Density‐ and size‐dependent mortality in fish early life stages

The importance of survival and growth variations early in life for population dynamics depends on the degrees of compensatory density dependence and size dependence in survival at later life stages. Quantifying density‐ and size‐dependent mortality at different juvenile stages is therefore important to understand and potentially predict the recruitment to the population. We applied a statistical state‐space modelling approach to analyse time series of abundance and mean body size of larval and juvenile fish. The focus was to identify the importance of abundance and body size for growth and survival through successive larval and juvenile age intervals, and to quantify how the dynamics propagate through the early life to influence recruitment. We thus identified both relevant ages and mechanisms (i.e. density dependence and size dependence in survival and growth) linking recruitment variability to early life dynamics. The analysis was conducted on six economically and ecologically important fish populations from cold temperate and sub‐arctic marine ecosystems. Our results underscore the importance of size for survival early in life. The comparative analysis suggests that size‐dependent mortality and density‐dependent growth frequently occur at a transition from pelagic to demersal habitats, which may be linked to competition for suitable habitat. The generality of this hypothesis warrants testing in future research.publishedVersio

Pathways between Primary Production and Fisheries Yields of Large Marine Ecosystems

The shift in marine resource management from a compartmentalized approach of dealing with resources on a species basis to an approach based on management of spatially defined ecosystems requires an accurate accounting of energy flow. The flow of energy from primary production through the food web will ultimately limit upper trophic-level fishery yields. In this work, we examine the relationship between yield and several metrics including net primary production, chlorophyll concentration, particle-export ratio, and the ratio of secondary to primary production. We also evaluate the relationship between yield and two additional rate measures that describe the export of energy from the pelagic food web, particle export flux and mesozooplankton productivity. We found primary production is a poor predictor of global fishery yields for a sample of 52 large marine ecosystems. However, chlorophyll concentration, particle-export ratio, and the ratio of secondary to primary production were positively associated with yields. The latter two measures provide greater mechanistic insight into factors controlling fishery production than chlorophyll concentration alone. Particle export flux and mesozooplankton productivity were also significantly related to yield on a global basis. Collectively, our analyses suggest that factors related to the export of energy from pelagic food webs are critical to defining patterns of fishery yields. Such trophic patterns are associated with temperature and latitude and hence greater yields are associated with colder, high latitude ecosystems

An Evaluation Of The Utility Of Spatially-Explicit Biophysical Models In Recruitment Studies: The Foci Example

No abstracts are to be cited without prior reference to the author.We have developed a set of coupled, spatially explicit biological and physical models designed to examine the recruitment process for walleye pollock in the western Gulf of Alaska. Numerical biophysical modelling can help us to explore mechanisms affecting recruitment and the interactions of these mechanisms. Due to the complexity of the involved processes, 'progress in understanding how selective forces shape recruitment will depend increasingly on modeling, such as the individual-based approaches now being applied to examine how habitat, environment and predation affect recruitment variability' (Pope et al. 1994)

Sixty volumes of the Journal du Conseil / ICES Journal of Marine Science

Article 1 of the ICES Convention of 1964 sets out the fundamental purposes of the Council and includes as the third and final point: "to publish or otherwise disseminate the results of research and investigations carried out under its auspices or to encourage the publication thereof". Publication and dissemination of information about the living resources and their environment have been at the heart of ICES from the first then, but not until the early 1920s was it mooted that a platform was needed solely for the publication of formal scientific papers. The decision to establish the Journal du Conseil was taken at the Council Meeting of 1925, and the first volume appeared between February and November 1926 as four issues, with a total of 383 pages. In 2002, six issues as well as a supplement were published as the 59th volume, with a total of 1732 pages, six issues a year having become the norm in 1995 when ICES Symposium papers became part of the Journal's remit. Accordingly, the current issue signals the completion of the 60th volume in this serie

Fecundity of walleye pollock from the Shelikof Strait, Gulf of Alaska

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Climate change causing phase transitions of walleye pollock (Theragra chalcogramma) recruitment dynamics

In 1976 the North Pacific climate shifted, resulting in an average increase of the water temperature. In the Gulf of Alaska the climate shift was followed (i.e. early 1980s) by a gradual but dramatic increase in the abundance of groundfish species that typically prey on pre-recruitment stages of walleye pollock. In the present study we used a previously parameterized model to investigate the effect of these climate and biological changes on the recruitment dynamics of walleye pollock in the Gulf of Alaska. Simulations covered the 1970–2000 time frame and emphasized the medium-to-long temporal scale (i.e. about 5–10 years) of environmental variability. Results showed that during periods characterized by high sea surface temperature and high predation on juvenile pollock stages, recruitment variability and magnitude were below average, and recruitment control was delayed to stages older than the 0-group. Opposite dynamics (i.e. high abundance and variability, and early recruitment control) occurred during periods characterized by low temperature and predation. These results are in general agreement with empirical observations, and allowed us to formulate causal explanations for their occurrence. We interpreted the delay of recruitment control and the reduction of variability as an effect of increased constraint on the abundance of post age-0 stages, in turn imposed by high density dependence and predation mortality. On the other hand, low density-dependence and predation favoured post age-0 survival, and allowed for an unconstrained link between larval and recruitment abundance. Our findings demonstrate that the dominant mechanisms of pollock survival change over contrasting climate regimes. Such changes may in turn cause a phase transition of recruitment dynamics with profound implications for the management of the entire stock