The fat laid down as a winter reserve by 0-group sand smelt, Arherina boyeri, was found to be sizedependent. The larger, earlier-spawned fish lay down more fat prior to the onset of winter. During the winter the fish do not feed for some 100 days and rely on this fat for energy; laterspawned 0-group fish (< 59 rnms.L. in November) have insufficient fat reserves and starve to death in a normal winter. This loss of the smallest 46% of the 0-group is shown as an increase in the mean size of the 0-group over the winter period. Older sand smelt age classes have more than sufficient fat reserves for overwintering. There is thus a clear advantage in spawning early in the season, and any restriction on spawning ground availability at that time will result in overall population regulation. This conclusion supports the hypothesis that the density-dependent control on population size in the sand smelt is a limitation on the number of fish which can spawn at the optimum time.

A 5-year study of the population biology of the common shrimp Crangon crangon (L.) in the Bristol Channel is reported. Samples were collected from three commercial fishermen and from the filter screens of five power stations taking water from the estuary.The Severn Estuary and Bristol Channel were found to contain a single shrimp population which undertook regular seasonal migrations in order to avoid low salinities and to reproduce. Previous studies had assumed that the regions held separate populations which migrated to avoid low winter temperatures.

A general description of the trophic inter-relationships and the food web within Bridgwater Bay, Somerset, England, is presented. This Bay is a lower estuarine site within the Bristol Channel. Particular attention is given to the use of the area by fish and larger crustaceans which are the dominant animal groups. Using data collected over a ten-year period of monthly sampling, the relative average biomass of all fish and larger crustaceans found within the Bay is calculated for each month of the year. These averages are used to produce food webs which also show species relative abundance for each calendar month.It is shown that the trophic structure follows a seasonal cycle due to migration. The application of the concept of stability within such a dynamic system is discussed. The role of compartmentalization of the ecosystem into a number of loosely connected groups as an aid to overall stability is discussed. While there is no evidence that contemporaneous compartments exist, it is argued that inter-specific interactions are temporally limited because of the different periods of annual residence of the species. It is suggested that the stability and resilience to interference shown by these estuarine systems is enhanced by temporal partitions.

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