Svein Sundby

Observation-based evaluation of surface wave effects on currents and trajectory forecasts

Knowledge of upper ocean currents is needed for trajectory forecasts and is essential for search and rescue operations and oil spill mitigation. This paper addresses effects of surface waves on ocean currents and drifter trajectories using in situ observations. The data set includes colocated measurements of directional wave spectra from a wave rider buoy, ocean currents measured by acoustic Doppler current profilers (ADCPs), as well as data from two types of tracking buoys that sample the currents at two different depths. The ADCP measures the Eulerian current at one point, as modelled by an ocean general circulation model, while the tracking buoys are advected by the Lagrangian current that includes the wave-induced Stokes drift. Based on our observations, we assess the importance of two different wave effects: (a) forcing of the ocean current by wave-induced surface fluxes and the Coriolis–Stokes force, and (b) advection of surface drifters by wave motion, that is the Stokes drift. Recent theoretical developments provide a framework for including these wave effects in ocean model systems. The order of magnitude of the Stokes drift is the same as the Eulerian current judging from the available data. The wave-induced momentum and turbulent kinetic energy fluxes are estimated and shown to be significant. Similarly, the wave-induced Coriolis–Stokes force is significant over time scales related to the inertial period. Surface drifter trajectories were analysed and could be reproduced using the observations of currents, waves and wind. Waves were found to have a significant contribution to the trajectories, and we conclude that adding wave effects in ocean model systems is likely to increase predictability of surface drifter trajectories. The relative importance of the Stokes drift was twice as large as the direct wind drag for the used surface drifter.

Retention of Coastal Cod Eggs in a Fjord Caused by Interactions between Egg Buoyancy and Circulation Pattern

Abstract Norwegian coastal cod form a stationary population of Atlantic cod Gadus morhua consisting of several genetically separated subpopulations. A small-scale differentiation in marine populations with pelagic eggs and larvae is made possible by local retention of early life stages in coastal environments. A numerical model was used to simulate the circulation in a fjord system in northern Norway over 2 years with different river runoff patterns. The dispersal of cod eggs was calculated with a particle-tracking model that used three-dimensional currents. The observed thickness of the low-salinity surface layer was well reproduced by the model, but the surface salinity was generally lower in the model than in the observations. The cod eggs attained a subsurface vertical distribution, avoiding the surface and causing retention. Interannual variations in river runoff can cause small changes in the vertical distribution of cod eggs and larger changes in the vertical current structure. Retention in the fjord system was strong in both years, but some eggs were subjected to offshore transport over a limited time period. The timing of offshore transport depended on the precipitation and temperatures in adjacent drainage areas. A possible match between maximized spawning and offshore transport may have a negative effect on local recruitment.

Real-Time Ichthyoplankton Drift in Northeast Arctic Cod and Norwegian Spring-Spawning Herring

Background Individual-based biophysical larval models, initialized and parameterized by observations, enable numerical investigations of various factors regulating survival of young fish until they recruit into the adult population. Exponentially decreasing numbers in Northeast Arctic cod and Norwegian Spring Spawning herring early changes emphasizes the importance of early life history, when ichthyoplankton exhibit pelagic free drift. However, while most studies are concerned with past recruitment variability it is also important to establish real-time predictions of ichthyoplankton distributions due to the increasing human activity in fish habitats and the need for distribution predictions that could potentially improve field coverage of ichthyoplankton. Methodology/Principal Findings A system has been developed for operational simulation of ichthyoplankton distributions. We have coupled a two-day ocean forecasts from the Norwegian Meteorological Institute with an individual-based ichthyoplankton model for Northeast Arctic cod and Norwegian Spring Spawning herring producing daily updated maps of ichthyoplankton distributions. Recent years observed spawning distribution and intensity have been used as input to the model system. The system has been running in an operational mode since 2008. Surveys are expensive and distributions of early stages are therefore only covered once or twice a year. Comparison between model and observations are therefore limited in time. However, the observed and simulated distributions of juvenile fish tend to agree well during early fall. Area-overlap between modeled and observed juveniles September 1st range from 61 to 73%, and 61 to 71% when weighted by concentrations. Conclusions/Significance The model system may be used to evaluate the design of ongoing surveys, to quantify the overlap with harmful substances in the ocean after accidental spills, as well as management planning of particular risky operations at sea. The modeled distributions are already utilized during research surveys to estimate coverage success of sampled biota and immediately after spills from ships at sea.

Egg buoyancy variability in local populations of Atlantic cod (Gadus morhua).

Previous studies have found strong evidences for Atlantic cod (Gadus morhua) egg retention in fjords, which are caused by the combination of vertical salinity structure, estuarine circulation, and egg specific gravity, supporting small-scaled geographical differentiations of local populations. Here, we assess the variability in egg specific gravity for selected local populations of this species, that is, two fjord-spawning populations and one coastal-spawning population from Northern Norway (66–71°N/10–25°E). Eggs were naturally spawned by raised broodstocks (March to April 2009), and egg specific gravity was measured by a density-gradient column. The phenotype of egg specific gravity was similar among the three local populations. However, the associated variability was greater at the individual level than at the population level. The noted gradual decrease in specific gravity from gastrulation to hatching with an increase just before hatching could be a generic pattern in pelagic marine fish eggs. This study provides needed input to adequately understand and model fish egg dispersal.

The Principles of Buoyancy in Marine Fish Eggs and Their Vertical Distributions across the World Oceans

Buoyancy acting on plankton, i.e. the difference in specific gravity between plankton and the ambient water, is a function of salinity and temperature. From specific gravity measurements of marine fish eggs salinity appears to be the only determinant of the buoyancy indicating that the thermal expansions of the fish egg and the ambient seawater are equal. We analyze the mechanisms behind thermal expansion in fish eggs in order to determine to what extent it can be justified to neglect the effects of temperature on buoyancy. Our results confirm the earlier assumptions that salinity is the basic determinant on buoyancy in marine fish eggs that, in turn, influence the vertical distributions and, consequently, the dispersal of fish eggs from the spawning areas. Fish populations have adapted accordingly by producing egg specific gravities that tune the egg buoyancy to create specific vertical distributions for each local population. A wide variety of buoyancy adaptations are found among fish populations. The ambient physical conditions at the spawning sites form a basic constraint for adaptation. In coastal regions where salinity increases with depth, and where the major fraction of the fish stocks spawns, pelagic and mesopelagic egg distributions dominate. However, in the larger part of worlds’ oceans salinity decreases with depth resulting in different egg distributions. Here, the principles of vertical distributions of fish eggs in the world oceans are presented in an overarching framework presenting the basic differences between regions, mainly coastal, where salinity increases with depth and the major part of the world oceans where salinity decreases with depth. We show that under these latter conditions, steady-state vertical distribution of mesopelagic fish eggs cannot exist as it does in most coastal regions. In fact, a critical spawning depth must exist where spawning below this depth threshold results in eggs sinking out of the water column and become lost for recruitment to the population. An example of adaptation to such conditions is Cape hake spawning above the critical layer in the Northern Benguela upwelling ecosystem. The eggs rise slowly in the onshore subsurface current below the Ekman layer, hence being advected inshore where the hatched larvae concentrate with optimal feeding conditions.

The North Atlantic Spring-Bloom System—Where the Changing Climate Meets the Winter Dark

The North Atlantic with its spring-bloom ecosystem has its particular responses to climate change, many of them different from the other parts of the world’s oceans. The system is strongly influenced by anthropogenic climate change as well as to strong decadal to multidecadal natural climate variability. In particular, the northernmost part of the system and the Arctic is exposed to higher increase in temperature than any other ocean region. The most pronounced examples of poleward migration of marine species are found in the North Atlantic, and comprise the recent warming phase after the 1970s. The latitudinal asymmetric position of the Arctic Front and its nature of change result in a considerably larger migration distance and migration speed of species in the Northeast Atlantic part of the system. However, we here hypothesize that there is a limit to the future extent of poleward migration of species constrained by the latitudinal region adjacent the Polar Circle. We define this region the critical latitudes. This is because the seasonal light cycle at high latitudes sets particular demands on the life cycle of planktivore species. Presently, boreal planktivore species at high latitudes deposit lipids during the short spring bloom period and overwinter when phytoplankton production is insufficient for feeding. Unless invading temperate species from farther south are able to adapt by developing a similar life cycle future poleward migration of such species will be unlikely.

Experimental parameterisation of principal physics in buoyancy variations of marine teleost eggs

It is generally accepted that the high buoyancy of pelagic marine eggs is due to substantial influx of water across the cell membrane just before ovulation. Here we further develop the theoretical basis by applying laboratory observations of the various components of the fertilized egg in first-principle equations for egg specific gravity (ρegg) followed by statistical validation. We selected Atlantic cod as a model animal due to the affluent amount of literature on this species, but also undertook additional dedicated experimental works. We found that specific gravity of yolk plus embryo is central in influencing ρegg and thereby the buoyancy. However, our established framework documents the effect on ρegg of the initial deposition of the heavy chorion material in the gonad prior to spawning. Thereafter, we describe the temporal changes in ρegg during incubation: Generally, the eggs showed a slight rise in ρegg from fertilization to mid-gastrulation followed by a gradual decrease until full development of main embryonic organs just before hatching. Ontogenetic changes in ρegg were significantly associated with volume and mass changes of yolk plus embryo. The initial ρegg at fertilization appeared significantly influenced by the chorion volume fraction which is determined by the combination of the final chorion volume of the oocyte and of the degree of swelling (hydrolyzation) prior to spawning. The outlined principles and algorithms are universal in nature and should therefore be applicable to fish eggs in general.

The Northeast Greenland Shelf as a Potential Habitat for the Northeast Arctic Cod

Observations (1978-1991) of distributions of pelagic juvenile Northeast Arctic cod (Gadus morhua L.) show that up to 1/3 of the year class are dispersed off the continental shelf and into the deep Norwegian Sea while on the way from the spring-spawning areas along the Norwegian coast to the autumn-settlement areas in the Barents Sea. The fate of this variable fraction of pelagic juveniles off-shelf has been an open question ever since Johan Hjort’s (1914) seminal work. We have examined both the mechanisms causing offspring off-shelf transport, and their subsequent destiny using an individual-based biophysical model applied to quantify growth and dispersal. Our results show, consistently with the observations, that total off-shelf transport is highly variable between years and may be up to 27.4 %. Offspring from spawning grounds around Lofoten have a higher chance of being displaced off the shelf. The off-shelf transport is dominated by episodic events where frequencies and dates vary between years. Northeasterly wind conditions over a 3-7-day period prior to the off-shelf events are a good proxy for dispersal of offspring off the shelf. Offspring transported into the open ocean are on average carried along three following routes: back onto the adjacent eastern shelves and into the Barents Sea (36.9 %), recirculating within the Lofoten Basin (60.7 %), or drifting northwest to the northeast Greenland shelf (2.4 %). For the latter fraction the transport may exceed 12 % depending on year. Recent investigations have discovered distributions of young cod on the northeast Greenland shelf indicating that conditions may support survival for NEA cod offspring.

Oogenesis and reproductive investment of Atlantic herring are functions of not only present but long-ago environmental influences as well

Significance Fish stock advice and management have a focus on protecting the reproductive capacity of a stock. Central to the associated research undertaken is whether spawning stock biomass is a proxy for total egg production. Here we demonstrate for the planktivorous Atlantic herring that the standard use of season-independent potential fecundity formulas may cause flaws in these interpretations. In particular, this would happen when the adults have experienced poor feeding conditions that negatively affect the reservoir of primary oocytes and thereby the future fecundity. Hence, females in excellent condition may show significantly lower-than-expected fecundity that could lead to errors in the judgment of reproductive capacity, especially under “red flag” situations. Following general life history theory, immediate reproductive investment (egg mass × fecundity/body mass) in oviparous teleosts is a consequence of both present and past environmental influences. This clarification questions the frequent use of season-independent (general) fecundity formulas in marine fish recruitment studies based on body metrics only. Here we test the underlying assumption of no lag effect on gametogenesis in the planktivorous, determinate-fecundity Atlantic herring (Clupea harengus) displaying large plasticity in egg mass and fecundity, examining Norwegian summer–autumn spawning herring (NASH), North Sea autumn-spawning herring (NSAH), and Norwegian spring-spawning herring (NSSH). No prior reproductive information existed for NASH. Compared with the 1960s, recent reproductive investment had dropped markedly, especially for NSAH, likely reflecting long-term changes in zooplankton biography and productivity. As egg mass was characteristically small for autumn spawners, although large for spring spawners (cf. different larval feeding conditions), fecundity was the most dynamic factor within reproductive investment. For the data-rich NSSH, we showed evidence that transient, major declines in zooplankton abundance resulted in low fecundity over several subsequent seasons, even if Fulton’s condition factor (K) turned high. Temporal trends in Kslope (K on total length) were, however, informative. These results clarify that fecundity is defined by (i) dynamics of primary (standing stock) oocytes and (ii) down-regulation of secondary oocytes, both processes intimately linked to environmental conditions but operating at different timescales. Thus, general fecundity formulas typically understate interannual variability in actual fecundity. We therefore argue for the use of segmented fecundity formulas linked to dedicated monitoring programs.

Per Solemdal (1941–2016) – an unconventional marine scientist

Per Solemdal (Figure 1) passed away on 4 April 2016 at the age of 75. He leaves behind a daughter and two sons, and two grandsons. Per worked as a scientist at the Institute of Marine Research (IMR) in Bergen, Norway for his entire career, i.e. from the time he was appointed in 1967 to retiring in 2011, altogether for more than 43 years. Over the course of his career, he influenced two generations of fishery and marine biologists in Norway, both as a supervisor and as a colleague. Because of his deep appreciation of the history of marine science he would remind his students to always look at the original source because, as he used to state with great authority, but with a smile: ‘If you think you have a new idea, it is because you have not read the literature well enough’. He loved good, open academic discussions, and often approached scientific problems with a different and unexpected point of view. His extrovert personality often placed him in the centre of gatherings, both scientific and social. Per was not a mainstream marine scientist. Despite writing many influential scientific papers, he was not really concerned with publishing on ‘silk paper’, as he put it. He had a broad interest in marine science, but focused on fish reproductive ecology and links to recruitment dynamics. His research interests followed an almost circular path; his studies into ‘maternal effects’ early in his career (1967–74), were picked up again from 1990 to 2011, when he came at it with much more knowledge and, not least, new ideas to present. Between these two periods, he made some notable contributions on early life stages of fish. Below we recollect these periods in his working life, and also include some of the other issues that captured this enthusiastic researcher’s interest.