Merluccius capensis spawn in Namibian waters, but do M. paradoxus?

Spawning time and areas, and the length of the spawning season of shallow-water (Merluccius capensis) and deepwater (M. paradoxus) hake, were investigated from bottom trawl collections taken in Namibian waters between September 1998 and October 2000 and from August to November 2001. The gonadosomatic index (GSI) and the incidence of advanced maturity stages of M. paradoxus were low along the entire coast throughout the year, but with a few exceptions in the south (between Lüderitz and Orange River). By contrast, M. capensis spawn in the area throughout the year, but mainly between July and October. Evaluating the accuracy of visual maturity staging by comparing results with those of image and histological analyses revealed few errors in classifications of maturity stage in the field, with the exception of Stage 5 (spent and resting) in M. paradoxus. Specific GSI values were intercalibrated with the appearance of developing oocytes. As the GSI is quickly estimated, this new concept, designated here as a ‘maturity reference line', could enhance understanding of the spawning biology of other species with a similarly complex, indeterminate spawning strategy. It is concluded that M. paradoxus do not appear to spawn in Namibian waters. Keywords: GSI, hake, image analysis, maturity staging, Namibia, spawning timeAfrican Journal of Marine Science 2007, 29(3): 379–39

Changes in the fish fauna of the Oosterschelde estuary: a ten-year time series of fyke catches

Frequency of occurrence of fish species was monitored on a fortnightly basis in four fykes and a weir in the Oosterschelde estuary from 1979 through 1988. This was done in order to record changes in the fish fauna that may have occurred as a response to the construction of a storm-surge barrier in the mouth of the Oosterschelde (1984-1986) and the concomitant building of compartmentalization dams in the landward part. These compartmentalization dams reduced the freshwater inflow into the system. Principal component analysis using the annual averages in frequency of occurrence suggests a slight shift occurred in the fish community separating a cluster of years 1979-1984 from the cluster 1985-1988. Many of the changes in individual species could be attributed to fluctuations in yearclass strength or were part of changes occurring on a wider geographical scale. The only impact of the construction works seems to be the decrease in a number of anadromous fish. Fish traps seem to be useful as a monitoring tool for a number of species. The value of the data collected could be improved if catch size and length-frequency data are recorded