ALBERT

Description: Here we present a new, pan-North-Atlantic compilation of data on key mesozooplankton species, including the most important copepod, Calanus finmarchicus. Distributional data of eight representative zooplankton taxa, from recent (2000-2009) Continuous Plankton Recorder data, are presented, along with basin-scale data of the phytoplankton colour index. Then we present a compilation of data on C. finmarchicus, including observations of abundance, demography, egg production and female size, with accompanying data on temperature and chlorophyll.

Description: Here we present a new, pan-Atlantic compilation and analysis of data on Calanus finmarchicus abundance, demography, dormancy, egg production and mortality in relation to basin-scale patterns of temperature, phytoplankton biomass, circulation and other environmental characteristics in the context of understanding factors determining the distribution and abundance of C. finmarchicus across its North Atlantic habitat. A number of themes emerge: (1) the south-to-north transport of plankton in the northeast Atlantic contrasts with north-to-south transport in the western North Atlantic, which has implications for understanding population responses of C. finmarchicus to climate forcing, (2) recruitment to the youngest copepodite stages occurs during or just after the phytoplankton bloom in the east whereas it occurs after the bloom at many western sites, with up to 3.5 months difference in recruitment timing, (3) the deep basin and gyre of the southern Norwegian Sea is the centre of production and overwintering of C. finmarchicus, upon which the surrounding waters depend, whereas, in the Labrador/Irminger Seas production mainly occurs along the margins, such that the deep basins serve as collection areas and refugia for the overwintering populations, rather than as centres of production, (4) the western North Atlantic marginal seas have an important role in sustaining high C. finmarchicus abundance on the nearby coastal shelves, (5) differences in mean temperature and chlorophyll concentration between the western and eastern North Atlantic are reflected in regional differences in female body size and egg production, (6) regional differences in functional responses of egg production rate may reflect genetic differences between western and eastern populations, (7) dormancy duration is generally shorter in the deep waters adjacent to the lower latitude western North Atlantic shelves than in the east, (8) there are differences in stage-specific daily mortality rates between eastern and western shelves and basins, but the survival trajectories for cohort development from CI to CV are similar, and (9) early life stage survival is much lower in regions where C. finmarchicus is found with its congeners, C. glacialis and/or C. hyperboreus. This compilation and analysis provides new knowledge for evaluation and parameterisation of population models of C. finmarchicus and their responses to climate change in the North Atlantic. The strengths and weaknesses of modeling approaches, including a statistical approach based on ecological niche theory and a dynamical approach based on knowledge of spatial population dynamics and life history, are discussed, as well as needs for further research.

Description: Fourteen years of data collected by the Atlantic Zone Monitoring Program served to model the habitat of four dominant copepod species ( Calanus glacialis , Calanus hyperboreus , Calanus finmarchicus , Paracalanus sp.) on the continental shelf and slope waters in the Northwest Atlantic Ocean. Generalized additive mixed models (GAMMs) were applied to abundance and presence–absence data for C. hyperboreus , C. glacialis and Paracalanus sp. and abundance for C. finmarchicus , to describe the optimal environmental productivity envelopes associated with the occurrence and/or the net productivity of these species. The models for Calanus species considered two main phases of their life cycle: (i) an active population growth phase dominated by early stages that occur primarily in surface layers, and (ii) a dormant phase dominated by overwintering stages generally found in deeper layers. GAMMs identified a marked contrast in environmental envelopes occupied by arctic and temperate species. Our analyses underline the importance of using data representative of all the copepodid developmental stages and occupied habitats in order to accurately model the distribution of Calanus species. The value of our models as tools to understand past events in the Northwest Atlantic or to predict future distributions of the species is also discussed.

Description: Variations in larval fish growth rates are largely the result of variability in biotic and abiotic characteristics of the feeding environment experienced by each individual. An assessment of an individual's overall feeding success (i.e. accumulation of utilizable organic matter) can best be achieved at the time of capture when the relationships among environment, short-term feeding success as defined by gut content and long-term feeding success as defined by accumulated growth can be contrasted. Here, we investigated the relationships between average growth, feeding success, and variability in individual growth and feeding rates across a range of taxa based on a synthesis of studies in which stomach content and otolith growth were measured in the same individuals. Instantaneous measures of feeding success were highly variable and demonstrated a positive yet somewhat limited association with growth rates across all taxa. The strength of the feeding-growth relationships among taxa, and cohorts within taxa, was reflected in the autocorrelation of individual growth rates, suggesting that stable growth was achieved through consistent feeding success. However, when viewed at the individual level, faster growth was achieved in individuals with more variable growth rates, and by inference more variable past feeding success. The dichotomy in these underlying relationships may point to the importance of stochastic events in the development of exceptional individuals in a population, and may be linked to how surplus energy is allocated to individual growth rates. The positive correlation found between feeding success and growth in all taxa is consistent with the growth-survival paradigm for the larval stage of fish. However, both the correlation between feeding success and growth and the serial correlation of growth time-series was greatest in fast-growing species, suggesting that the potential for an early "critical period" regulating survival varies among species, reaching a maximum in fast-growing fish.

Description: This essay contrasts the inferences about the patterns of size-dependent mortality in larval fish based on the traditional catch-curve approach with that achieved through the vertical life table method in an application to data from coastal Newfoundland. Although both approaches reveal that the average mortality rates decline with increasing body size, the rate of decline estimated using the vertical life table approach is much less pronounced than estimated from the catch-curve method. More important, however, is that on a case-by-case assessment the vertical life table reveals that mortality increases with increasing body size in 70% of the cases and declines in the remainder. Instances with greater rates of loss in larger individuals are consistent with larvae becoming more susceptible to the dominant planktivore in the study region. The contrasting results indicate that the patterns of change in mortality rates need to be measured over relatively short-time and/or length intervals. Such inferences have important implications for the development of studies dealing with larval fish dynamics. To be effective and applicable, comparative analyses that aim to develop macroscopic principles for the early life stages of fish must take the local food web structure into consideration to gain appropriate understanding of the trophic interactions that most strongly affect losses from larval fish populations.