Improvement of rearing conditions for juvenile lobsters (Homarus gammarus) by co-culturing with juvenile isopods (Idotea emarginata)
Introduction
European lobsters (Homarus gammarus Linnaeus, 1758) form valuable fisheries along the coastline of the northeast Atlantic. In 2006, the annual European landings from the major lobster exporting countries, i.e. UK, Norway, Greece, Ireland and France reached approximately 3400 metric tons (t) with a value of 45 million Euros (Fishery Statistics, 1999–2006). Along the German coast, a sustainable lobster population is only present at the rocky subtidal of the island of Helgoland (North Sea, German Bight), where the lobster fishery was important during the 1920s and 1930s with catches of up to 80,000 animals (38 t) per year (Klimpel, 1965). Since the 1960s, the catch rates decreased drastically and reached a minimum of a few hundred lobsters per year in the 1980s (Goemann, 1990, Anonymus, 1980–2008). The reasons for the collapse of the Helgoland lobster population may include the destruction of the habitat by the bombing of the island during and after the second world-war, extensive fishing pressure in the 1950s and 1960s, pollution of the North Sea waters by oil spills, chemicals, and industrial wastes, and interspecific competition for food and shelter with the crabs, Cancer pagurus (Linnaeus, 1758) (Klimpel, 1965, Anger and Harms, 1994, Harms et al., 1995).
In order to provide specimens for biological research, a lobster rearing facility was established in 1997 at the Marine Station at Helgoland. Berried females were provided by local fishermen and kept in aquaria until the larvae hatched. The larvae were raised to post-larvae and juveniles. Based on the experience in lobster rearing, a restocking programme was initiated to enhance the natural lobster population around Helgoland with laboratory raised juveniles. Within a 5-year programme it is anticipated to raise and release more than ten thousand juveniles per year. The local rearing capacities have to be extended accordingly.
Since the maintenance of lobsters is time and cost intensive, the present work was aimed to optimize the breeding conditions for juveniles to reach maximum productivity at lowest cost. A principal goal of our study was to utilize local resources. In this respect discards of the crab fishery around the island of Helgoland provide a suitable and cheap food for the juvenile lobsters. Preliminary feeding trials with minced crab meat showed good acceptance and satisfactory growth rates. However, remains of the crab meat frequently blocked the water circulation system and drastically increased the effort for cleaning and maintenance of the rearing facilities and sometimes caused complete loss of the lobster stock. Despite the initial failure, crab meat from C. pagurus seems to be a suitable, abundant, and inexpensive feed for lobsters at Helgoland. Therefore, we focussed our attempts on improving the rearing systems and rearing conditions to minimise the adverse effects when feeding minced crab meat. In order to increase the survival rates we tried to reduce food waste and other debris by co-culturing juvenile lobsters with live juvenile isopods, Idotea emarginata (Fabricius, 1793). These isopods are known as debris feeders and thus appeared useful to keep the rearing system clean and thus to maintain water quality.
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Materials and methods
The experiments were carried out in summer 2006 at the Marine Station at Helgoland. The feeding experiments were run at 20.5 ± 0.6 °C, at ca. 33 psu and maintained under the natural light/dark-cycle.
Diet
The gross compositions and energy contents of the three diets (Artemia sp. nauplii, minced crabs (whole carcasses of C. pagurus) and juvenile I. emarginata) were significantly different (P < 0.001, Table 1). Artemia sp. nauplii showed the highest carbon and nitrogen contents and, thus, the highest energy content of 454 J·mg− 1AFDW. All parameters were significantly higher than in C. pagurus and I. emarginata.
Growth and survival
The growth rates of all juvenile lobsters ranged from 0.069 to 0.091 mm·day− 1 CL-R− 1 or
Discussion
The major challenges in rearing juvenile lobsters (H. gammarus) are the supply with clean seawater, feeding with high quality but reasonable food, and the prevention of cannibalisms. These aspects require expensive or technically ambitious solutions and may entail high staff costs. We established a cost-efficient rearing and feeding method for easiest handling and maintenance but highest possible productivity of juvenile lobsters.
Conclusions
An extensive restocking programme for the endangered Helgoland lobster population requires laboratory rearing and subsequent release of large numbers of juveniles lobsters. For the implementation of such a programme we developed a rearing method which is optimized from both economic and ecological viewpoints.
Minced crab meat from discards of the Helgoland crab fisheries proved to be a suitable and inexpensive food for juvenile lobsters but it blocked the rearing system and impaired water
Acknowledgements
This work is part of the PhD thesis of I.S. This study was supported financially by the Ministry of Fisheries and Agriculture of the State of Schleswig-Holstein, Germany, project title: 1317-11-68511005. Thanks are due to the local fishermen for providing berried females and to Michael Janke for technical support and maintenance of the lobsters.
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