Quantitative molecular detection of larval Atlantic herring (Clupea harengus) in stomach contents of Atlantic mackerel (Scomber scombrus) marks regions of predation pressure.

Mortality rates in the early life-history stages of fishes are generally high yet identifying the causes remain unclear. Faltering recruitment rates of Atlantic herring (Clupea harengus) in the Norwegian Sea indicate a need to identify which mortality factors influence larval herring survival. Previous research suggests that increased predation pressure by Atlantic mackerel (Scomber scombrus) may contribute to the disconnect between spawning stock biomass and recruitment. To quantify the contribution of predation pressure by Atlantic mackerel to herring larval mortality, two research cruises were conducted within a probable "hot spot" (67-72° N) for intensified mackerel predation based on particle drift simulations. Mackerel stomach contents were analysed for herring larvae content using droplet digital polymerase chain reaction (ddPCR) with a quantitative molecular detection assay specific for herring. The ddPCR results demonstrate clear predation by mackerel on herring larvae and also suggest that the alternative use of visual examination may give misleading results. Our results show that mackerel should be considered a potentially important predator on herring larvae. The quantitative molecular assay presented here shows great promise as an efficient and specific tool to correctly identify and quantify predation pressure on early life-history stages of fishes.

Characteristics of the Norwegian Coastal Current during Years with High Recruitment of Norwegian Spring Spawning Herring (Clupea harengus L.).

Norwegian Spring Spawning herring (NSSH) Clupea harengus L. spawn on coastal banks along the west coast of Norway. The larvae are generally transported northward in the Norwegian Coastal Current (NCC) with many individuals utilizing nursery grounds in the Barents Sea. The recruitment to this stock is highly variable with a few years having exceptionally good recruitment. The principal causes of recruitment variability of this herring population have been elusive. Here we undertake an event analysis using data between 1948 and 2010 to gain insight into the physical conditions in the NCC that coincide with years of high recruitment. In contrast to a typical year when northerly upwelling winds are prominent during spring, the years with high recruitment coincide with predominantly southwesterly winds and weak upwelling in spring and summer, which lead to an enhanced northward coastal current during the larval drift period. Also in most peak recruitment years, low-salinity anomalies are observed to propagate northward during the spring and summer. It is suggested that consistent southwesterly (downwelling) winds and propagating low-salinity anomalies, both leading to an enhanced northward transport of larvae, are important factors for elevated recruitment. At the same time, these conditions stabilize the coastal waters, possibly leading to enhanced production and improved feeding potential along the drift route to Barents Sea. Further studies on the drivers of early life history mortality can now be undertaken with a better understanding of the physical conditions that prevail during years when elevated recruitment occurs in this herring stock.

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 1(st) 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.