Survival and growth of
hatchery-reared individuals of the European lobster, Homarus gammarus
(L.), in field-based nursery cages on the Irish west coat
B.F. Beal, J.P. Mercer, A. O'Conghaile-2002
Aquaculture, 210(1-4): 137-157
Abstract:
At present, one of two strategies is employed by
fisheries managers for enhancing wild stocks of homarid lobsters using
hatchery-reared individuals. The first is repeated releases of large numbers
(>5000 at a time) of postlarvae (stage IV and V; carapace length [CL]=5-7
mm) to selected bottom locations. This option exists primarily because these
programs lack space, time, and/or the finances to rear animals to larger
sizes that would most likely have initially higher survival rates. The
second is to rear animals in the laboratory for 5-8 months to stage XII+
(CL=12-16 mm) and then release small numbers (<1000) of these relatively
large juveniles. To date there has been no attempt to release large numbers
of relatively large juveniles because the costs are too prohibitive.
We have developed a low-cost, low-maintenance,
field-based nursery caging system for rearing cultured lobsters, Homarus
gammarus (L). Individuals (780 and ranging in CL from 5.2 to 7.2 mm)
were reared in pre-fouled and unfouled containers (360 cm3)
fabricated from an extruded plastic netting (3.2 mm aperture) and in
pre-fouled plastic petri dishes (200 cm3) that were deployed in
five near-bottom cages for 10 months (September 2000 to June 2001) at two
subtidal sites located in a shallow, relatively exposed embayment on the
west coast of Ireland. Animals apparently were able to survive and grow by
suspension feeding on the plankton and/or foraging on the fouling community
that settled on and within individual containers. Mean recovery rate (±95%
CI) was independent of a priori fouling treatments, but was site-specific
(42.1±7.9% and 27.8±13.7%; n=5). These rates are minimal estimates
of survival because we found that at least 20% of the animals were capable
of escaping from the mesh containers. Mean recovery in petri dishes that
prohibited emigration was 53.3±37.02% at one site and 75.0±23.1% (n=5)
at the other. These recovery rates compare favorably with survival rates of
fed conspecifics held in the laboratory over the same time (54/81=66.7%). At
the end of the experiment, animals in field cages had mean CLs that were
significantly smaller than the fed controls. Because of costs incurred with
maintaining small lobsters under laboratory conditions, results of this
short-term, manipulative field experiment indicate that field-based
nurseries represent an economically viable, third option for managers of
lobster stock enhancement programs.