Salmon lice from fish farms in open net pens pose a threat to the survival of wild salmon post-smolts migrating through areas with high farm and lice densities. Reliable estimation of this mortality is fundamental for the sustainable management of aquaculture in such areas but is challenged by considerable uncertainty about several of the processes that link reported lice numbers in fish farms to post-smolt mortality. Utilising recent access to lice data from post-smolt trawling, we revised a virtual post-smolt model that estimates salmon lice-induced mortality of seaward-migrating wild salmon post-smolts. We also assessed the sensitivity of model results to model assumptions that differ among the virtual post-smolt models currently being used as management support in Norway. The spatiotemporal variation in infestation pressure along the entire Norwegian coast was calculated based on monitoring data for salmon lice, temperature and fish abundance in all Norwegian salmonid farm locations. The revised model integrates lice data from sentinel cage experiments and post-smolt trawling to quantify the spatiotemporal variation in infestation rate of wild post-smolts from the spatiotemporal infestation pressure, while also estimating the effect of calibration data type on infestation rate. Results show 10 times higher infestation rate in trawl-caught post-smolts than in post-smolts from sentinel cage experiments. Mortality estimates calibrated to trawl data are more sensitive to assumptions about possible density dependence in lice infestation rate than to assumptions about migration speed. These findings contribute to explaining why different virtual post-smolt models provide different estimates of salmon lice-induced mortality.

Salmon lice are a severe problem in salmonid aquaculture and also affect wild salmon smolts migrating through fjord systems in spring. To keep the lice burden within acceptable limits, frequent use of chemical delousing has resulted in the parasite becoming resistant to these treatments. Alternative thermal and mechanical delousing practices induce welfare problems and loss of farmed fish. To avoid losing the constant arms race with the parasite, we need a new approach to the problem. Inspired by the natural host-parasite balance, we propose a change in salmonid aquaculture practices by combining and improving existing management strategies for a more holistic and sustainable vision of the industry. Before salmonids were farmed in open cages, few hosts were available for salmon lice during winter, which reduced the salmon lice populations to a minimum when the wild smolts were migrating to sea. Thus, the natural strategy is to decrease host availability for a sufficient period of time to allow salmon lice nauplii to die of starvation. Due to the long survival and drift of the nauplii, it is important to significantly increase the distance between open farming units, either by organising and aggregating farms in extensive fallowing areas with significant distance to other corresponding areas or by controlling water flow through active production units during extensive fallowing. Here we primarily address environmental interactions. Economic and juridical implications of the proposed strategy are not discussed in detail. However, we do briefly suggest relevant current regulations and possible costs and benefits of reducing delousing treatment intensity.

The African grey bichir Polypterus senegalus is a popular ornamental fish in Indonesia. Pet trade with this species is increasing, and aquaculture production is well established. Here we present a detailed market survey and volume of domestic trade, export and import between January 2018 and February 2021. Climate similarity between the native range of P. senegalus and Indonesia was analysed by the MaxEnt algorithm. A significant number of areas of numerous Indonesian islands were identified as suitable for survival and establishment of this fish. This was confirmed by the records of 3 likely established populations in rivers in Java and Sumatra, where both wild type and albino juveniles were captured. The occurrence of more feral populations was suggested by local fishermen in Kalimantan, Java, and Lombok islands; however, verification via future field trips is required. The culture of P. senegalus is unregulated in Indonesia, and the potential risk of establishment of this predatory fish and its potential spread in this Southeast Asian country is alarming for wildlife managers. Although a total ban seems the best solution, an alternative risk mitigation strategy with minimal negative effects on the socio-economic situation in local communities is more feasible. The albino phenotype of P. senegalus is probably less of a risk because of its easier detection by predators, higher sensitivity to disease and stress, and disrupted social behaviour. Since albinos are popular in Indonesia, replacing the wild phenotype with this potentially less invasive phenotype could be a recommendation after experimental confirmation of the lower invasiveness of albinos.

Salmon lice Lepeophtheirus salmonis infestation of sea trout Salmo trutta results in both additional marine mortality and behavioral changes which may contribute to sea trout population decline. For effective management of activities that increase exposure to salmon lice, such as salmon aquaculture, it is necessary to have a full understanding of how salmon lice may affect sea trout populations. An individual-based model (IBTRUTTA) was therefore developed to investigate the potential effects of salmon lice infestation on sea trout population abundance and dynamics based on data from the River Halselva and Altafjord system in northern Norway. This model allowed investigation of the effect of lice-induced mortality and also the compensatory salmonid behavioral mechanisms of premature return to freshwater, either persistent for overwintering or transitory after which sea trout could go back to sea. It was found that, in the absence of compensatory mechanisms, even low rates of lice infestation could lead to marked declines in sea trout abundance. Compensatory behavioral mechanisms had the potential to reduce these declines, but persistent premature return resulted in reduced body mass of returning adults. The shape of the stock-recruitment relationship was also shown to strongly affect how lice-induced mortality impacted the population.

Freshwater pond polyculture faces many challenges in Europe. Appropriate tools must be developed to better understand and manage trophic interactions in pond ecosystems. The objective of our study was to understand the trophic interactions and make inference on the fish diet in common carp polyculture through a combination of experiments and trophic web modeling. We conducted an experiment in small fishponds of common carp polyculture reared with roach and perch and used Ecopath with Ecosim software to characterize the food web. Two replicates of 3 treatments were performed: a semi-extensive pond with low fish density and no formulated feed, an intensive pond with twice the fish density and formulated feed and an intensive pond coupled with a planted lagoon. Ten trophic groups were defined to describe the food web. The modeling procedure enabled us to estimate the diets of each trophic group. The fish diet in fed and non-fed treatments differed greatly since the carp fed mainly on formulated feed when available. The roach exhibited trophic plasticity by adapting their diet to the available resources. The benthic macroinvertebrates and zooplankton were preyed upon intensively; they became the limiting factors for fish production and depended on phytoplankton availability. Detritus and phytoplankton were the main sources of nutrients in all treatments but were not used efficiently. These results provide several insights for improving polyculture. In particular, they promote better management of zooplankton and macroinvertebrates as food sources for target species and a better balance in fish assemblages for more efficient use of resources.

This paper reviews the impacts of shellfish and finfish aquaculture on eelgrass Zostera marina, the most widely distributed seagrass species in the northern hemisphere. Shellfish aquaculture can have positive, neutral, and negative effects on eelgrass. Positive interactions can be generated by the filtering activity of cultured bivalves, which may improve water quality and reduce epiphyte loads, and shellfish biodeposits may provide more nutrients to eelgrass and other vegetation. However, negative responses are more commonly reported and can be caused by shading and sedimentation. These negative effects tend to occur directly under and immediately surrounding shellfish farms and rapidly diminish with increasing distance. In contrast to shellfish aquaculture, only one field study has investigated the effects of finfish aquaculture on eelgrass in a temperate setting, and the results were inconclusive. However, many studies have investigated the effects of Mediterranean finfish farms on 2 other species of seagrass (Posidonia oceanica and Cymodocea nodosa). These studies reported clear negative interactions, which have been linked to increased nutrient concentrations, sulphides, sedimentation, epiphyte loads, and grazing pressure. It is unknown if these studies are relevant for finfish aquaculture in temperate regions due to differences in environmental conditions, and because the studies focused on different species of seagrass. Thus, further study in a temperate setting is warranted. We conclude by highlighting key research gaps that could help regulators establish unambiguous operational and siting guidelines that minimize the potential for negative interactions between aquaculture and eelgrass.

Commercial off-bottom aquaculture of the eastern oyster Crassostrea virginica is challenged by repeated summer mortality events that appear to affect triploid oysters disproportionally. Many farmers believe common farming practices, especially when performed during hot summer months, may increase triploid mortality. The goal of this study was to investigate how diploid and triploid oysters react to common stressors imposed by farmers: tumbling during size grading and desiccation to prevent biofouling. Triploid and diploid oysters were deployed in floating cages at 3 farm sites along the US Gulf of Mexico coast. In May and July, oysters in 7 cages were subjected to 1 of 6 stress treatments: 1 of 3 levels of desiccation (18, 24, or 48 h) mixed with 1 of 2 levels of tumbling (tumbled or not tumbled), along with a never handled, submersed control. The mortality and growth rate of oysters were assessed in June, August, and September. Growth rates of both ploidies were affected by compounding stressors; that is, treatments with both tumbling and the longest desiccation period exhibited slower growth. Triploid oysters exhibited a greater increase in mortality in response to farm stress than diploid oysters, and mortality increased in all oysters subjected to extreme stress treatments when compared to the control. Based on this study, farmers should limit the desiccation time of oysters (particularly triploids) in the summer months to avoid any mortality or reduced growth resulting from compounding stressor effects.

Vibrio aestuarianusinfection in oyster populations causes massive mortality, resulting in losses for oyster farmers. Such dynamics result from host-pathogen interactions and contagion through water-borne transmission. To assess the spatiotemporal spread ofV. aestuarianusinfection and associated oyster mortality at a bay scale, we built a mathematical model informed by experimental infection data at 2 temperatures and spatially dependent marine connectivity of oyster farms. We applied the model to a real system and tested the importance of each factor using a number of modelling scenarios. Results suggest that introducingV. aestuarianusin a fully susceptible adult oyster population in the bay would lead to the mortality of all farmed oysters over 6 to 12 mo, depending on the location in which infection was initiated. The effect of temperature was captured by the basic reproduction number (R0), which was >1 at high seawater temperatures, as opposed to values <1 at low temperatures. At the ecosystem scale, simulations showed the existence of long-distance dispersal of free-living bacteria. The western part of the bay could be reached by bacteria originating from the eastern side, though the spread time was greatly increased. Further developments of the model, including the consideration of the anthropogenic movements of oysters and oyster-specific sensitivity factors, would allow the development of accurate maps of epidemiological risks and help define aquaculture zoning.

For successful evaluation of the overall effects of salmon louse infestation on brown trout population dynamics, it is crucial to have a realistic understanding of how lice infestation distributions are generated and how they should be interpreted. Here, we simulated the potential effects of spatio-temporal variance in lice larvae densities, temporal variance in sea trout marine migration timing and spatial variance in marine habitat use on lice infestation distributions. We show that, when sampling populations with individual variation in marine behaviour, e.g. from post-smolts to veteran migrants, we must expect multi-modal mixture lice infestation distributions. Applying standard statistical distributions, such as the Poisson, negative binomial or zero-inflated distributions, can be too simplistic and give biased results. Temporary increases in salmon lice load in a given area may have inconsistent effects among individuals of a population and may be critical for vulnerable groups such as post-smolts, dependent on timing. For many analyses, it will be necessary to resolve the contributions from groups of fish with different lice infestation expectations due to spatio-temporal differences in habitat use within the overall mixture distribution. Another consequence is that different data sources, obtained by different methods or sampled at different locations and periods, must be expected to give different lice infestation distributions, even when sampling the same population. We also discuss additional factors that may complicate the interpretation of salmon lice infestation distributions on sea trout, such as lice-induced mortality, and behavioural changes, such as premature return to less saline water for delousing.

Closed recirculation aquaculture systems (RAS) in combination with integrated multi-trophic aquaculture (IMTA) are considered best management practices, but high material costs and difficult maintenance still hinder their implementation, especially in developing countries and the tropics. Few case studies of such systems with tropical species exist. For the first time, an extremely low-budget system was tested combining the halophyte sea purslane Sesuvium portulacastrum and a detritivore, sandfish Holothuria scabra, with finfish milkfish Chanos chanos over 8 wk on Zanzibar, Tanzania. In a 2 m3 RAS, milkfish and sea purslane showed good growth, producing an average (±SD) of 1147 ± 79 g fish and 1261 ± 95 g plant biomass, while sea cucumber growth was variable at 92 ± 68 g. The system operated without filter units and did not discharge any solid or dissolved waste. Water quality remained tolerable and ammonia levels were reliably decreased to <1 mg l-1. A NO2- peak occurred within the first 30 d, indicating good biofilter performance of the different system compartments. Changes in dissolved inorganic nitrogen (DIN) species support the notion that the sea cucumber tank was the main site of nitrification, while the hydroponic halophyte tank acted as a net sink of NO3-. A nitrogen budget accounted for 63.7 ± 5.3% of the nitrogen added to the system as fish feed. Increasing the plant to fish biomass ratio to 5:1 would fully treat the DIN load. The experiment provides proof-of-concept of a simple pilot-scale RAS, integrating tropical species at 3 trophic levels.