Observations on the density of pelagic marine fish eggs have indicated that they are near neutral buoyancy for most of development and have a tendency to become denser towards hatching (see Russell, 1976). However, most of this information has been based on limited laboratory observations on the relative buoyancy of eggs; only a few experiments have given more precise estimates of the absolute density of eggs based on the salinity of water in which they are neutrally buoyant (e.g. Franz, 1910; Sundnes, Leivestad & Iversen, 1965). Recently, a new technique (Coombs, 1981), based on the use of a stable column of continuously graded sea-water concentration has allowed more precise and routine measurements of the density of fish eggs throughout development.

Against the backdrop of warming of the Northern Hemisphere it has recently been acknowledged that North Atlantic temperature changes undergo considerable variability over multidecadal periods. The leading component of natural low-frequency temperature variability has been termed the Atlantic Multidecadal Oscillation (AMO). Presently, correlative studies on the biological impact of the AMO on marine ecosystems over the duration of a whole AMO cycle (∼60 years) is largely unknown due to the rarity of continuously sustained biological observations at the same time period. To test whether there is multidecadal cyclic behaviour in biological time-series in the North Atlantic we used one of the world's longest continuously sustained marine biological time-series in oceanic waters, long-term fisheries data and historical records over the last century and beyond. Our findings suggest that the AMO is far from a trivial presence against the backdrop of continued temperature warming in the North Atlantic and accounts for the second most important macro-trend in North Atlantic plankton records; responsible for habitat switching (abrupt ecosystem/regime shifts) over multidecadal scales and influences the fortunes of various fisheries over many centuries.

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Understanding and modelling the vertical distribution of fish eggs in the water column is a major challenge for future use of underway‐continuous egg samplers as estimators of the total egg abundance. This study presents modelling of field data of anchovy (Engraulis encrasicolus) and sardine (Sardina pilchardus) egg vertical distribution obtained from LHPR sampling in the Bay of Biscay. Starting from Sundby's model [Deep‐Sea Res.30 (1983) 645], improvements were achieved through successive modifications concerning egg buoyancy and vertical propagation of wind‐induced turbulence. In addition, measurements of egg settling velocity and buoyancy by stages were included as inputs for the model. The best model fitting was achieved through the adoption of a gradual turbulence vertical decay model (proportional to the inverse of the water density profile), a Gaussian variability of egg density and adaptability of the egg density to the surrounding water by means of permeability of the chorion. This led to improvements over the Sundby original model. The coefficient of determination (R2) of the modelled egg abundance profiles compared with the observed ones was around 80% for both sardine and anchovy. The model described successfully the vertical distribution of eggs for waters of high surface salinity (R2 of almost 90%), but less so for waters of low surface salinity (R2 of about 70%).

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