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Changes in structural connectivity as it can affect functional connectivity, the biological and behavioural responses of an organism, has been examined here over 2 contrasting years when the spatial distribution of larval and juvenile cod and their prey shifted from the flank to the crest on Georges Bank. New data on the gut contents of pelagic juvenile cod are compared with known prey distributions, potential growth and how climate warming can change connectivity in this region. Centropages spp. was the most important prey for pelagic juveniles, especially in June 1987 when they had high abundance on the crest and were dominant in the cod stomachs. In June 1986, copepod abundance was low where the juveniles were distributed along the flank. The potential growth of juvenile cod was greater in June 1987, consistent with the higher abundance of Centropages spp., and with higher recruitment survival, compared with June 1986. Annual changes in the spatial distribution of cod early life stages within the environment of cold or warm years can have different impacts on their growth and survival. Whereas the small copepods, Pseudocalanus spp., are primary prey for cod larvae and very abundant in cold years, larger copepods, Calanus finmarchicus and Centropages spp., are important prey for the pelagic juveniles and the latter species can have a high impact in warm years on the crest. The different spatial structure during cold or warm years provides an explanation why different year classes respond differently to environmental change. Depending on the presence or absence of specific prey, the functional connectivity response changes pathways that determine the growth and survival of early life stages and ultimately a role in recruitment.

Hollardia hollardi (Reticulate Spikefish) is reported from two specimens caught in lobster traps along the southern edge of Georges Bank, far north of its previously known northern distribution from southern Florida, the Bahamas, and Bermuda.

Cruises were conducted in spring 1999 to describe the interaction between tidal-front processes and the transport, retention, and growth of cod larvae and their prey during the seasonal transition to a stratified water-column along the southern flank of Georges Bank. All the physical and biological observations were integrated in coupled circulation-trophodynamic simulations. The three-dimensional circulation fields were modeled using data assimilation methods described in Aretxabaleta et al. [2005. Data assimilative hindcast on the Southern Flank of Georges Bank during May 1999: frontal circulation and implications. Continental Shelf Research 25, 849–874]. The individual-based model (IBM) of Lough et al. [2005. A general biophysical model of larval cod growth applied to populations on Georges Bank. Fisheries Oceanography 14, 241–262] was used to consider trophodynamic effects on the growth and survival of larval cod. Prey fields were specified for mixed and stratified water columns from field surveys and allowed to adjust in the circulation model. Encounter and ingestion rates of larvae were functions of prey concentration, larval search patterns, light, swimming speeds of predator and prey, and turbulence. Model outputs provide hourly depth-dependent estimates of growth rates, prey biomass ingested, and larval length and weight. Simulations were conducted along a 2-D transect across the tidal front, from mixed to stratified water columns, before and after a wind event. Pre-storm, observed larval cod growth rates, based on RNA–DNA analysis, were highest in the surface 20 m at the stratified and front stations. Post-storm, larval growth rates decreased 1–2% d −1 at the stratified and front stations, corresponding with a

In late May 1992, cod and haddock eggs and larvae were sampled at discrete depths using MOCNESS on the southern flank of Georges Bank when seasonal stratification of the water column was developing. A deeper, stratified site (80-m bottom) was sampled over 7 days in relation to the temporal evolution of vertical structure and compared to a nearby shoal, mixed site (50 m). Sampling also was conducted to the west along two transects in the Great South Channel. Larval biochemical composition was measured and nutritional condition inferred from RNA:DNA ratios. During the period 22–24 May 1992, surface warming resulted in a temperature gradient from 6 to 10°C (0.6 δt units) in the upper 20 m at the stratified site. Gadid eggs (93% haddock) were most abundant (6–14 per 100 m3, on average) in the surface 20 m, with maximum density just above the base of the thermocline at 20-10 m. The few recently-hatched larvae (5–6 mm) caught were broadly distributed in the water column, with maximum average densities (1–3 per 100 m3) deeper at 10–40 m. The naupliar and copepodite stages of Pseudocalanus spp. were principal prey of these larvae, and also were more abundant (3–7 prey 1−1) in the surface 20 m, with a maximum density at 20-10 m. A moderate storm on 25 May mixed the upper part of the water column to 5–6°C. The day after the storm, gadid eggs and copepods had highest densities in the surface 10 m. At the shoal (mixed site) egg densities were low (1–3 per 100 m3) and distributed evenly through the water column. Cod and haddock larvae were larger (7–8 mm modal length) and more abundant than at the stratified site. At night, they were evenly distributed at 4–6 per 100 m3 on average, but by day, maximum density increased to 6–16 per 100 m3 in the deepest strata sampled (40–30 m). The larger larvae at this site preyed predominantly on copepodite stages of Pseudocalanus spp., which were uniformly distributed in the water column. Of all the larvae sampled for biochemical analysis, 10% of haddock and 1.5% of cod had ratios below 4.1. Based on laboratory studies, fish larvae with RNA:DNA ratios below 4.1 are considered to have been in poor condition. There were no larvae in poor condition at the Great South Channel Transect, where their mean size was greatest; no larvae were collected at the stratified site for comparison. At the mixed site, cod larvae sampled before and after the storm had identical ratios, while haddock larvae had significantly higher ratios after the storm. Larvae from different sampling depths had significantly different ratios, inferring that they remained together long enough to acquire a unique signature.

Understanding larval fish survival dynamics is essential to determining variability in future adult population structure. Realistic modeling of larval fish feeding ecology depends on incor- porating both the biotic and abiotic conditions that affect predator-prey interactions. We used an individual-based model (IBM) to test which variables drive Atlantic larval cod Gadus morhua feeding preferences. The IBM included a bioenergetics component that incorporated metabolic parameters and growth and a mechanistic prey selection component that depended on larval development and behavior, prey size and behavior, depth, light, and physical oceanographic conditions. We applied our model to Georges Bank and incorporated high-resolution field data on environmental conditions and prey abundance to analyze larval cod feeding ecology. Based on simulated selectivity indices, we found that cod prey selection was determined by differential encounter of prey due to the abundance of suitably sized prey, their visibility, and larval cod ability to capture these prey items. The model suggested that Pseudocalanus spp. were the dominant prey species for larval cod because of their abundance in the water column and their large image area. Centropages spp. were also modeled to be an important part of larval diet, but no copepodite stages of this taxon were found in gut samples. Lack of Centropages spp. in the gut samples indicated that they are more elusive in their behavior than Pseudocalanus spp. Overall our results suggest larval cod feeding ecology on Georges Bank is a consequence of the physical and biological conditions rather than active prey selection.