Life in floating weed : the ecology of juvenile lumpsuckers, Cyclopterus lumpus (L).
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
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