Salmonid Fish Farming Research Articles

Trapping and relocating seals from salmonid fish farms in Tasmania, 1990-2000: was it a success?

In an effort to reduce the impact of seals on fish farms, the trapping and relocation of seals at Tasmanian salmonid farms began in 1990. To the end of May 2000,353 identified individual seals had been trapped in 672 capture events. Most were non-breeding male Australian fur seals (Arctocephalus pusillus doriftrus). The number of seals captured increased (from four in 1990 ro a peak of 164 in 1998) with the size and extent of the farms, and an increase in salmon production from 55 tonnes in 1986/87 to almost 10000 tonnes in 1999/2000. Of 586 capture events 52% were of seals that had been captured more than once. When seals are recaptured following trapping and relocation, this occurs on average 25 days after capture. Capture-mark-recapture calculations show that many seals in the vicinity of fish farms are not 'trappable', suggesting that trapping is only effective for certain individuals. Some individuals are recaptured many times, reflecting the predisposition of some individuals to be captured ('trap-happy'). Interaction is seasonal, with most seals trapped during winter, between May and September. The assessment of trends in capture rates is problematic, due to the lack of capture effort information from the farms. A further confounding factor has been the change in management practice both between farms and over time, as the use of predator nets has become more widespread. Two seals trapped at fish farms and fitted with satellite transmitters before relocation have either not returned to the farm or returned to the vicinity of farms and not interacted with them, although on one occasion the individual was trapped. The effectiveness of the relocation program as a management tool to reduce seal interactions cannot be quantified from the relocation data per se, but relocation does not stop seals interacting with farms.

Geographical and temporal variation in abundance of salmon lice (Lepeophtheirus salmonis) on salmon (Salmo salar L.)

Surveys in the area around Austevoll Aquaculture Research station have failed to reveal the presence of a large number of the free-swimming stages of salmon lice in the water column. However, lice were abundant on salmon held in cages in this area. Lice-free salmon were used as collectors of lice at three sites chosen at varying distances from other known sources of lice (salmonid fish farms). The abundance of lice on salmon at particular sites was found to vary during the year and also between years. During the study period, variations were correlated with temperature, increased abundance occurring at higher temperatures (>6°C). Comparisons between sites during the period with the highest temperatures showed that lice abundance was negatively correlated with distance to other salmon farms in the area. During periods of colder temperatures (<6°C) lice abundance was positively correlated with distance to salmonid fish farms. In January 1995, it was possible to predict that 1995 would be a year with heavy lice burdens. The basis of the prediction was the accumulated difference between average temperature during the period 1978-1996 and the sea temperature for the 4-month period preceding the settling period.

Prevalence of Infectious Pancreatic Necrosis Virus on Salmonid Fish Farms in Spain

In Spain, salmonid fish farming was commercially developed in the 1960s, and now there are 140 private farms that depend heavily on imported embryonate eggs. Infectious pancreatic necrosis was first clinically diagnosed in Spain in 1970, but the virus (IPNV) was not isolated and identified until 1980. Since that time, researchers have isolated IPNV from other samples in Spain. A diagnostic survey was conducted to determine how prevalent IPNV is on fish farms in Spain and whether the virus has been responsible for some of the major financial losses occurring every year on these farms. In total, 236 samplings of rainbow trout Oncorhynchus mykiss from 31 farms in eight hydrographic areas were done over a 3-year period. Infectious pancreatic necrosis virus was isolated in 94 cases, and serotyping of the viral strains revealed that 81% of these isolates were strain Sp and 19% were strain Ab. Neither IPNV strain VR-299 nor rhabdovirus (as infectious hematopoietic necrosis virus or viral hemorrhagic septicemia virus) was detected in any of the samples.

The usage of vaccination and antimicrobial agents for control of Yersinia ruckeri

. Selected salmonid fish farms in England and Wales were sent a questionnaire designed to collect data on enteric redmouth disease (ERM) vaccination and the usage of antimicrobial agents. The response to the survey was 41·5%. The results indicated that 22% of responding farms have had ERM diagnosed since 1982, half of them have experienced the disease every year since the initial confirmation and it occurs primarily between April and September; mainly in rainbow trout, Oncorhynchus mykiss (Walbaum), of 222/kg-200g. Although larger fish were also affceted. The incidence of ERM coincides with a rise or fall in water temperature, over-crowding, poor water quality or grading. Vaccination against ERM is used on 53% of the responding farms. In addition, 45% of these farms considered immunization to have failed, mainly due to the poor condition of the fish and the low water temperatures at vaccination. On those farms whieh employed vaccination, the commonest ehemotherapeutant used against ERM is oxolinic acid. However, oxytetracycline is the most frequently used antibiotic on farms not employing a vaccine. The use of an antimicrobial agent as a prophylactic or the use of another compound when the first treatment is completed occurred on some farms. The average annual cost for vaccination and treatment has been calculated.

Veterinary aspects of salmonid fish farming: husbandry diseases

Most disease problems affecting farmed salmonid fish are either caused or exacerbated by bad husbandry and poor environmental conditions. The aspects of water quality and nutrition which are significant in causing husbandry problems are considered. The clinical aspects of these diseases are decribed in terms of the life cycle of salmonids, and methods of prevention are discussed.

Veterinary aspects of salmonid fish farming: husbandry

A knowledge of environmental requirements is important in order to obtain good growth and health when farming salmonid fish. Biological requirements are discussed, including water quality parameters and nutritional aspects. The main techniques used for farming salmonids are described together with the annual cycle of operations undertaken on a commercial trout farm.