Mass die-offs, reduced species richness and local extinctions of freshwater mussels have resulted from river drying events, which often co-occur with high ambient temperatures. These events are predicted to increase in frequency and severity under the influence of climate change. We aimed to identify the desiccation tolerance of two freshwater mussel species (the river mussel Alathyria jacksoni and the floodplain mussel Velesunio ambiguus) across a range of temperatures by simulating river drying events in laboratory conditions. Freshwater mussels were buried in sediment heated to 29, 32, 35, 38 and 41°C. Lethal times and lethal temperatures at which 50

Light is a fundamental environmental cue which influences the migration of many marine organisms. For the salmon louse Lepeophtheirus salmonis , light is believed to drive the diel vertical migration behaviour of their planktonic larvae. Salmon lice are of critical importance to the salmonid industry due to the damage they cause to wild and farmed hosts. Salmon lice larvae have an eyespot and are positively phototactic, yet how light intensity alters their vertical distribution remains unclear. Here, we tested how light intensity (0, 0.5, 10 and 80 µmol m -2 s -1 ), dispersal duration (1, 4 and 12 h) and release point (surface or bottom) influenced the vertical migration of salmon lice nauplii and copepodids under controlled conditions in experimental columns. Overall, higher light intensity increased the proportion of nauplii that aggregated at the surface. Copepodid behaviour differed from that of nauplii, as they swam upwards in both light and fully dark conditions, and surface aggregations increased with dispersal duration. Results from the experiments did not support the existing view that light strongly influences the vertical position of copepodids in the water column. Combined with previous work, our results reveal that salmon lice larval stages display different vertical responses to light, temperature and salinity, which may be explained by the different strategies of nauplii (maximise survival and dispersal) and copepodids (maximise host-finding success). Our results have implications for salmon lice dispersal models, where responses of copepodids and nauplii to light are currently parametrised by the same equations. Implementing stage-specific behaviours towards light may improve the outputs of dispersal models.

Reports of infestation by marine parasitic tapeworms (Eubothrium sp.) and an associated growth reduction in Norwegian farmed salmon are on the rise. With few acceptable treatment options available, due to drug resistance evolution in tapeworms or negative drug impacts on fish, alternative controls against the parasite are in demand. In a 10-month commercial-scale study involving standard sea cages and lice barrier snorkel sea cages of different depths (4, 8, 12 and 16 m), we examined if this depth-based preventive technology primarily used against salmon lice (Lepeophtheirus salmonis) also reduced tapeworm infestation. A submerged net roof opening to a central barrier tube (snorkel) was added to standard cages to move salmon deeper but retain surface access; a cage manipulation that avoids contact with mostly surface-dwelling salmon lice larvae and may also separate fish from calanoid copepods, the intermediate hosts of Eubothrium sp. Salmon populations in unmodified standard cages had higher tapeworm prevalence (63-93%) and abundances (4.6-5.7 Eubothrium sp. fish-1) than those in snorkel cages (20-36% and 0.2-0.6 Eubothrium sp. fish-1). Based on these observations, tapeworm prevention could be another beneficial parasite management outcome of snorkel cage technology or other depth-based prevention techniques against salmon lice.

Cleaner fish used as a biological control agent against salmon lice is rapidly increasing in Atlantic salmon aquaculture. However, concerns have been raised about the welfare and mortality of cleaner fish in salmon cage systems, which could in turn affect their performance in controlling salmon lice. In a 4-month autumn-winter study, we monitored growth, welfare, mortality and daytime depth distribution of the most commonly used cleaner fish, farmed ballan wrasse and lumpfish, in six salmon production sea cages where thermo- and halo-clines were present. Ballan wrasse did not grow (SGR: small: -0.01% day(-1), large: -0.06% day(-1)), while lumpfish significantly doubled in size (SGR: 0.87% day(-1)) during the study. High losses (registered mortality + unregistered loss) were observed in both species (57 and 27% of ballan wrasse and lumpfish, respectively). The welfare status of remaining individuals generally improved over the study period, regardless of species. Brief daytime camera observations at hides found ballan wrasse were typically deeper at warmer (median 12.4 degrees C) more saline (median 31.7 ppt) depths, where salmon were expected to reside during day periods, compared to lumpfish generally occupying colder (median 7.3 degrees C), brackish (median 18.9 ppt) water in surface layers. Considerable mortalities, minimal feeding (inferred from ceased growth) by ballan wrasse and a possible mismatch in lumpfish and salmon depths (inferred from limited daytime camera observations) suggest that cleaner fish may have low long-term effectiveness against salmon lice in stratified salmon sea cages over autumn-winter. Similar studies across seasons, locations and cage types (e.g. depth-based cage technologies) are vital to understand the extent of these issues in salmon aquaculture more broadly.

Buoyancy regulation is a fundamental process by which pelagic fish maintain their position in the water column in an energy efficient way. Buoyancy has been largely overlooked as an important factor in Atlantic salmon production as salmon have a physostomous swim bladder that they fill by gulping air from the surface. The speed and ease at which the swim bladder can be filled and emptied by salmon has made understanding how much it contributes to buoyancy difficult to calculate. Here, we used an "increased excess mass WA" to investigate the maximum neutral buoyancy depth in seawater of farmed post-smolt Atlantic salmon across a range of sizes. The WA involved adding small weights to salmon over time to make them heavier. The salmon compensate for the extra weight via gulping air at the water's surface to fill their swim bladders and maintain neutral buoyancy. We monitored the swimming behaviour of salmon to determine the weight at which they could not maintain neutral buoyancy and used this to calculate the maximum neutral buoyancy depth (MNBD). For postsmolt salmon of 175-2400 g, the MNBD in seawater ranged from 21 to 24 m, with some variation with fish size: the average MNBD of 175 g salmon was 3.2 m shallower than 2400 g salmon. Fish body density also influenced MNBD. The individual with the lowest recorded body density (1.043 g/cm(3)) could achieve neutral buoyancy 11.2 m deeper than the individual with highest recorded body density (1.065 g/cm(3)). We found that salmon weigh approximately 2.65% of their weight in air in seawater. Depth-modified cages are becoming increasingly popular in Atlantic salmon aquaculture; farming fish deeper requires knowledge of the basic limits of salmon buoyancy and our results will inform guidelines that ensure optimal welfare in these new cage types.

Effective fisheries management requires fish size, growth and mortality information representative of the population and location of interest. Golden perch Macquaria ambigua is long lived, potamodromous and widespread in the Murray–Darling Basin (MDB), Australia. Using a sample spanning 13 river systems and 10° of latitude, we examined whether the maximum size of golden perch differed by latitude and whether growth and mortality varied between northern and southern MDB regions. The length, weight and age ranges of golden perch sampled (n=873) were 52–559mm, 2–3201g and 0+ to 26+ years respectively, and maximum length and weight were unaffected by latitude. Length and age–length distributions represented by age–length keys varied by region, with greater variability in age-at-length and a larger proportion of smaller individuals in northern MDB rivers, which generally exhibit greater variability in discharge. Growth and mortality rates were similar between regions, and an MDB-wide von Bertalanffy growth model (L∞=447, k=0.32 and t0=–0.51) and instantaneous mortality rate (Z=0.20) best described the data. An MDB-wide length–weight equation also provided the best fit (W=6.76×10–6 L3.12). Our data suggest that the MDB can be treated as one management unit in terms of golden perch maximum size, growth and mortality parameters.

Understanding how salinity affects marine parasites is vital to understanding their ecology and treatment, particularly for host-parasite systems that traverse marine and freshwater realms such as the globally important Atlantic salmon (Salmo salar), salmon louse (Lepeophtheirus salmonis) system. Growing concerns for wild fish populations, and decreased efficiencies and burgeoning costs of lice treatments for farmed fish has necessitated more environmentally and socially acceptable delousing procedures, such as hyposaline treatments. The effect of brackish water on L. salmonis following primary attachment is largely unknown, with experimental evidence derived mostly from unattached or newly attached copepodids, or adult stages. We aimed to understand how attached lice respond to hyposaline environments to assess effectiveness as a parasite management strategy and to help better define delousing areas used by wild fish. Louse development at 4, 12, 19 and 26 ppt, and survival at 4 ppt, decreased as exposure times increased, but survival was otherwise unaffected. Subjecting salmon to fluctuating, repeat exposures did not influence efficacy. We confirm that free-swimming stages are susceptible, and show that attached copepodids were more tolerant than previously predicted based on experiments on alternate development stages. These results improve our understanding of the utility of hyposaline treatments in aquaculture and self-treating in wild fish, and could apply to other fish-lice parasite systems. Further, these data are important for models predicting host-parasite interactions and can contribute to predictive models on the transmission dynamics of sea lice from farm to wild fish.

High salmon lice (Lepeophtheirus salmonis) infestation levels resulting from intensive salmonid sea-cage aquaculture can threaten populations of wild salmonid hosts. This includes anadromous Arctic char (Salvelinus alpinus), which rely on short migrations into more productive seawater environments to build energy stores for maturation, spawning and over-wintering in freshwater. Elevated salmon lice burdens may limit the benefits of migration by constraining osmoregulation, growth, survival and reproduction. To test for these effects, we simulated anadromous migration in tanks by transferring individually tagged Arctic char smolts (n = 352, averaging 133 g) to seawater where they were infected with salmon lice or left as uninfected controls for 1 month, and then transferring them back to freshwater for 2 months. After the seawater phase, infected post-smolts had a mean of 0.33 (range of 0.09-0.91) mobile lice g(-1) fish weight. At this point, specific growth rates (SGRs) dropped in infected compared to control fish (0.1% vs. 1.6% day(-1)). Higher plasma Na+ and osmolality in infected fish also indicate osmoregulatory impairment. Throughout the study, mortality was 18.2% and 1.7% in infected and control groups, but sexual maturation was low and comparable between groups. Infection intensity correlated positively with mortality rate and plasma Cl-, and correlated negatively with SGR and condition factor (CF). CF dropped (Delta CF < 0) at intensities of >0.09 lice g(-1) fish weight, and intensities of >0.3 causing zero or negative SGRs and increased mortality were particularly concerning. If infection intensities reach these levels in the wild, char could be impacted by growth restrictions and increased mortality rates, which potentially cause shorter migration durations, lowered reproductive success and possibly also selection against anadromy. This study provides vital information for conservation practitioners wanting to understand the physiologically derived burden salmon lice can have on Arctic char populations, and can be used to define thresholds in the monitoring and conservation of Arctic char populations affected by aquaculture-driven salmon lice infestations.

Warm water thermal treatments dominate delousing operations in Norwegian Atlantic salmon (Salmo salar) aquaculture for the removal of ectoparasitic salmon lice (Lepeophtheirus salmonis). However, treatments can lead to poor welfare outcomes for fish. Reverse thermal delousing by rapidly reducing ambient temperatures to very low treatment temperatures may also prove effective if there is a sufficient safety margin in the tolerances of salmon to cold water exposure such that negative side effects do not emerge. We tested the effects of immediate transfer of salmon from ambient temperature (15 degrees C) to cold water at different temperatures and durations on lice removal and short-term impacts on fish welfare. Treatments of -1 degrees C water for 10 min and 1 degrees C for 240 min treatments reduced mobile lice loads, but created more skin and eye damage than controls. Our results on delousing effect and welfare outcomes require ground-truthing at industry scale, as crowding, pumping, and the passage of fish through delousing units add additional and possibly synergistic lice removal effects and pose further welfare risks.

In trying to deal with the problematic salmon louse Lepeophtheirus salmonis in salmon aquaculture, strategies to better prevent infestations are gaining traction. Successful prevention requires an accurate understanding of the environmental influences that alter the distribution of the planktonic stages of lice in the water column in space and time. Here, we tested the salinity preferences of nauplii and copepodid larval stages using step salinity column experiments. Under consistent temperature and lighting conditions, we created step gradients using a bottom layer of full salinity (34.7 ppt), with an upper layer of equal or lower salinity (~34.7 to 16 ppt). Lice entered the column in the lower layer and dispersed for 1 h before their position was recorded. Both nauplii and copepodids increasingly avoided the overlying layers as they became more brackish. However, the strength of avoidance differed between nauplii and copepodids. Nauplii almost completely avoided salinities below 30 ppt. For copepodids, there was a more gradual decline in the proportion preferring the less saline overlying layer, and the presence of some individuals occurred even at 16 to 20 ppt. Both stages aggregated at or just below the halocline, with no aggregation evident in isohaline columns at the same depth. For nauplii, clustering within the halocline was particularly strong. When integrated into a sea lice dispersal model, the new salinity preferences we determined markedly altered dispersal patterns in scenarios when salinity gradients were present. Our results have implications for the mapping of salmon lice larval behaviour and dispersal, with benefits for aquaculture planning and management.

Salmon louse Lepeophtheirus salmonis, a key parasite of salmonids, is managed by multiple methods at both salmon farm- and ecosystem-scale that are informed by an understanding of the abundance and distribution of the infective, planktonic stage of the lice. Dispersal modelling using hydrodynamic models relies on accurately estimating larval depth and how environmental variables modify distributions. Larval responses to temperature could modify dispersal distances by altering their depth in the water column and thus exposure to depth-dependent oceanographic processes and the duration of their temperature-dependent development. Using column experiments, we tested how L. salmonis nauplii and copepodids responded to different thermoclines by establishing a bottom layer of 12°C with an overlaying layer varying from 6 to 18°C in 2°C steps. Nauplii moved upwards in high proportions and aggregated in the surface layer when the overlying layer was 10°C or cooler. In contrast, nauplii moved downwards and aggregated at the thermocline when the overlying layer exceeded 12°C. Temperature did not influence the vertical distribution of copepodids. When nauplii behaviour towards temperature was integrated into a dispersal model, dispersal distances increased. Temperature should be considered when calculating depth distributions. Further, nauplii and copepodids behave differently and should be configured separately in dispersal models.

Automatic visual inspection (AVI) is a popular tool for object detection and analysis in many fields. In fisheries, AVI systems designed for fish egg development analysis are used for fish egg assessments in commercial and experimental fish production systems and in ecological monitoring. The first step in AVI systems is to subdivide an image into meaningful regions. The accuracy of this segmentation directly influences the success of subsequent image analysis operations such as feature selection and classification. At present, a lot of image segmentation techniques are available, but there is no universal segmentation technique suitable for all kinds of images. This paper presents a cascading automatic segmentation method for microscopic digitised images of live common carp (Cyprinuscarpio) eggs in Petri dishes filled with water. The proposed method involves five main steps:1) image pre-processing, 2) image initial segmentation by Otsu's method, 3) segmentation post-processing based on morphological operations, 4) image edge detection and morphological operations to remove incomplete eggs, and finally 5) a Watershed method was used to resolve the problem of separating adjoining fish eggs. Testing the method in Matlab software verified that fish eggs were recognized, segmented and counted with 100% accuracy compared to manual observation in all 96 test cases.

Methods to prevent parasite infestations in farmed fish are becoming widespread, yet tests of their effectiveness often lack commercial relevance and statistical power, which may lead to technology misuse. Here, we examined salmon louse infestation on Atlantic salmon in triplicate commercial snorkel louse barrier and standard cages over a 12 month production cycle. Barrier cages reduced newly settling lice on Atlantic salmon by 75%, with variability in parasite reduction over time depending upon environmental variables. The commercial, triplicate, long-term study design serves as a template to validate performance and detect weaknesses in anti-parasite techniques in fish mariculture.

Surface environment modification is a potential parasite control strategy in Atlantic salmon sea-cage farming. For instance, a temporary low salinity surface layer in commercial-scale snorkel sea-cages has coincided with reduced amoebic gill disease (AGD) levels after an outbreak. We tested if a permanent freshwater (FW) surface layer in snorkel sea-cages would lower AGD and salmon lice levels of stock relative to snorkel cages with seawater (SW) only and standard production cages with no snorkels. Triplicate cages of each type with 2000 post-smolts were monitored in autumn to winter for 8 wk and sampled 4 times. Lower proportions of individuals with elevated AGD-related gill scores were registered in SW and FW snorkel cages compared to standard cages; however, these proportions did not differ between SW and FW snorkel cages. Individuals positive for AGD-causing Paramoeba perurans were reduced by 65% in FW snorkel relative to standard cages, but values were similar between SW snorkel cages and other types. While total lice burdens were reduced by 38% in SW snorkel compared to standard cages, they were unchanged between FW snorkel and other cage types. Fish welfare and growth were unaffected by cage type. Surface activity was detected in all cages; however, more surface jumps were recorded in standard than snorkel cages. Overall, fish in FW snorkel cages appeared to reside too little in freshwater to consistently reduce AGD levels and salmon lice compared to SW snorkel cages. Further work should test behavioural and environmental manipulations aimed at increasing freshwater or low salinity surface layer use.

Freshwater bathing is one of the main treatment options available against amoebic gill disease (AGD) affecting multiple fish hosts in mariculture systems. Prevailing freshwater treatments are designed to be long enough to kill Neoparamoeba perurans, the ectoparasite causing AGD, which may select for freshwater tolerance. Here, we tested whether using shorter, sublethal freshwater treatment durations are a viable alternative to lethal ones for N. perurans (2-4 hr). Under in vitro conditions, gill-isolated N. perurans attached to plastic substrate in sea water lifted off after ≥2 min in freshwater, but survival was not impacted until 60 min. In an in vivo experiment, AGD-affected Atlantic salmon Salmo salar subjected daily to 30 min (sublethal to N. perurans) and 120 min (lethal to N. perurans) freshwater treatments for 6 days consistently reduced N. perurans cell numbers on gills (based on qPCR analysis) compared to daily 3 min freshwater or seawater treatments for 6 days. Our results suggest that targeting cell detachment rather than cell death with repeated freshwater treatments of shorter duration than typical baths could be used in AGD management. However, the consequences of modifying the intensity of freshwater treatment regimes on freshwater tolerance evolution in N. perurans populations require careful consideration.