Following a short summary of the contribution of Hjort's 1914 paper to the subsequent studies on population richness of fish species, the literature on herring larval drift and transport in the northeast Atlantic is reviewed (including the early studies contributing to the Migration Triangle hypothesis, process oriented studies in the field, and oceanographic modelling work). The review indicates that the concept of larval transport, within the context of the Migration Triangle hypothesis (which addresses the location of spawning) and the Common Pool/Adopted Migrant hypotheses (which interpret the degree of population richness of Atlantic herring), is generally supported. In contrast, analyses of the ICES large scale herring larval surveys in the Norwegian Coastal Current, the North Sea, and the shelf seas north and west of Scotland and Ireland (as well as the similar survey off southwest Nova Scotia in Atlantic Canada) indicate that there is limited displacement in the centres of distribution of different length classes of larvae. Given that larval length is a proxy for age, the analyses suggest that during the first two to three months of the larval phase the Centres of Mass (CoMs) for the progeny of the different spawning areas are relatively fixed. The results support the interpretation that location of spawning is not defined in relation to transport of larvae from the spawning area to the juvenile nursery area, but rather due to the minimisation of drift and transport (i.e. "larval retention") at the early life history stage (as outlined in the Member/Vagrant hypothesis). The minimisation of drift and transport in the areas of spawning are interpreted as being important to the definition of the spatial scale of philopatry (and thus life cycle closure) in herring populations. (C) 2015 Elsevier B.V. All rights reserved.

•The evidence for respectively top-down control and bottom-up control of the ecosystem features of the Eastern Scotian Shelf off Atlantic Canada from the 1970s to the present is evaluated.•There is limited evidence in support of the fisheries induced trophic cascade hypothesis.•Stratification/nutrient/plankton trends are generally consistent with climate variability hypothesis.•It is concluded that a combination of concurrent top-down forcing by grey seals and bottom forcing due to climate variability is consistent with the aggregate observations on the ecosystem decadal trends.

The cod stock on the Eastern Scotian Shelf (the 4VsW management area) collapsed in the early 1990s, and has experienced high natural mortality since then, while the abundance of grey seals foraging in the area has doubled about every seven years since the 1960s. The causes of the high cod natural mortality are not well understood, but seals are not considered to have played a significant role. This study takes a fresh look at the impact of seals on 4VsW cod abundance. Abundance trends of the Sable, Eastern Shore and Gulf seal herds which forage on the Scotian Shelf are estimated to 2020. The Sable herd is projected to stabilize at about 350,000 individuals. If their exponential growth continues, the Eastern Shore and Gulf herds could in aggregate reach about 200,000 individuals. However, density dependent processes are likely to slow population growth of these two herds sometime in the coming decade. Total annual food consumption of the three herds is estimated. In 2010, in excess of 550,000 t of fish are consumed annually by the Sable and Eastern herd, with the Gulf herd consuming about 138,000 t. The literature on seal diets is summarized and three scenarios of type II predator-prey functional response are defined. In an ADAPT analysis of the 4VsW cod stock, seals are treated as an additional fishing gear sector under two scenarios: respectively a "flat-top" and "domed" partial recruitment vectors. Model results infer that seals have contributed to increases in natural mortality since the late 1980s, and have contributed to the lack of recovery of the stock since 1993. However, predictions by the functional models are not consistent with estimate of recent increases in abundance of cod in trawl surveys. Present levels of grey seal abundance have not occurred on the Scotian Shelf since at least the 1800s. (c) 2011 Elsevier B.V. All rights reserved.

The drift and exchange of sea scallop larvae (Placopecten magellanicus) on Georges Bank is investigated by tracking particles in three-dimensional flow fields consisting of the semidiurnal tidal current and autumn mean circulation on realistic topography. Three composite flow fields are considered, each forced by non-linear tidal current interactions, seasonal-mean density gradients and seasonal-mean wind stress. The around-bank flow rates are in approximate agreement with the observed residual gyre, while the cross-isobath currents in the flow fields are consistent with observations only in being generally weak. In most cases it is unclear whether the discrepancies arise from observational uncertainties or from model approximations.In the simulations the particles are given the behavior and planktonic period expected of sea scallop larvae. Particle star-ting positions correspond to the three major scallop aggregations: the Northeast Peak (NEP), the Southern Flank (SF), and the Great South Channel (GSC). Simulations are run to examine the sensitivity of the particle trajectories and settlement numbers to aspects of larval biology (vertical distribution, ascent and descent rates, search times, growth and mortality rates), and to various flow field components. The pattern and extent of larval exchange and settlement are most sensitive to the duration and depth of planktonic drift, gyre strength, weak cross-isobath flow, and mortality rate. The simulations indicate significant larval exchange among the three aggregations, with self-seeding possible for the GSC and NEP, and unlikely for the SF. Given the high retention of particles on Georges Bank as a whole (10-73% before mortality), Georges Bank scallops should be considered self-sustaining.

ABSTRACTResults of a modeling study designed to explore the influences of physical advection and certain biological mechanisms on the distribution of cod (Gadus morhua) and haddock (Melarwgrammus aeglefinus) early life stages on Georges Bank are described. Using a late‐winter/early‐spring 3‐D circulation field driven by the M2 tidal current, mean wind stress and Scotian Shelf inflow, we examine the distribution of cod and haddock larvae spawned on the Northeast Peak of the Bank. The sensitivity to a March‐April baroclinic field is also explored. Results indicate that larvae remaining in the surface Ekman layer are generally advected off‐bank. However, downwelling associated with Ekman layer convergence near the shelf break provides a mechanism for larvae to exit from the off‐bank surface drift. Larvae below the surface layer are transported south‐westward along the southern flank of Georges Bank and are retained on the Bank if their position immediately upstream of the Great South Channel is shoalward of (roughly) the 70 m isobath. Within the Great South Channel region and between the 50 and 70 m isobaths, retention can depend on the phase of the tide. Spawning shoalward of the 50 m isobath on the Northeast Peak greatly increases the chances of retention. These results apply to passive larvae and to those with specified vertical distributions and migration based on observations. Directional on‐bank swimming at rates of 0.5 to 1 body length per second would substantially enhance shoalward displacement, resulting in larval distributions during the first 2 months that are consistent with field observations.

The relation of zooplankton distributions to the physical environment across the frontal system on the northern flank of Georges Bank is described for early summer. The goal of this study was to determine if the frontal system is transparent or a barrier to the transport of zooplankton on and off the Bank. Physical characteristics of the frontal system included a tidal front on the Bank, and processes associated with the Bank edge such as a thermohaline front and a strong residual current jet. Persistent patterns in the cross-bank distributions of zooplankton were related to the locations of these physical features such that separate zooplankton communities occurred on and off the Bank. Zooplankton on the Bank were most abundant in the mixed area and often maximal in the vicinity of the tidal front. The separation into Georges Bank and Gulf of Maine communities was related to the edge of the Bank, although some zooplankton from the latter community did occur on the Bank as far as the tidal front. A particle advection model demonstrates that the strength of the current jet at the Bank edge and slope is sufficient to act as a barrier to cross-bank exchange. However, in the lower layer at the Bank edge the along-bank current is greatly reduced, resulting in increased cross-bank transport for plankton which spend time in this deep layer. This contributes to local retention of plankton already on the Bank and provides a mechanism for transport onto the Bank of plankton in deeper waters at the Bank edge. It may represent an important mechanism for supplying zooplankton prey to the important fish habitats of north-eastern Georges Bank.

Within the distributional range of a species the number of self-sustaining populations can vary, from one population for panmictic species such as American eel to many populations as observed in Atlantic salmon. This "species-level" characteristic is defined here as "population richness." Aspects of geographic patterns in population richness in the northern Atlantic for Atlantic salmon, American shad, rainbow smelt, Atlantic herring, Atlantic cod, haddock, winter flounder, yellowtail flounder, Atlantic mackerel, and European eel are described. It is concluded that events at the early life-history stages, involving "retention" of the eggs and larvae in relation to particular physical oceanographic features, arc involved in the definition of population richness.