Julia Lynne Overton

Rationale for Restocking the Eastern Baltic Cod Stock

The Danish Institute for Fisheries Research and Bornholms Salmon Hatchery examined the potential for restocking Baltic cod (Gadus morhua callarias L.) in the eastern Baltic Sea. This cod population has adapted to the unique brackish water conditions where successful spawning depends on regular inflows of oxygenated saltwater from the North Sea. Hydrographical conditions are therefore considered to constitute the principal bottleneck for recruitment of this population. Successful recruitment is also dependent upon food availability and predation pressure from mainly herring (Clupea harengus L.) and sprat (Sprattus sprattus L.). A 2-to 3-month delay in the spawning period compared to 20–30 years ago has altered feeding conditions and predation susceptibility in a way that may have exacerbated the decline in recruitment. Producing and releasing cod larvae during spring would mimic the spawning period recorded in previous times and would coincide with the spring peak in copepod production. An evaluation of 3 different release scenarios showed that a release of 474 million first-feeding larvae over 5 months would enhance the average population of 2-year-olds by 10% and be biologically and economically the most feasible scenario.

Current Status of Eurasian Percid Fishes Aquaculture

This chapter presents the actual status and the perspectives of development for Eurasian perch and pikeperch in different countries, mainly in Europe (Denmark, Finland, Sweden, Ireland, The Netherlands, France, Czech Republic and Hungary) but also in Iran and Tunisia. For each country, main culture techniques are summarized and production types are specified, according to the local or international markets.

Chemical surface disinfection of eggs of Baltic cod, Gadus morhua L.

The effect of two disinfectants on eggs and larvae of Baltic cod, Gadus morhua, was investigated. The eggs were disinfected for 10 min using various concentrations of either glutaraldehyde (100, 200, 400, 600 and 800 mg L(-1)) or iodophor (10, 50, 100 and 150 mg L(-1)), 1-4-days post-fertilization. Bactericidal effect of disinfection, survival to hatching, hatching success and larval abnormalities were assessed. Larval survival was recorded at 5-, 10- and 15-days post-hatch (dph). Although Baltic cod eggs have an unusually thin chorion, they could tolerate surface disinfection. A reduction in bacterial growth was observed with increased concentrations of disinfectant (3.0 x 10(7)-1.6 x 10(1) CFU mL(-1)). Abnormalities in newly hatched larvae were not related to disinfection. Survival of the yolk sac larvae was significantly better for eggs treated with 400 mg L(-1) glutaraldehyde for 10 min at 10 and 15 dph. Effective disinfection was also recorded using 100 mg L(-1) Actomar K30. Egg batch effect rather than initial bacterial concentration, disinfectant type or incubation method determined the survival of the eggs to hatching and survival of larvae. Because of the carcinogenic effect of glutaraldehyde, iodophor is recommended for routine disinfection of cod eggs.

Commercial Production: Factors for Success and Limitations in European Percid Fish Culture

Percid farming is still in its infancy; however there are already a handful of commercial ventures successfully producing percids. The underlying factors for success for these companies include the introduction of technology that allows a more intensified production that is independent of season, such as RAS systems and the development of out of season spawning. General know how in these companies and research institutions is also accumulating over time. Continued investment in “learning” has resulted in some companies being able to break the barrier of pilot production and move into commercial production. There are still limitations for further upscaling of production. For established companies these include, domestication, stabilizing and streamlining production, slow growth in larger fish and nutrition. New enterprises find it difficult to find financial backing when there is a lack of general information on Percid markets; basing their sales projections on local traditional market prices. Lack of veterinary knowledge of percids and a working knowledge of RAS systems are also limiting in some countries. Finally, there are recommendations for future development necessary for improving production.

Ribosomal RNA gene sequences confirm that protistan endoparasite of larval cod Gadus morhua is Ichthyodinium sp.

An enigmatic protistan endoparasite found in eggs and larvae of cod Gadus morhua and turbot Psetta maxima was isolated from Baltic cod larvae, and DNA was extracted for sequencing of the parasites small subunit ribosomal RNA (SSU rRNA) gene. The endoparasite has previously been suggested to be related to Ichthyodinium chabelardi, a dinoflagellate-like protist that parasitizes yolk sacs of embryos and larvae of a variety of fish species. Comparison of a 1535 bp long fragment of the SSU rRNA gene of the cod endoparasite showed absolute identity with I. chabelardi, demonstrating that the 2 parasites are very closely related, if not identical. This finding is discussed in relation to some morphological differences that appear to exist between I. chabelardi and the cod endoparasite.