David Brickman

Contemporary management issues confronting fisheries science

Abstract Stock collapses have occurred worldwide. The most frequently cited cause is over-fishing, suggesting that fisheries management has been ineffective in controlling exploitation rates. The progression of a fishery from an over-exploited to a collapsed state involves impairment of the reproductive capacity of the target species, i.e. recruitment over-fishing. In many cases, this occurs by reduction of the spawning stock biomass (SSB) through the systematic elimination of spawning components within a stock complex. While operational definitions of minimum levels of SSB have been developed, they have seldom been applied and never adopted in a Canadian groundfish management context. The answer to the question of how much is enough to perpetuate a stock under exploitation has been illusive. Serebryakov [J. Cons. Int. Explor. Mer, 47 (1990) 267] has advocated definition of critical levels of SSB based on survival rates (R/SSB). We review his method and discuss the utility of the approach. An alternative approach to the problem of estimating minimum SSB is through a fundamental revision of the traditional stock and recruitment relationship. Explicit theoretical SSB thresholds below which reproduction/recruitment is severely impaired based upon density-dependent mating success (or Allee effects) is considered a superior approach to the question of how much is enough because of its ecological grounding. However, the successful application of this approach will require re-definition of the space/time scales of the management unit. Finally, support is growing for the establishment of closed areas or “no-take zones” as an alternative approach to managing the problems of fishing a stock complex by enabling sub-populations to escape fishing. While the expected benefits of areas protected from fishing are numerous, clear demonstrations of benefits of such areas in marine temperate ecosystems are lacking. In fact, unintended negative consequences may result from such actions.

Detailed structure of currents and hydrography on the northern side of Georges Bank

A suite of observations from July 2–3, 1988, is used to describe the spatial structure and temporal evolution over the tidal period of currents and hydrography across the northern side of Georges Bank in summer under light winds. The data set includes moored current and hydrographic observations at four cross-bank positions, fast response thermistor chain observations at two of the sites, a conductivity-temperature-depth section, 10 repeated sections over the tidal period using a towed Batfish and ship-mounted acoustic Doppler current profiler, and two surface drifter trajectories. The observations provide a detailed description of previously identified features such as the strong semidiurnal tidal currents, an internal tide, a tidal front, and a residual current jet and also reveal a hierarchy of energetic smaller-scale structures. These include an internal hydraulic jump during off-bank tidal flow and subsequent internal waves propagating onto the bank and also a surface convergence in the frontal zone. The physical oceanographic regime on the northern side of Georges Bank during spring-fall can be conceptualized as a hybrid of a stratified shelf break with strong tidal advection and a tidal(mixing) front. Key factors to the regime are the strong tidal currents and abrupt topographic variation over the banks side. The result is a nonlinear and baroclinic tide-topography interaction at the bank edge and a frontal zone with strong variability over the tidal period associated with tidal advection and large-amplitude internal waves. The along-bank transport in the residual jet is estimated to be 0.91 Sv, indicating that it may be the largest summertime transport feature on the northwestern Atlantic shelf between Cape Hatteras and the Grand Bank.

Energetics of the Internal Tide on Northern Georges Bank

Abstract This paper discusses the energetics of the internal waves generated by the tide on the northern edge of Georges Bank in the outer Gulf of Maine–-a region of strong tidal flow, a density/temperature front, and abrupt topography. A series of bank-edge cross sections in early July 1988, using the towed vertically profiling Batfish, shows that during off-bank flow a large internal hydraulic jump develops over the bank slope. This disturbance propagates on bank during the reverse phase of the tide, evolving into two internal solitonlike features that impinge upon the frontal zone separating stratified bank-edge water from the homogeneous water on bank. Thermistor chain data from the bank edge show that two internal wave packets per tidal cycle are characteristic of the region during this time of year. The available potential energy plus kinetic energy of the internal disturbances, estimated using the Batfish and thermistor chain data, is found to be 35 J m−3 in a plug of fluid 60 m deep and 4 km long ...

RANGE CONTRACTION MAY NOT ALWAYS PREDICT CORE AREAS: AN EXAMPLE FROM MARINE FISH

Our goal was to identify the core areas of six severely depleted marine fish species on the Scotian Shelf, Canada, using the theory of ideal free distribution (IFD). We tested for density-dependent distribution on both regional and local scales. At the regional scale, a density-dependent response was observed in the majority of populations. At the local scale, density was expected to remain stable in areas of high density and to change more rapidly in marginal areas, in response to changes in regional abundance. Lower local density responses were associated with areas of higher density, but deviations were evident, in part due to the magnitude of decline and in part due to fishing effects. Former areas of high density can be eroded if the population has severely declined. Fishing directed at an area of high density can cause local depletion when recolonization rates are low relative to the intensity of fishing. Thus, areas occupied during periods of low regional abundance do not of necessity reflect the historical array of core areas. We do not recommend the use of IFD theory to identify core areas of heavily exploited species. Instead, we recommend a precautionary approach that assumes the existence of low-mixing populations that can be differentially affected by fishing. For species at risk, only data derived before significant population declines should be used to identify high-density areas. Such areas would rep- resent those with the potential to support higher densities as well as the historical array of subpopulations. Our study provides insight into the practical aspects of analyzing exploited species using ecological theory.

Heat Flux Partitioning in Open-Ocean Convection

Abstract Cold air blowing out over a warm ocean leads to convection over an isolated region of the ocean basin. This phenomenon, known as open-ocean convection, is often simulated by convective forcing from a circular disk much smaller than the dimension of the domain. An important feature of these simulations is the development of eddies at the edge of the disk that serve to transport heat horizontally. This has sparked interest in the heat-flux characteristics of such a system. This paper deals with the thermodynamic properties of this type of convective flow, using a rotating tank with bottom-mounted hotplate as the experimental apparatus. Experiments are performed, in an initially unstratified fluid, for a set of values of the nondimensional forcing parameter Rc = B1/2/(Hf3/2), where B is the buoyancy flux, H is the fluid depth, and f is the Coriolis parameter. The ratio of the frontal eddy size to the hotplate radius, ϵ, is shown to be an important parameter. The experiments reported are for the regi...

An abrupt shift in the Labrador Current System in relation to winter NAO events

The behavior of the Labrador Current during the period from 1990 to 2007 is investigated with an eddy-resolving circulation model for the North Atlantic Ocean. An EOF analysis of the model output suggests that the variability in the Labrador Current can be partitioned into a western Labrador Current (WLC; from the 300-2500 m isobaths), and an eastern Labrador Current (ELC; from the 2500-3300 m isobaths). The model results demonstrate that the WLC transport experienced an abrupt increase during 2000-2002, consistent with data. This differed significantly from the ELC transport which was strong during the high winter NAO (North Atlantic Oscillation) years (1990-95) and then steadily declined. This ELC trend is consistent with changes in the modelled Atlantic Meridional Overturning Circulation and convection depth. Our study proposes that the change in the WLC is due to a southwestward shift of the atmospheric circulation pattern starting in 2001, coincident with a change in the 2001 NAO index, and also in a westward shift of the action centers of the winter NAO events. This article is protected by copyright. All rights reserved.

Could ocean currents be responsible for the west to east spread of aquatic invasive species in Maritime Canadian waters

The circulation in the shelf seas of Maritime Canada is predominantly in the northeast-southwest direction. Despite the mean northeast-southwest flow, a number of AIS invasions have been observed to proceed in the opposite direction - from the Gulf of Maine, around Nova Scotia, and into the southern Gulf of St. Lawrence. Flow fields from a numerical circulation model are used to investigate whether these invasions could be due to drift in ocean currents. Particle tracking experiments are performed and probability density functions (PDFs) derived that describe the probability of drifting a given upstream distance in a given drift time. Analysis of these PDFs revealed that for invasions that took 20-40 y to occur, propagule drift in ocean currents could be responsible for the upstream spread, while this was not the case for short timescale invasions (<10 y). Rafting could be responsible for both short and long timescale invasions.

A climate-associated multispecies cryptic cline in the northwest Atlantic

Cryptic multispecies genetic structure reflects ocean climate and is associated with response to climate change. The spatial genetic structure of most species in the open marine environment remains largely unresolved. This information gap creates uncertainty in the sustainable management, recovery, and associated resilience of marine communities and our capacity to extrapolate beyond the few species for which such information exists. We document a previously unidentified multispecies biogeographic break aligned with a steep climatic gradient and driven by seasonal temperature minima in the northwest Atlantic. The coherence of this genetic break across our five study species with contrasting life histories suggests a pervasive macroecological phenomenon. The integration of this genetic structure with habitat suitability models and climate forecasts predicts significant variation in northward distributional shifts among populations and availability of suitable habitat in future oceans. The results of our integrated approach provide new perspective on how cryptic intraspecific diversity associated with climatic variation influences species and community response to climate change beyond simple poleward shifts.