The role of oceanographic conditions and plankton availability in larval fish assemblages off the Catalan coast (NW Mediterranean)

In the northwestern Mediterranean, most fish species reproduce in early summer and fewer in the autumn mixing period. This study analyses and compares larval fish assemblages (LFA) in both seasons, and is the first attempt to characterize LFA structure for the autumn period. We analyze horizontal and vertical distribution of fish larvae and the micro- and mesozooplankton biomass and abundance of the main zooplankton groups. The oceanographic situation was analyzed through the study of data from CTD, Nν-Shuttle and ADCP surveys. LFA were determined by ordination analyses based on larval abundance, and the relationships between larval assemblages and environmental variables were investigated through canonical correspondence analysis. The importance of some hydrographic variables (temperature, salinity and stability of the water column), current fields (along-shelf and across-shelf transport) and the abundance of zooplankton are discussed as important factors shaping the structure of larval assemblages. In early summer, LFA were mainly structured by a combination of bathymetry and trophic components, although sea surface temperature also played a role in shaping the horizontal larval distributions. In autumn, trophic variables were the main factors influencing the shelf-dwelling species assemblage. Larvae of oceanic species, on the other hand, were not related to trophic variables but were more affected by current fields.     

DENSIMETRIC FRACTIONATION OF SOIL ORGANO-MINERAL COMPLEXES

A rapid, simple, and precise density fractionation of soil mineral-organic complexes, suitable for routine work, is described. Special features of the method are: low sample weight, use of glass-fibre paper to collect the lighter fractions, and use of a modified Walkley-Black procedure to determine the carbon content of the light fraction in test tubes. Density-distribution curves of organo-mineral soil complexes are shown to yield useful basic information on degradation processes of soil organic matter caused by cultivation. The results suggest that complexes between clay and organic matter tend to build up preferentially with a certain proportion between the components.     

Relationships between anchovy (Engraulis encrasicolus) recruitment and environment in the Bay of Biscay (1967–1996)

Various hypotheses have been put forward to explain the mechanisms leading to recruitment variability in fish populations. These have been based on either physical (wind stress, upwelling) or biological (food and predation) processes. In the Bay of Biscay, the hypothesis of a physical influence on anchovy recruitment has been confirmed. Oceanographic conditions in the Bay of Biscay in the spring and summer, influenced by north-easterly winds of medium and low intensity, seem to induce good levels of recruitment to the anchovy ( Engraulis encrasicolus ) population. An index of upwelling was significantly correlated with annual recruitment of anchovy ( P  < 0.0001) for the period 1967–1996. This physical parameter explains about 59% of the variability in the recruitment of the Bay of Biscay anchovy. Two factors affecting productivity in the Bay of Biscay may be linked to north-easterly winds, namely weak upwelling and an extension of the area influenced by river outflows. Both of these factors, together with low turbulence and stability, may act to enhance survival of the early life-history stages of anchovy by increasing food availability. The potential use of this upwelling index to forecast the recruitment of the Bay of Biscay anchovy offers possibilities for improving the management of this population.     

From egg to juvenile in the Bay of Biscay: spatial patterns of anchovy (Engraulis encrasicolus) recruitment in a non‐upwelling region

In this study the spatial distribution of eggs, larvae and juveniles of European anchovy (Engraulis encrasicolus) was followed in 2004 and 2005 during three consecutive cruises each year in spring–summer to test what the limits of retention are in a non‐upwelling area. Eggs, small larvae and large juveniles were mainly distributed over the shelf, whereas large larvae and small juveniles were found mainly off the shelf. Although overall distributions were similar, the 2 yr differed in that there was more of a coastal distribution of individuals in 2004, whereas in 2005 more individuals were found off the shelf. There were no significant differences in the length–weight relationships for individuals found on and off the shelf or between years. The correspondence in circulation patterns and the lack of difference in the length–weight relationships suggest that a single population is present, larvae drifting off the shelf due to currents and returning as mobile juveniles. Quantile regression analysis of the long‐term recruitment index suggests that transport off the shelf may favour good recruitments. This would suggest that in non‐upwelling regions the retention area resulting in good recruitment may not be restricted to the shelf.