Effects of temperature on amoebic gill disease development: Does it play a role?

A relationship between increasing water temperature and amoebic gill disease (AGD) prevalence in Atlantic salmon (Salmo salar) has been noted at fish farms in numerous countries. In Scotland (UK), temperatures above 12°C are considered to be an important risk factor for AGD outbreaks. Thus, the purpose of this study was to test for the presence of an association between temperature and variation in the severity of AGD in Atlantic salmon at 10 and 15°C. The results showed an association between temperature and variation in AGD severity in salmon from analysis of histopathology and Paramoeba perurans load, reflecting an earlier and stronger infection post‐amoebae exposure at the higher temperature. While no significant difference between the two temperature treatment groups was found in plasma cortisol levels, both glucose and lactate levels increased when gill pathology was evident at both temperatures. Expression analysis of immune‐ and stress‐related genes showed more modulation in gills than in head kidney, revealing an organ‐specific response and an interplay between temperature and infection. In conclusion, temperature may not only affect the host response, but perhaps also favour higher attachment/growth capacity of the amoebae as seen with the earlier and stronger P. perurans infection at 15°C.     

Early development of resistance to the salmon louse, Lepeophtheirus salmonis (Krøyer), in juvenile pink salmon, Oncorhynchus gorbuscha (Walbaum)

This study examined the effect of fish weight on the susceptibility of post-emergent pink salmon to Lepeophtheirus salmonis (Krøyer). Three trials were conducted, each with two stocks of pink salmon, Oncorhynchus gorbuscha (Walbaum), at starting weights of c . 0.3, 0.7 and 2.4 g, respectively. In each trial, duplicate tanks of fish were exposed to 0, 25 (only in Trial 1), 50 or 100 copepodids per fish. Mortality in Trial 1 was c . 37%, regardless of stock following exposures to 50 or 100 copepodids. Mortalities occurred up to 26 days after exposure, and more than 80% of the lice on the dead fish were chalimus stages. Infections with adult or preadult lice were observed on c . 35% of fish surviving to 37 days after exposure. Mortality was 5% in Trial 2 and there was no mortality in Trial 3. The abundance of L. salmonis was lower in Trial 3 compared with Trials 1 or 2. Histological changes in the skin coincident with fish growth included a thickening of the epidermis, infiltration of the dermis with fibroblasts by the end of Trial 1 and the first evidence of scales by the end of Trial 2; scales were evident throughout Trial 3. These results showed that the previously reported innate resistance to L. salmonis displayed by pink salmon develops in fish heavier than 0.3 g and appears to be functional by 0.7 g. This resistance coincided with changes to the epidermis and dermis, including the formation of scales. The present results indicate that elevated risk associated with L. salmonis infection among migrating post-emergent pink salmon may occur during a relatively brief period before the fish reaches 0.7 g.     

The effects of hydrogen peroxide treatment on different life-cycle stages of the salmon louse, Lepeophtheirus salmonis (Krøyer 1837)

The effects of hydrogen peroxide administered at a concentration of 1500 ppm for 20 min at 7.5 °C on different life-cycle stages of Lepeophtheirus salmonis were examined experimentally. The mobile adult and pre-adult stages of L. salmonis readily reattached to Atlantic salmon after hydrogen peroxide treatment. Adult female lice, but not adult males or pre-adults, reattached in significantly lower numbers than untreated controls. Survival of early chalimus stages (I and II) was not affected by hydrogen peroxide treatment, but their subsequent development to chalimus III and IV was delayed compared with untreated controls. Nauplii and copepodid larvae of L. salmonis were almost all dead by 1 hour post-treatment. A very few copepodids survived 24 h post-treatment. Egg strings of L. salmonis at an early stage of development failed to hatch after hydrogen peroxide treatment, but those with pigmented eggs did hatch, although in significantly reduced numbers. Treated egg strings did produce viable copepodids, although in significantly reduced numbers compared with controls.     

Spatial and temporal distribution of Lepeophtheirus salmonis (Kr?yer) larvae in a sea loch containing Atlantic salmon, Salmo salar L., farms on the north-west coast of Scotland.

A plankton survey investigated spatial and temporal distributions of larval sea lice in a sea loch on the north-west coast of Scotland. Plankton tows were collected approximately weekly at two depths (0 and 5 m) at five sample stations over a 2-year period. The survey began in March 2002 when Atlantic salmon, Salmo salar L., farms in the study area stocked with fish and finished in March 2004, after farms completed harvesting. Over 90% of lice recovered were Lepeophtheirus salmonis (Kr?yer); therefore, the results of this paper refer largely to that species. The data were analysed using generalized additive models with explanatory variables for time, depth and station. Greatest densities of nauplii were recovered at stations adjacent to farms indicating that the local salmon farms were a likely source of larvae at times during the production cycle. There were significant temporal trends in larval densities and the stocking and harvesting of farms can possibly account for these increases and decreases, particularly at the start and at the end of the production cycle. The distribution of copepodids was more widespread than that of nauplii, indicating that the larvae can be transported several kilometres from the point of release.     

Where art thou louse? A snapshot of attachment location preferences in salmon lice on Atlantic salmon hosts in sea cages

Problematic sea lice infestations on farmed Atlantic salmon (Salmo salar) have motivated extensive research and development into new methods to prevent, monitor and control sea lice. Most of these technologies require detailed information on the behaviour, spatial distribution and demography of lice on host fish. This study investigated how salmon lice (Lepeophtheirus salmonis) infestation density varies across the host's surface under sea cage farming conditions. Lice abundance, demography and attachment location were tracked over time, with repeated sampling of 300 individually tagged salmon across three replicate experimental sea cages. The data reveal clear differences in attachment locations according to sex and stage, but with an overall preference for the dorsal surface among mobile stages—dorsal head for adult females and dorsal-posterior section for males and pre-adults. Total lice abundance was highly variable between repeated measures of individual fish, consistent with frequent host-switching or mortality. Total lice numbers also declined between sampling dates, likely due to handling, with lost mobile lice being almost exclusively adult males. As the distribution of sea lice on hosts is likely determined by numerous factors, future image-based automated detection systems should be validated in settings that reflect the complex host–parasite interactions that occur in open farming systems.     

Do habitat measurements in the vicinity of Atlantic salmon (Salmo salar) parr matter?

Atlantic salmon, Salmo salar L., parr habitat characterisation is usually performed by in situ measures of key environmental variables taken at the exact fish location if the fishing gear allows precise pinpointing of this location, or in large sampling sections covering a river reach or mesohabitat, often ignoring variability in the immediate vicinity around individual fish. These data may be critically important in the development and validation of habitat preference models. The influences of seven increasing distances of measures, the variation of the number of considered measures and the depth of velocity measurement (bottom or 0.6 of the depth) in the calculations of HSI (Habitat Suitability Index) from a multiple‐experts fuzzy model of Atlantic salmon parr habitat were tested. When a parr was present, six measures collected in a 50‐cm radius around the fish to provide an average measure as input data and velocity measured at 60% of the depth gave the highest HSI values. These results show some potential for the use of an intermediate study scale, between micro‐ and mesohabitat, and questions how fish habitat conditions are currently measured.     

Does the AquasmartTM feeding system reduce sea louse [Lepeophtheirus salmonis (Krøyer)] infestation on farmed Atlantic salmon (Salmo salar L.) in winter?

Infections of sea lice [Lepeophtheirus salmonis (Krøyer)] were studied over winter on a working, commercial salmon farm in relation to two types of feeding system: a regular, pneumatic spray feeder and an irregular, ‘demand feed’ sensor system (Aquasmart^TM). Contrary to prediction, the Aquasmart^TM system alone had no significant effect on chalimus infestation compared with fish fed by pneumatic spray. However, it was associated with significantly reduced numbers of mobile stages, both at the beginning of the study (P < 0.05) and, more particularly, in conjunction with treatment of all pens using hydrogen peroxide (P < 0.001). It is suggested that feeding regime may represent a useful tool in reducing lice infestation when co‐ordinated with other methods of control, such as peroxide treatment, and that further work in this field is merited.     

Decline and fall of the salmon fisheries in the Netherlands: is restocking the Rhine a reality?

Abstract. The Atlantic salmon, Salmo salar L., was once of major importance to the fishing industry along the River Rhine. This paper discusses the Dutch and German salmon catches over the years 1863–1950. Even up to the end of the last century, it was not uncommon for Dutch and German fishermen to land 100000 salmon a year. However, factors such as the increased use of locks and weirs along the Rhine, coupled with growth in pollution, soon led to a rapid decline in numbers. By 1933, the salmon fishing industry in the Netherlands had virtually ceased to exist. Analysis of the available catch statistics suggests that the decline in the salmon population could have started before official records began. Although the degree of scatter in the data and uncertainties in the assumptions preclude the possibility of drawing firm conclusions about the survival rate of salmon, these figures illustrate how difficult it will be to maintain a stable population in the Rhine. Moreover, a number of changes have taken place since the heyday of salmon in western Europe, which could compound the problem. Of particular importance in the context of the Rhine are:

• 1 the closure of two of the major migration routes to the sea (Haringvliet and Zuiderzee);
• 2 morphological changes in the river;
• 3 chemical and thermal pollution;
• 4 the loss of accessible spawning and nursery areas of the required quality;
• 5 the disappearance of salmon from other rivers that flow into the North Sea such as the Rivers Elbe, Weser and Ems. If salmon were only reintroduced into the Rhine, a certain proportion would probably stray and infiltrate these other rivers.

The fact that the impact of these changes is difficult to quantify increases the uncertainty associated with maintaining a stable stock of salmon in the Rhine.     

Marine mortality in the river? Atlantic salmon smolts under high predation pressure in the last kilometres of a river monitored for stock assessment

The River Bush (Northern Ireland) is an index river for the estimation of Atlantic salmon, Salmo salar L., stock size, population dynamics and marine survival rates. Marine survival estimates are based on the number of smolts counted at a trap 3.5 km upstream of the river outlet. The survival from release to coastal inshore waters for acoustic‐tagged smolts released at the Bushmills trap varied between 32% and 68%, with both year and brightness during river exit playing a significant role in explaining the variations in survival. This constitutes an important survival bottleneck. Contrary to true marine mortality, this significant loss of smolts in the river and nearshore environments could be reduced by focused management actions. More studies on other rivers, where smolts are enumerated above the head of tide, could further partition smolt and post‐smolt mortality, help differentiate true marine survival and help understand fluctuations in adult returns.     

Dynamics of Caligus rogercresseyi (Boxshall & Bravo, 2000) in farmed Atlantic salmon (Salmo salar) in southern Chile: Are we controlling sea lice?

Caligus rogercresseyi generates the greatest losses in the salmon industry in Chile. The relationship between salmon farming and sea lice is made up of various components: the parasite, host, environment and farming practices, which make it difficult to identify patterns in parasite population dynamics to define prevention and control strategies. The objectives of this study were to analyse and compare the effect of farming, sanitary practices and environmental variables on the abundance of gravid females (GF) and juveniles (JUV) of C. rogercresseyi on Salmo salar in three Salmon Neighborhood Areas (SNAs) in Region 10, south of Chile. Linear mixed‐effects models of the negative binomial distribution were used to evaluate the effect of the different explanatory variables on GF and JUV. Productive variables were the key drivers explaining the abundance of GF and JUV. Results suggest that C. rogercresseyi is not controlled and JUV are persistent in the three SNAs, and sanitary practices do not control the dissemination of the parasite among sites. Environmental variables had a low impact on sea lice abundance. There is a need to perform analysis for modelling of parasite population dynamics to improve Integrated Pest Management, including changes in the governance to achieve an effective prevention and control.     

An outbreak of visceral white nodules disease caused by Pseudomonas plecoglossicida at a water temperature of 12°C in cultured large yellow croaker (Larimichthys crocea) in China

Visceral white nodules disease (VWND) caused by Pseudomonas plecoglossicida is a common disease in cage-farmed large yellow croaker (Larimichthys crocea) in China. VWND usually occurred at water temperature of 16–19℃, resulting in high mortality in farmed large yellow croaker. Now, P. plecoglossicida as its pathogen has been considered nonpathogenic at 7–12℃. During February 2019, an infectious disease outbreak was observed in cage-farmed large yellow croaker at a water temperature of 12℃ in Ningde, China. This disease is characterized by white granulomatous lesions in internal organs of the diseased fish, which was similar with the symptoms of the VWND in large yellow croaker. Then, we isolated a bacterial strain named PQLYC4 from visceral lesions of the diseased fish. The experimental infection studies demonstrated that the strain PQLYC4 was the pathogen of the disease, which was further identified as P. plecoglossicida by the analysis of morphology, 16s rRNA gene homology and average nucleotide identity based on the whole genome sequence. Our results revealed that P. plecoglossicida strain PQLYC4 could cause the outbreak of the VWND at 12℃, a water temperature lower than that reported previously, thus providing new knowledges of prevalence and prevention of the VWND in large yellow croaker.     

Assessment of changes in the ichthyofauna in a tropical reservoir in south-eastern Brazil: Consequences of global warming?

Increases in temperatures over the last century, more intensively after the eighties, were recorded in several databases for the south-eastern Brazil. These increases are likely to change fish communities in aquatic systems by decreasing abundances, biomass and sizes of some species more sensitive to climate change. Reservoirs may be particularly susceptible to the effects of climate change, as they isolate previously connected areas limiting fish dispersal and migration, as well as increasing water temperature and thermal stratification. We assessed temporal changes in the abundance and biomass of the ichthyofauna in an isolate reservoir (inflowing waters from small tributaries in a highland region) aiming to associate changes with climate effects over three decades (1990, 2000, 2010). Two abundant native species, a loricariid Loricariichthys castaneus (Castelnau, 1855) and a cichlid Geophagus brasiliensis (Quoy & Gaimard, 1824), were selected to assess eventual response to climate changes in their geographical distribution in the Southeast Atlantic Hydrographic Region, considering different carbon emission scenarios using ecological niche models. A decrease in the densities and biomass of the ichthyofauna and of the two selected species were observed in both summer and winter over the three decades, coinciding with increases in temperatures. These decreases were correlated with increases in positive anomalies of temperature, which may be an indicator of climatic changes, although with low explanation of the data variation. The mean size of L. castaneus decreased between the nineties and 2010, whereas no significant trend was found for the size of G. brasiliensis. Our predictions of new area for future adequacy indicated a loss in the distribution area for both species (mainly for G. brasiliensis), considering the most pessimistic scenario for 2050 and 2070, possibly due to climatic changes. Both hydroelectric dams and global warming pose threats to freshwater fish diversity, and both will interact. Changes observed in the fish assemblage over the last decades were essential for an assessment of the consequences of global warming in this type of reservoir combining larger scale studies with future projections.     

Do active‐dispersing insects dominate the invertebrate fauna of rock pools in the wet–dry tropics,Kimberley, Australia?

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Regulation of oocyte maturation in the rainbow trout,Salmo gairdneri: role of cyclic AMP in the mechanism of action of the maturation inducing steroid (MIS), 17α-hydroxy, 20β-dihydroprogesterone

In fish, oocyte maturation (resumption of meiosis after completion of vitellogenesis and before ovulation) is triggered by maturation inducing steroids (MIS) which generally appear to be secreted in the ovary in response to stimulation by a pituitary maturational gonadotropin. Converging data from different laboratories show that 17-hydroxy, 20-dihydroprogesterone (17, 20-OH-P) is the principal MIS in salmonoids; but clear identification remains to be done in other taxonomic groups.The experiments reported here in the rainbow troutSalmo gairdneri examine the possible involvement of oocyte cAMP on the mechanism of MIS action. The action of 17, 20-OH-P, on germinal vesicle breakdown (GVBD) in oocytes incubatedin vitro within the follicle, was inhibited by various substances expected to elevate the intraoocyte concentrations of cAMP: cAMP ( 1 mM) or dibutyril cAMP ( 2 mM), phosphodiesterase inhibitors such as theophylline ( 0.2 mM) or 3-isobutyl-1 methylxanthine (IBMX 0.1 mM), adenylate cyclase activators such as cholera toxin (> 100 nM) or forskolin ( 0.03 mM). In fact, the combined action of IBMX (1 mM) and forskolin (0.01 or 0.05 mM)in vitro was to promote accumulation of intraoocyte cAMP within 1 to 5 hours. Oocyte cAMP concentrations exhibited a large variability between different females, depending on the stage of oocyte development; a significant positive correlation between oocyte cAMP concentration and the follicular weight, and a significant negative correlation between oocyte cAMP concentration and the median efficient dose of 17, 20-OH-P for induction of GVBD, were observed. Finally, when intrafollicular oocytes were incubatedin vitro, the addition of a maturation-inducing concentration of 17, 20-OH-P (3×10^–6M) induced a significant decrease of oocyte cAMP within the first 10 hours of incubation. These results show that cAMP appears to play a central role in the regulation of oocyte sensitivity to 17, 20-OH-P and in the intraoocyte mechanisms leading to GVBD in trout.These data are discussed together with the few indications available in fish concerning the mechanism of MIS action which can be compared to some extent with the amphibian model.     

The GABA a agonist muscimol enhances locomotor activity, but does not alter the behavioural effects of CRH in juvenile spring chinook salmon (Oncorhynchus tshawytscha)†

The present study investigated the effects of manipulating the GABAergic system on locomotor activity in juvenile spring chinook salmon, Oncorhynchus tshawytscha. In addition, we evaluated whether the GABAergic system is important for mediating the behavioural effects of corticotropin-releasing hormone (CRH). An intracerebroventricular (ICV) injection of the GABA_a agonist muscimol caused an acute and dose dependent increase in locomotor activity in juvenile spring chinook salmon. ICV injections of the GABA_a antagonist bicuculline prevented the increase in activity when administered concurrently with muscimol. The GABA_b agonist baclofen had no effect on locomotor activity in this study. Furthermore, we found no evidence that the locomotor response to exogenous CRH was altered by the concurrent administration of muscimol or bicuculline. These results provide evidence to support the hypothesis that endogenous GABA within the central nervous system is involved in the control of locomotor activity in fish. The data also suggest that there is no interaction between the GABAergic system and CRH with regards to the control of locomotor activity in this species.