| Summary: | Ph. D. thesis submitted to the University of London for the degree of Doctor in Philosophy. Sampling of the surface waters of the Firths of Clyde and Lorne, on the Scottish west coast, established the seasonal presence of detached floating macrophytes from April to October. The neuston of the Firth of Lorne was characteristic of coastal waters and relatively impoverished. In contrast the neuston of the Firth of Clyde tended to be abundant and was often dominated by the calanoid copepod Anomalocera patersoni, characteristic of oceanic neuston. The presence of floating weed provided an additional assemblage of weed associated species to those of the neuston. The association of juvenile C. lumpus with detached floating weed was confirmed. When floating weed was absent (November to March) trawling over rough ground established the year round presence of juvenile C. lumpus in inshore waters. Changes in the size distribution of juvenile C. lumpus caught were negligible compared to the growth of captive fish and the capture of larger juveniles from around moored rafts indicates that the association with floating weed is probably size dependent. The incidental capture of larvae in December in addition to the Summer months indicates a breeding season more protracted than previously reported with the probability that some level of breeding occurs year-round. The diet of juvenile C. lumpus differed fundamentally between the two study areas. The principal foods were harpacticoid copepods in the Firth of Lorne and the calanoid copepod Temora longicornis in the Firth of Clyde. Neither food type was well represented in the neuston samples described but complementary samples obtained with a fine meshed plankton dipnet indicated the numerical dominance of these relatively small components of the macrofauna. Comparison of the composition of the diet with that of the macrofauna showed that lumpsuckers feed as size generalists. The energetic costs of sucker attachment and continuous swimming were compared by respirometry. Although swimming was more costly than attachment, as lumpsucker weight increased the weight specific cost of swimming decreased at a higher rate than that of attachment so that the energetic advantage declined. Behavioural aspects of the lumpsuckers association with floating weed were investigated in the laboratory with plastic structures as weed analogues. The lumpsuckers preferred black structures located at the surface. Structural complexity was also preferred, particularly when concealment was possible. The balance between feeding motivation and predation risk was investigated at two hunger levels in a series of habitat selection experiments. The predicted use of shelter and activity were not fully realised in the results.
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