge Site: Cultural Change and the Neoglacial in the Eastern Aleutians

Evidence from a series of excavations in Unalaska Bay indicates that the onset of the Neoglacial had begun to alter the cultural and natural landscape of the Aleutian Islands as early as 4500 rcybp. By 3500 rcybp deep semi-subterranean houses with stone walls and elaborate hearth systems were constructed at the Margaret Bay site. The nearby Amaknak Bridge site was occupied between 3300 and 2700 rcybp and faunal data reflect a very significant increase in the extent and duration of ice in the Bering Sea. Multiple room houses, elaborate labrets, complex burials, and other evidence of changes in social organization were among the cultural responses of the ancient Unangan to the colder conditions of the Neoglacial. We also find evidence for technological innovation, aggregated settlements, and resource intensification during this time.

Temperature explains settlement patterns of the introduced bryozoan Membranipora membranacea in Nova Scotia, Canada

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 344:95-106 (2007) - DOI: https://doi.org/10.3354/meps06924 Temperature explains settlement patterns of the introduced bryozoan Membranipora membranacea in Nova Scotia, Canada Megan Saunders*, Anna Metaxas Department of Oceanography, Dalhousie University, Halifax, Nova Scotia B3H 4J1, Canada *Email: msaunders@dal.ca ABSTRACT: Settlement is a critical life-history stage for benthic invertebrates with a planktonic larval form, and is important for understanding mechanisms of larval supply and population dynamics. We examined spatial (metre to kilometre scales) and temporal (weekly to yearly scales) patterns in abundance of settlers of the introduced bryozoan Membranipora membranacea on the numerically dominant kelp Saccharina longicruris, at 2 sites in St. Margarets Bay, Nova Scotia, Canada. Relationships between patterns in settlement and indices of the temperature regime, such as average weekly temperature, temperature fluctuations, water column stratification and thermal history (growing degree-day) were determined using multiple linear regressions. Although settler abundance varied interannually, it was highest in early autumn, and was generally higher at depth (12 and 8 m) than in the shallows (4 m). Abundance was an order of magnitude greater after a significantly warmer winter. Growing degree-day accounted for up to 81% of the variability in settler abundance, whereas average temperature, temperature fluctuations and stratification were less important. The strong positive relationship between thermal history and settlement of this ecologically important species suggests that abundance of established introduced marine species may increase with the increased temperatures predicted to result from climate change. KEY WORDS: Larval settlement · Temperature · Thermal history · Stratification · Bathymetric patterns · Benthic invertebrates · Introduced species · Climate change Full text in pdf format PreviousNextCite this article as: Saunders M, Metaxas A (2007) Temperature explains settlement patterns of the introduced bryozoan Membranipora membranacea in Nova Scotia, Canada. Mar Ecol Prog Ser 344:95-106. https://doi.org/10.3354/meps06924 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 344. Online publication date: August 23, 2007 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2007 Inter-Research.

Community resilience or unidentified health risk?: health professional perceptions on the impact of the Swissair Flight 111 disaster on surrounding communities.

On September 2nd, 1998 Swissair Flight 111 crashed in Saint Margaret's Bay, Nova Scotia. Surrounding coastal communities were immediately transformed into disaster response sites. Sixteen community health professionals were interviewed that identified several types of individual and community exposure, including exposure to human remains. The interviews revealed that the coastal communities have responded with silence and stoicism. This silence has been viewed by some health professionals as resilience. The interface of a major disaster, community silence, low help-seeking behaviours, and limited disaster health responses raises the critical question whether this is a profile of resilience or a community silently enduring.

Peri‐urban residential development in the Halifax region 1960–2000: magnets, constraints, and planning policies

The post 1960 locational sequence of peri‐urban residential development in the Halifax city region is interpreted in terms of three sets of factors: magnets or attractors, constraints or inhibitors, and planning policies designed to direct or control development. Most peripheral housing has been suburban (i.e., on small, centrally‐serviced lots) rather than exurban (on large lots, serviced on‐site). Suburban development patterns have been strongly shaped by the 1963 Halifax Housing Survey and the 1975 Regional Development Plan, both of which promoted planned satellite communities on government land assemblies, in areas of cheap land and low development costs.After 1980 effective regional planning was phased out, which encouraged excessive large‐lot development in country districts beyond the service boundary. Magnets for country residential development operate at a variety of scales, and include highway access, elementary schools, localized site conditions, and appropriate zoning. The major regional constraint is road‐distance from the urban core, but socio‐cultural and cadastral constraints are locally important.The paper concludes with a survey of policy options to promote efficient patterns of suburban development and to curb exurban sprawl. Many of these options were recommended in earlier plans, but have lacked fiscal and political support in the last two decades. They must now be re‐assessed in a new regional plan.La séquence géographique post‐1960 du développement résidentiel péri‐urbain dans la région de Halifax est interprétée par l'entremise de trois ensembles des facteurs: les aimants ou les attracteurs, les contraintes ou les inhibiteurs, et les politiques de planification conçues pour diriger ou contrôler le développement. La plupart des logements périphériques ont été sub‐urbains (c'est‐à‐dire sur des petits lots dotés de services centraux) plutôt qu'exurbains (sur des grands lots, dotés sur place). Des configurations suburbaines de développement ont été fortement conditionnées par le Halifax Housing Survey (1963) et par le plan de développement régional (1975), tous deux ayant promu des communautés satellites prévues sur les terres de gouvernement, dans les zones de terrain bon marché avec des coûts de développement relativement bas.Après 1980, la planification régionale a étééliminée, ce qui a encouragé le développement excessif de lots de grandes dimensions dans des zones au‐delà de la limite des services. Les aimants pour le développement résidentiel exurbain fonctionnent à diverses échelles. Ils incluent notamment l'accès routier, les écoles primaires, l'état des sites locaux, et la répartition appropriée en zones. La contrainte régionale principale est la distance routière par rapport au noyau urbain, mais les contraintes socio‐culturelles et cadastrales sont localement importantes.En conclusion, l'article fournit un résumé des options politiques favorisant les configurations efficaces du développement suburbain et limitant le développement des grands lots exurbains. Plusieurs de ces options ont déjà fait l'objet de recommandations dans le cadre de plans passés, mais ont manqué de support fiscal et politique au cours des deux dernières décennies. Elles doivent maintenant être réévaluées dans un nouveau plan régional.

A major settlement event associated with minor meteorologic and oceanographic fluctuations

Settlement of ophiuroids (Ophiopholis aculeata, Ophiura spp.) was measured using artificial collectors at 3-day intervals during their annual 2-week settlement period in July-August 1993 in St. Margaret's Bay, Nova Scotia. Hydrographic (temperature, salinity, current velocity, wave height and period) and meteorologic conditions (atmospheric pressure and wind velocity) were recorded concurrently at the site or at nearby locations. A major settlement pulse occurred over one 3-day period, with declining settlement over the following 6 days. This pulse was associated with a shift in current direction and preceded by rapid temperature and salinity fluctuations. Similar changes in temperature occurred at a second site (4.5 km away), indicating at least bay-scale forcing. This period was characterised by the passage of weak atmospheric pressure gradients and a low-energy sea state. These meteorologic and oceanographic fluctuations were within the normal range for this time of year, indicating that major settlement events can occur at scales of days in association with minor fluctuations in the physical environment.

A major settlement event associated with minor meteorologic and oceanographic fluctuations

Settlement of ophiuroids (Ophiopholis aculeata, Ophiura spp.) was measured using artificial collectors at 3-day intervals during their annual 2-week settlement period in July-August 1993 in St. Margaret's Bay, Nova Scotia. Hydrographic (temperature, salinity, current velocity, wave height and period) and meteorologic conditions (atmospheric pressure and wind velocity) were recorded concurrently at the site or at nearby locations. A major settlement pulse occurred over one 3-day period, with declining settlement over the following 6 days. This pulse was associated with a shift in current direction and preceded by rapid temperature and salinity fluctuations. Similar changes in temperature occurred at a second site (4.5 km away), indicating at least bay-scale forcing. This period was characterised by the passage of weak atmospheric pressure gradients and a low-energy sea state. These meteorologic and oceanographic fluctuations were within the normal range for this time of year, indicating that major settleme...

Effects of conspecific density on settlement, growth and post-settlement survival of a temperate reef fish

Settlement, growth and post-settlement mortality of the cunner ( Tautogolabrus adspersus Walbaum) was investigated on isolated boulder reefs in St. Margaret's Bay, Nova Scotia. Populations of cunner on these reefs were subjected to experimental manipulations of adult and recruit population density. Settlement of cunner was unaffected by the density of prior resident adults. Elevation of adult densities to approximately twice natural levels resulted in complete mortality of newly settled fish, while removal of adults resulted in enhanced growth and recruitment success over natural populations. This clearly demonstrates that natural populations of cunner in St. Margaret's Bay are sufficiently high to result in density-dependent growth and post-settlement mortality. Growth and mortality of newly settled fish were also suppressed by high recruit densities and enhanced by low densities of conspecific recruits. On all study reefs, newly settled cunner formed social dominance hierarchies, and both growth rate and post-settlement survival were positively correlated with fish size. Recruitment success of species such as cunner, for which post-settlement mortality is size-dependent, may be strongly influenced by processes affecting growth rate.

Effects of habitat on settlement, growth, and postsettlement survival of Atlantic cod (Gadus morhua)

Settlement and growth of age 0+ cod were monitored using snorkel and self-contained underwater breathing apparatus (SCUBA) in four distinct habitat types (sand, seagrass, cobble, and rock reef) in St. Margaret's Bay, Nova Scotia. Newly settled cod were marked with acrylic dye, allowing repeated visual length estimates of individual fish. Settlement of cod did not differ between habitat types, but postsettlement survival and subsequent juvenile densities were higher in more structurally complex habitats. These differences appear to be due to increased shelter availability and decreased predator efficiency in structurally complex habitats. Growth rate was highest in seagrass beds, while the efficiency of cod predators was lowest and cod survival was highest on rocky reefs and cobble bottoms. Thus, trade-offs occur between energy gain and predation risk. In St. Margaret's Bay, the population structure of Atlantic cod may be less influenced by patterns of larval supply than by postsettlement processes such as habitat-specific growth and mortality.

Evaluation of relative density, diameter growth, and stem form in a red spruce (Picearubens) stand 15 years after precommercial thinning

Thirty red spruce (Picearubens Sarg.) trees were harvested from a precommercial thinning trial near St. Margaret's Bay, Nova Scotia. Total height was measured, and samples were removed from the following five stem heights: stump height, breast height, 25, 50, and 75% of total height. Relative density and growth rate were measured using X-ray densitometry, and taper was calculated for log and tree lengths. Growth rates were significantly greater on the treatment plot than on the control plot. No significant differences in relative density were found between trees from the treatment plot (2.4 × 2.4 m spacing) and trees from the unthinned control. The relative density of the treatment and control trees was also compared with the species average relative density for red spruce. No significant differences were found, except for the relative density at stump height in the after-treatment portion of the trees from the thinned plot. The difference was not considered great enough to cause a deterioration of the mechanical properties of lumber or the yield of pulp produced from this material. The taper factor was significantly different between the treatment and control plots, but the taper factor for the treatment trees was similar to that for the spruce resource currently being processed in eastern Canada.