The patterns of decapod larvae occurrence and abundance were studied from weekly time-series data of 8 years from Stonehaven (north-east Scotland) and 4 years and 8 months from Loch Ewe (north-west Scotland). The annual cycle observed was similar in the two locations and characterized by abundance peaks, the first in spring and another in the summer, extending into autumn. During the coldest months (December to February) decapod larvae were virtually absent in the plankton. Differences in abundance and occurrence of decapod larvae between locations and the influence of temperature, salinity and chlorophyll-a in the patterns observed, were analysed by generalized least-square functions. The results showed significant differences in the abundance of decapod larvae between locations, with higher larval abundances and an earlier appearance in the plankton in Loch Ewe (west coast). In Stonehaven (east coast), from 2003 onwards, a general increasing trend in the abundance of decapod larvae was observed, related to the increasing temperatures recorded at that site. The data demonstrate the high variability of decapod larval abundance on an annual basis and the high importance of temperature and chlorophyll-a to the occurrence and abundance of decapod larvae.

Herring Clupea harengus L. viscera were examined for endoparasitic infections as part of a multidisciplinary stock identification project (WESTHER, EU Contract no. Q5RS-2002-01 056) which applied a range of stock discrimination techniques to the same individual fishes to obtain comparable results for multivariate analysis. Spawning and non-spawning adults, and juvenile herring were caught, over 3 years, by commercial and research vessels from numerous locations to the west of the UK and Ireland, along with control samples of spawning fish from the eastern Baltic Sea, and juveniles from sites in the eastern and western North Sea, and the north of Norway. The metacercariae of two renicolid digeneans (Cercaria pythionike and Cercaria doricha), one larval nematode (Anisakis simplex s.s.) and one larval cestode (Lacistorhynchus tenuis) were selected as tag species. Results were compared with those from herring collected between 1973 and 1982, which suggested remarkable stability in the parasite fauna of herring in the study area. These species were used to compare the parasite infracommunities of spawning herring. A significant variation in infracommunity structure was observed between different spawning grounds. These results suggest that the parasite fauna of herring are spatially variable but remain temporally stable in both the short and long term. Significant differences in prevalence and abundance of infections and comparisons of parasite infracommunity enabled the separation of putative herring stocks west of the British Isles. Distinctive patterns of parasite infection in two different spawning groups off the north coast of Scotland suggest that this area is occupied by two spawning populations, one recruiting from the west of Scotland, the other from outside this area, and most likely from the eastern North Sea. The distribution patterns of L. tenuis, C. doricha and C. pythionike suggest the potential for fish that spawn in three distinct International Council for the Exploration of the Seas (ICES) management units to be present in mixed aggregations found over the Malin Shelf, with significant implications for management in this area.