Iodophor disinfection of non-hardened Lota lota, Salmo trutta, and Thymallus thymallus eggs: Tolerance levels and iodine permeability
Introduction
Iodophors are polyvinylpyrrolidone–iodide–iodine complexes in aqueous solution, which have a broad range of application as disinfectants. They are also used as disinfectants for fish eggs before water hardening to prevent transmission of viruses and bacteria from the broodstock to the offspring. Protocols for egg disinfection during water hardening have been developed for different species of the Salmonidae, in particular for the North American genus Oncoryhnchus. It is recommended to disinfect eggs 10–15 min after fertilization at active iodine concentrations of 50–100 mg/l for 15–60 min depending on the used iodine concentration (Ontario Ministry of Natural Resources, 2009, Fisheries and Oceans Canada, 1984; U.S. Fish and Wildlife Service). Several studies indicate that iodophor tolerance of salmonid eggs is species specific and depends on individual egg quality. Alderman (1984) tested iodophor solutions with 75–200 mg/l iodine on salmonid eggs and obtained highly varying tolerance levels depending on the individual egg quality. Fowler and Banks (1990, 1991) observed increased mortality of eggs and larvae of fall chinook salmon (Oncorhynchus tshawytscha) eggs after disinfection with 50–75 mg/L iodine for 30 min. Rainbow trout eggs disinfected in 125 mg/L active iodine and Arctic grayling eggs (Thymallus arcticus) disinfected in 50–100 mg/l active iodine for 30 min had a reduced hatching success (Brown and Shrable, 1994). Fish larvae are extremely sensitive to iodophor exposure (Fowler and Banks, 1990).
To get better knowledge about iodophor tolerance of eggs of different fish species, further studies are necessary. In particular, there exist no systematic investigations on Eurasian Salmonidae, as the brown trout, Salmo trutta, and the European grayling, Thymallus thymallus. Further, no data are available for a non salmonid coldwater species, the burbot, Lota lota, although it is a promising new species for aquaculture and also cultured for restocking purposes (Lahnsteiner et al., 2012, Worthington et al., 2010).
The permeability of eggs of teleost fish to iodophors is unclear too, as eggs have efficient permeability barriers to maintain internal solute concentration independent from the environment, i.e., the cell membrane, the yolk syncytial layer (Hagedorn et al., 1997, Hagedorn et al., 1998), the chorion, and the perivitelline fluid (Shephard, 1989, Shephard and McWilliams, 1989). Iodophor influx into the eggs may vary when the disinfection is performed in iodophor solution diluted in water or isosmotic NaCl, as the diffusion coefficients differ between non-activated and activated eggs (Peterson and Martin-Robichaud, 1993, Shephard and McWilliams, 1989). Moreover, in water, the cortical reaction is activated and a concentration gradient is generated between the pervitelline space and the external environment (Alderdice, 1988, Kitajima et al., 1985). Due to this gradient, water is osmotically drawn into the egg and increased iodophor influx could be expected too. Contrary, in isosmolar NaCl solutions, iodophors must be expected to enter the eggs solely by diffusion.
The present study focused on the addressed questions: It investigated the tolerance limits of non-hardened L. lota, S. trutta, and T. thymallus eggs to iodophor disinfection using the rate of eyed stage embryos and the rate of hatched larvae as viability parameters. In S. trutta, and T. thymallus the egg percentile mass increase during hardening was determined as it reflects the physiological and biochemical processes occurring during oocyte activation (Lahnsteiner and Patzner, 2002). As the permeability of eggs of teleost fish to iodophors is unclear, the egg internal active iodine concentrations were investigated for S. trutta, and T. thymallus eggs disinfected in water or isosmolar NaCl solution.
Section snippets
Materials and methods
Desamar K30 was obtained via a local supplier from DESAG AG, Hygiene for Food and Aquaculture (Fredenbeck/Stade, Germany). Its active iodine concentration was 1.35 mg/g. All other chemicals were reagent grade (purity ≥ 95%) and were obtained from VWR International (Vienna, Austria).
Brown trout (S. trutta) (450 ± 160 g, mean ± standard deviation) were from a commercial source and burbot (L. lota) (385 ± 175 g) from an experimental brood stock. European grayling (T. thymallus) (220 ± 50 g) derived from an
Effect of iodophor disinfection on the rate of eyed stage embryos and on the rate of hatched larvae in L. lota, S. trutta, and T. thymallus
In these experiments, the iodophor stock solution was diluted in 0.75% NaCl. Disinfection of non-hardened L. lota eggs in ≤ 1% iodophor solution (≤ 135 mg/l active iodine) for 15 min did not affect the rate of embryos developing to the eyed stage and the rate of hatched larvae in comparison to the control, while concentrations > 1% (> 135 mg/l active iodine) significantly decreased the described viability parameters (Table 1). After disinfection periods of 30 min, the rate of embryos developing to the
Discussion
The present study demonstrates that non hardened eggs of L. lota, S. trutta, and T. thymallus can be disinfected with 1% iodophor solution (≤ 135 mg/l active iodine) diluted in NaCl solution for 15 min without a significant decrease in embryo mortality. At longer disinfection periods of 30 min, 0.5% iodophor solution (67.5 mg/l) is tolerated as shown in S. trutta and L. lota. According to data of Skall and Olesen (2011), the tolerated iodine concentrations are efficient to kill most of the relevant
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