Noble Crayfish Aquaculture : Embryonic Development and Juvenile Performance
Freshwater crayfish play an important role in freshwater ecosystems and represent a highly appreciated food item. The European crayfish fauna comprises five indigenous species and noble crayfish Astacus astacus (L. 1758) is the only species native to Northern Germany. Once widespread and common in almost any kind of freshwater habitat, the crayfish plague, the spread of non-indigenous crayfish species and physical as well as chemical degradation of natural water bodies greatly threaten our largest mobile invertebrate and have locally driven it to extinction. Noble crayfish are produced in extensive and semi-intensive pond systems for restocking of natural water bodies and for consumption. Intensification of crayfish culture is challenging due to the cannibalistic behaviour, the low fecundity, the slow and intermittent growth and the seasonal growth pattern. A promising approach to an efficient production of table-size crayfish and stocking material is the extension of the animals’ growth period in the first year by an accelerated incubation of eggs that is followed by a nursery phase until ambient conditions are suitable for pond stocking. Intensification efforts of astacid culture depend on the appropriate assessment of the animal’s physiological condition both for research and application. I could show that RNA/DNA ratio is a suitable parameter to assess overall growth capability in juvenile noble crayfish. Based on RNA/DNA ratios different feeding regimes were detected earlier than by morphometric growth parameters while RNA per wet weight failed to detect any differences at all. Furthermore, RNA/DNA ratio explained 89 % of the variation found in specific growth rate. The results are applicable likewise for the assessment of physiological condition of natural and cultured stocks and ease the development of commercial feeds by reducing the necessity for long experimental periods. By using artificial incubation for an accelerated incubation of early stripped eggs of noble crayfish, I demonstrated that dormancy in this species represents a form of diapause that is regulated by temperature and can be accelerated beyond a threshold only at the expense of higher mortality and reduced juvenile fitness. A moderately long cold period of 6-7 weeks from gastrulation favoured high stage II survival and wet weight. Furthermore, this diapause period synchronised embryonic development and reduced hatching and moulting period to 7 days with no overlap between both. Applying this to culture, stage II juvenile crayfish can be available in late March which is an appropriate time point for the start of a nursery phase. Suitable rearing conditions for juvenile crayfish were investigated in an on-farm study for two starting points (early: 28th of March, late: 21st of May) each followed by an eight-week nursery. Lighting is crucial for the entrainment of a nocturnal locomotor activity pattern thereby affecting performance. Therefore, the effects of natural sunlight and different artificial light sources on crayfish performance and periphyton characteristics were investigated. Survival was higher if crayfish were stocked later and natural sunlight yielded highest specific growth rates in both trials. The importance of naturally occurring food items differed between natural sunlight and the artificial light sources and periphyton characteristics partly explained differences in crayfish performance. This illustrates the potential of periphyton as a complementary food source in nursery of noble crayfish and shows that its growth can be promoted by novel lighting techniques. The results obtained in my thesis favour the production of table-size crayfish within two summers, are of high value for the conservation of endangered stocks of indigenous astacid crayfish and deliver valuable information for the development of commercial feeds and novel approaches to juvenile rearing.
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