Lobster Ecology And
Recruitment – LEAR
Aquaflow Technical Leaflet 2002-53
European Network for the Dissemination of Aquaculture
RTD Information (Q5CA-2000-30105) and previously FAIR-3837, URL: http://www.aquaflow.org/
The European clawed lobster Homarus gammarus, is
widely but sparsely distributed in European coastal waters from Northern
Norway to the Mediterranean. The overall objectives of this study were to
obtain field and experimental data on the occurrence and ecology of small,
early benthic phase (EBP) lobsters as a comparison with its considerably
more plentiful cousin H. americanus. Investigations took place in known
lobster habitats in Ireland, Norway, England, and Italy to allow comparison
between the centre and extremities of the geographic range. The abundance of
EBP H. gammarus and other benthos was investigated by airlift suction
sampling using the same methodology as for similar studies on H. americanus.
Various methods of netting took place at the sites to sample potential fish
predators to determine if EBP lobsters featured as food items. Further
extended sampling for EBP lobsters was made with static gear (traps,
shelters, collectors etc.), the design and selection of these being
complemented by mesocosm behavioural studies. The direct impact of predators
on small lobsters was examined in a series of field trials. Interaction
among EBP lobsters and crustacean co~titors was examined in field studies as
well as in mesocosrn and laboratory trials with a selection of competing
decapods commonly occurring on lobster fishing grounds.
A comprehensive faunal description has thus been
produced for H. gammarus habitats in Europe. However, EBP lobsters below
20rnm carapace length (CL) were not obtained in any of the suction samples
nor from the stomachs of fish captured in the vicinity of lobster habitat.
Although occasional small lobsters were located in static gear and at some
intertidal sites, none were below 20rnm CL. As a result it was necessary to
use hatchery produced EBP lobsters instead of wild ones for predation and
competition investigations. Total removal of specimens by predators took
place within a short time unless some form of shelter was provided. In the
case of defaunation and reseeding trials, which showed recolonisation by
other species especially decapods, no EBP lobsters were recorded on plots
after 2-3 months. In laboratory, mesocosm and field trials on habitat
preference and competition, there was some evidence to suggest that EBP
lobsters were displaced from shelters by similar sized decapods.
Overall, the present evidence suggests that:- (a) EBP
H. gammarus were either very scarce, or (b) able to avoid sampling gear, or
(c) inhabited extra territorial areas spatially removed from juvenile and
adult lobster ground, or ( d) they occupied a specialised intra territorial
habitat within lobster grounds which were inaccessible to standard suction
sampling methodology .It seems that scarcity , gear avoidance or extra
territoriality may not be the overriding factors in determining EBP ecology
and it may be postulated that the stages from settlement to 20mm CL could
adopt an intra-territorial deep sheltering mode of life within adult and
juvenile lobster habitats, which is beyond the range of normal sampling
equipment. The present results support the hypothesis that predation plays a
role by restricting EBP European lobsters to shelter-providing substrate. It
appears even more than its American counterpart, that such habitat plays an
essential role at least in the early benthic Stages of the life cycle.
Clearly this factor must be taken into consideration in further management
policies for the species. It is suggested that future studies on the
juvenile lobster question should focus on additional examination of the
scarcity factor and in particular on methods for examination of alternative
deep sheltering habitats as well as on related behaviour and feeding
studies.
For more information:
Dr. John P. Mercer
Zoology Department,National University of Ireland
Galway
Ireland
Phone : +353 91 750438
Fax : + 353 91 525003
E-mail : john.mercer@nuigalway.ie