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Laser is the abbreviation of Light Amplification by the Stimulated Emission of Radiation. It is a device that creates a narrow and low-divergent beam [1] of coherent light, while most other light sources emit incoherent light, which has a phase that varies randomly with time and position. Most lasers emit nearly «monochromatic» light with a narrow wavelength spectrum. Fig.1 [1] is the spectrum of a helium neon laser, showing very high spectra purity.

Fig1. Spectrum of a helium neon laser

1 Principle of Lasers

The principle of a laser is based on three separate features: a) stimulated emission within an amplifying medium, b) population inversion of electronics and c) an optical resonator.

Spontaneous Emission and Stimulated Emission

According to the quantum mechanics, an electron within an atom or lattice can have only certain values of energy, or energy levels. There are many energy levels that an electron can occupy, but here we will only consider two. If an electron is in the excited state with the energy E2 it may spontaneously decay to the ground state, with energy E1, releasing the difference in energy between the two states as a photon. [2] (see Fig.2a) This process is called spontaneous emission, producing fluorescent light. The phase and direction of the photon in spontaneous emission are completely random due to Uncertainty Principle. The angular frequency ω and energy of the photon is:

where ћ is the reduced plank constant.

Conversely, a photon with a particular frequency satisfying eq(1) would be absorbed by an electron in the ground state. The electron remains in this excited state for a period of time typically less than 10 -6 second. Then it returns to the lower state spontaneously by a photon or a phonon. These common processes of absorption and spontaneous emission cannot give rise to the amplification of light. The best that can be achieved is that for every photon absorbed, another is emitted.

Fig.2 Diagram of (a) spontaneous Emission; and (b) stimulated Emission

Alternatively, if the excited-state atom is perturbed by the electric field of a photon with frequency ω, it may release a second photon of the same frequency, in phase with the first photon. The atom will again decay into the ground state. This process is known as stimulated emission. [3] (see Fig.2b)

The emitted photon is identical to the stimulating photon with the same frequency, polarization, and direction of propagation. And there is a fixed phase relationship between light radiated from different atoms. The photons, as a result, are totally coherent. This is the critical property that allows optical amplification to take place.

All the three processes occur simultaneously within a medium. However, in thermal equilibrium, stimulated emission does not account to a significant extent. The reason is there are far more electrons in the ground state than in the excited states. And the rates of absorption and emission is proportional the number of electrons in ground state and excited states, respectively. [2,3] So absorption process dominates.

Population Inversion of the Gain Medium

If the higher energy state has a greater population than the lower energy state, then the light in the system undergoes a net increase in intensity. And this is called population inversion. But this process cannot be achieved by only two states, because the electrons will eventually reach equilibrium with the de-exciting processes of spontaneous and stimulated emission. [4]

Instead, an indirect way is adopted, with three energy levels (E1 [4] The population of level 2 and 4 are 0 and electrons just accumulate in level 3. Laser transition takes place between level 3 and 2, so the population is easily inverted.

In semiconductor lasers, where there are no discrete energy levels, a pump beam with energy slightly above the band gap energy can excite electrons into a higher state in the conduction band, from where they quickly decay to states near the bottom of the conduction band. At the same time, the holes generated in the valence band move to the top of the valence band. [5] Electrons in the conduction band can then recombine with these holes, emitting photons with an energy near the band gap energy.(see Fig.4)

Fig.4 Diagram of electron transitions of semiconductor gain medium

Optical Resonator

Although with a population inversion we have the ability to amplify a signal via stimulated emission, the overall single-pass gain is quite small, and most of the excited atoms in the population emit spontaneously and do not contribute to the overall output [6] . Then the resonator is applied to make a positive feedback mechanism.

An optical resonator usually has two flat or concave mirrors, one on either end, that reflect lasing photons back and forth so that stimulated emission continues to build up more and more laser light. Photons produced by spontaneous decay in other directions are off axis so that they won’t be amplified to compete with stimulated emission on axis. The «back» mirror is made as close to 100% reflective as possible, while the «front» mirror typically is made only 95 — 99% reflective so that the rest of the light is transmitted by this mirror and leaks out to make up the actual laser beam outside the laser device. [7]

More importantly, there may be many laser transitions contribute in the laser, because of the band in solids or molecule energy levels of organics. Optical resonator also has a function of wavelength selector. It just make a standing wave condition for the photons:

where L is the length of resonator, n is some integer and λ is the wavelength. Only wavelengths satisfying eq(2) will get resonated and amplified.

2 Summary of Principles and Modes of Operation

As a summary, Fig.5 is the schematic diagram of the working process of lasers. [8]

Fig.5 Schematic Diagram of Laser Operation

The output of a laser may be a continuous constant-amplitude output (known as CW or continuous wave); or pulsed, by using the techniques of Q-switching, model-locking, or gain-switching. In many applications of pulsed lasers, one aims to deposit as much energy as possible at a given place in as short time as possible. Some dye lasers and vibronic solid-state lasers can produce light over a broad range of wavelengths; this property makes them suitable for generating extremely short pulses of light, on the order of a few femtoseconds (10 -15 s).[1] The peak power of pulsed laser can achieve 10 12 Watts.

3 Types of Lasers and Applications

According to the gain material, lasers can be divided into the following types. Several common used lasers are listed in each type. [1]

Gas Lasers:

Laser Gain Medium Operation Wavelength(s) Pump Source Applications and Notes
Helium-neon laser 632.8nm Electrical discharge Interferometry, holography, spectroscopy, barcode scanning, alignment, optical demonstrations
Argon laser 454.6 nm, 488.0 nm, 514.5 nm Electrical discharge Retinal phototherapy (for diabetes), lithography, confocal microscopy, spectroscopy pumping other lasers
Carbon dioxide laser 10.6 μm, (9.4 μm) Electrical discharge Material processing (cutting, welding, etc.), surgery
Excimer laser 193 nm (ArF), 248 nm (KrF), 308 nm (XeCl), 353 nm (XeF) Excimer recombination via electrical discharge Ultraviolet lithography for semiconductor manufacturing, laser surgery

Solid State Lasers:

Laser Gain Medium Operation Wavelength(s) Pump Source Applications and Notes
Ruby laser 694.3nm Flash Lamp Holography, tattoo removal. The first type of visible light laser invented; May 1960.
Nd:YAG laser 1.064 μm, (1.32 μm) Flash Lamp, Laser Diode Material processing, laser target designation, surgery, research, pumping other lasers. One of the most common high power lasers.
Erbium doped glass lasers 1.53-1.56 μm Laser diode um doped fibers are commonly used as optical amplifiers for telecommunications.
F-center laser Mid infrared to far infrared Electrical current Research
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Metal-vapor Lasers:

Laser Gain Medium Operation Wavelength(s) Pump Source Applications and Notes
Helium-cadmium (HeCd) metal-vapor laser 441.563 nm, 325 nm Electrical discharge in metal vapor mixed with helium buffer gas. Printing and typesetting applications, fluorescence excitation examination (ie. in U.S. paper currency printing)
Copper vapor laser 510.6 nm, 578.2 nm Electrical discharge Dermatological uses, high speed photography, pump for dye lasers

Other types of lasers:

Laser Gain Medium Operation Wavelength(s) Pump Source Applications and Notes
Dye lasers Depending on materials, usually a broad spectrum Other laser, flashlamp Research, spectroscopy, birthmark removal, isotope separation.
Free electron laser A broad wavelength range (about 100 nm — several mm) Relativistic electron beam Atmospheric research, material science, medical applications

4 History and Extension

In 1917 Albert Einstein first raised the concepts of probability coefficients (later to be termed ‘Einstein coefficients’) for the absorption, spontaneous emission, and stimulated emission of electromagnetic radiation. It was confirmed in 1940s. [1]

In 1953, Charles H. Townes and graduate students James P. Gordon and Herbert J. Zeiger produced the first microwave amplifier, a device operating on similar principles to the laser. In 1955 Prokhorov and Basov suggested an optical pumping of multilevel system as a method for obtaining the population inversion, which later became one of the main methods of laser pumping. Townes, Basov, and Prokhorov shared the Nobel Prize in Physics in 1964.

The first working laser was made by Theodore H. Maiman in 1960. [9] He used a solid-state flashlamp-pumped synthetic ruby crystal to produce red laser light at 694 nm wavelength. Maiman’s laser, however, was only capable of pulsed operation due to its three-level pumping scheme. Later in 1960 the Iranian physicist Ali Javan made the first gas laser using helium and neon. It was the first continuous-light laser. The concept of the semiconductor laser diode was proposed by Basov and Javan. And the first laser diode was demonstrated by Robert N. Hall in 1962.

Numerous types of lasers have been invented since then and applied widely. Today they are still being improved to get better features, such as maximum peak output power and minimum output pulse duration.

As an interesting extension, femtosecond laser can be used to change the color of metals. This is a pretty new discovery by Chunlei Guo and Anatoliy Vorobyev in 2007. [10] They permanently change the aluminum, which is normally silver-colored, to gold, grey and black. And they can also turn gold black.(See Fig.6)

Fig.6 From left, aluminum turned a gold color, titanium turned to blue, and platinum turned gold.

Their trick is to etch the metals’ surfaces with pits of different lengths and create other tiny shapes using a powerful laser. These tune the surfaces to absorb particular wavelengths of light, and reflect only the desired color — or almost no light in the case of black.

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Dichlorvos from lice — the principle of action and methods of application

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Marine Pollution Bulletin

Abstract

Nuvan 500 EC is an organophosphorus pesticide (active ingredient: dichlorvos), widely used in agriculture, animal husbandry, horticulture, food storage and even to control fleas on domestic pets. In the salmon farming industry it is used to control sea lice. Recent investigations have led to the conclusion that, given Nuvan’s known toxicity and the extensive gaps in our knowledge about its environmental effects, the widespread and growing use of this substance in the marine environment is unacceptable. Furthermore, the current controls of the use of Nuvan are inadequate and largely contrary to the UK government’s stated commitment to a precautionary approach to marine pollution.

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This work was supported by the World Wide Fund for Nature.

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Dichlorvos from lice — the principle of action and methods of application

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Chemosphere

Abstract

Pesticides are released intentionally into the environment and, through various processes, contaminate the environment. Three of the main classes of pesticides that pose a serious problem are organochlorines, organophosphates and carbamates. While pesticides are associated with many health effects, there is a lack of monitoring data on these contaminants. Traditional chromatographic methods are effective for the analysis of pesticides in the environment, but have limitations and prevent adequate monitoring. Enzymatic methods have been promoted for many years as an alternative method of detection of these pesticides. The main enzymes that have been utilised in this regard have been acetylcholinesterase, butyrylcholinesterase, alkaline phosphatase, organophosphorus hydrolase and tyrosinase. The enzymatic methods are based on the activation or inhibition of the enzyme by a pesticide which is proportional to the concentration of the pesticide. Research on enzymatic methods of detection, as well as some of the problems and challenges associated with these methods, is extensively discussed in this review. These methods can serve as a tool for screening large samples which can be followed up with the more traditional chromatographic methods of analysis.

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The drivers of sea lice management policies and how best to integrate them into a risk management strategy: An ecosystem approach to sea lice management

Marine Institute, Galway, Ireland

Dave Jackson, Marine Institute, Galway, Ireland.

Fisheries Directorate, Oslo, Norway

Fisheries Directorate, Oslo, Norway

Marine Institute, Galway, Ireland

Fisheries Directorate, Oslo, Norway

Marine Institute, Galway, Ireland

Dave Jackson, Marine Institute, Galway, Ireland.

Fisheries Directorate, Oslo, Norway

Fisheries Directorate, Oslo, Norway

Marine Institute, Galway, Ireland

Fisheries Directorate, Oslo, Norway

Abstract

The control of sea lice infestations on cultivated Atlantic salmon is a major issue in many regions of the world. The numerous drivers which shape the priorities and objectives of the control strategies vary for different regions/jurisdictions. These range from the animal welfare and economic priorities of the producers, to the mitigation of any potential impacts on wild stocks. Veterinary ethics, environmental impacts of therapeutants, and impacts for organic certification of the produce are, amongst others, additional sets of factors which should be considered. Current best practice in both EU and international environmental law advocates a holistic ecosystem approach to assessment of impacts and risks. The issues of biosecurity and ethics, including the impacts on the stocks of species used as cleaner fish, are areas for inclusion in such a holistic ecosystem assessment. The Drivers, Pressures, State, Impacts, Responses (DPSIR) process is examined as a decision‐making framework and potential applications to sea lice management are outlined. It is argued that this is required to underpin any integrated sea lice management (ISLM) strategy to balance pressures and outcomes and ensure a holistic approach to managing the issue of sea lice infestations on farmed stock on a medium to long‐term basis.

1 INTRODUCTION

Criticism of the salmon farming industry for how they have handled the salmon louse problem has influenced the design of regulations and licences to operate. For example in Norway, concerns with respect to salmon lice led to a postponed implementation and possibly abandonment of an increase in the maximum allowable biomass (Asche & Bjørndal, 2011 ; Torrissen et al., 2013 ). The strong negative publicity in respect of the sea lice issue may also influence the public opinion and have a negative effect on the public perception of aquaculture. In Norway inadvertent accumulation of sea lice from fish farms and genetic interactions with farmed escapees have been identified as the two primary challenges with respect to interactions with wild salmon populations (Glover et al., 2017 ; Taranger et al., 2015 ). These concerns have led to the requirement, based on the precautionary principle, to avoid and mitigate impacts of sea lice of fish farm origin on wild stocks being a major driver of sea lice control programmes for farmed salmon (Jackson, 2011 ) in many jurisdictions.

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Control of sea lice infestations continues to be a major issue in many regions of the world. Drivers of priorities and objectives vary for different regions and even between jurisdictions in the same ecoregions. Ecosystems vary in terms of host species and the species of parasite. In the Northern Hemisphere Lepeophtheirus salmonis is regarded as the more serious parasite of the two species which affect farmed salmon (Helgesen, Romstad, Aaen, & Horsberg, 2015 ; Jackson & Minchin, 1992 ; Rae, 2002 ). In the Pacific Northwest, whilst the parasite is L. salmonis and the farmed host is normally the Atlantic salmon (Salmo salar) the host parasite dynamic is complicated by the existence of large numbers of wild hosts comprising several different species of the genus Oncorhynchus (Brooks, 2009 ). In Chile the parasite species is Caligus rogercresseyi which infests a number of native species including the small eyed flounder (Paralichthys microps) and the Silverside smelt (Odontesthes regia) as well as farmed Atlantic salmon. This species is widely distributed in southern Chile, and is considered a serious threat to this industry (Hamilton‐West et al., 2012 ).

The context for management strategies has also been undergoing radical revision as the scale and complexity of the salmon farming industry has grown. There has been an evolution of management strategies from a focus on treating individual cages to synchronous treatments of whole sites and also sophisticated treatment plans covering whole bays (Jackson, 2011 ; Jackson, Hasett, & Copley, 2002 ; Murray & Gubbins, 2016 ). There have also been initiatives to develop integrated pest management strategies including the use of biological controls such as cleaner fish (Deady, Varian, & Fives, 1995 ) and targeted control at key times and in key locations (Brooks, 2009 ; Jackson et al., 2002 ). However, this evolution has rarely been strategic and in many cases has been reactive responding to a variety of pressures from fish health, to environmental concerns over the impacts of treatments on non‐target organisms such as commercially important species including shrimp and lobster (Burridge, 2013 ) and the potential impacts on stocks of wild salmonids. Salmon farming is a relatively new form of farming in a relatively new sector, aquaculture. Land based food culture has evolved over millennia and underwent a radical transformation during the agricultural revolution, leading to massive changes in technology, husbandry techniques and production volumes. These developments resulted in fundamental changes in the approach to a whole range of issues including parasite control and animal health. The scale of the aquaculture industry and in particular salmon farming has been rapidly increasing in recent decades (Figure 1). It has grown from a small niche industry to become what is now a global scale sector. Farmed Atlantic salmon production in 2014 exceeded 2.3 million tonnes and the species was ranked as number eight by amount for cultured fish species produced globally (Glover et al., 2017 ) accounting for more than 99% of all human consumption of salmon (FAO, 2016 ). Salmon farming is now one of the world’s most economically important industries in the marine food sector.

2 CURRENT MANAGEMENT PRACTICES AND THE NEED FOR A CONCEPTUAL EVOLUTION

Currently lice management strategies in most parts of the world including Ireland, Scotland (Anon. 2007 , 2015 ), Canada (Saksida et al., 2011 ), the Faroes (Anon. 2013 ) and Norway are governed by protocols or plans developed at a local, regional or even national level (Anon, 2008 ; Jackson, 2011 ). In some cases these national plans are guided by risk assessments (Department of Agriculture Fisheries & Food, 2008 ; Taranger et al., 2015 ). The requirement to ensure sustainability and mitigate environmental impacts features in most if not all management regimes but in most cases it is not integrated in a structured or ecosystem‐based management framework. The concept of ecosystem‐based management has been developed in respect of wild capture fisheries in response to the unintended negative consequences of the total allowable catch system and associated quotas which were for several decades the basis of international fisheries management. In essence the ecosystem approach seeks to ensure that all negative impacts, both on non‐target organisms and on sensitive habitats and food webs are taken into account in the setting of limitations on harvesting and/or management activities. Such frameworks are being developed (Arthur, 2008 ) and applied increasingly in other sectors and are leading to re‐evaluations of management and risk assessment criteria (Berg, Furhaupter, Teixeira, Uusitalo, & Zampoukas, 2015 ). As the industry continues to develop and in particular if it is to reach the goals set out by the FAO ( 2016 ) as part of their Blue Growth initiative where they forecast that surging demand for fish and fishery products will mainly be met by growth in supply from aquaculture production, which is expected to reach 102 million tonnes by 2025 the adoption of such frameworks will be key to addressing the challenges in adopting suitable governance and regulatory frameworks.

A component of this evolution will be a move away from a reliance on medicinal treatments and chemotherapeutants to manage sea lice infections. In most salmon farming areas, widespread use of chemotherapeutants even where this has been part of a well coordinated treatment plan has led to reduced sensitivity or even resistance to treatment (Carmichael, Bron, & Taggart, 2013 ; Jones, Sommerville, & Wotten, 1992 ; Lees, Baillie, Gettinby, & Revie, 2008 ; Rae, 2002 ; Roth, Richards, Dobson, & Rae, 1996 ; Sevatdal, Copley, Wallace, Jackson, & Horsberg, 2005 ; Treasurer, Wadsworth, & Grant, 2000 ). From 2009 to 2015, the use of chemotherapeutants for lice treatments has increased. This best illustrated by the figures from Norway (Table 1). Except for the flubenzurones, in Norway all approved chemotherapeutants now show reduced effect, including hydrogen peroxide. Concerns have also been voiced about increasing the sea lice’s freshwater tolerance through freshwater treatments and the potential for selecting for increased virulence through farm management practices (Ugelvik, Skorping, Moberg, & Mennerat, 2017 ). Genetic studies have shown that resistance has the potential to spread rapidly throughout the north Atlantic salmon lice populations (Besnier et al., 2014 ). Thus, it can be assumed that the resistance situation seen in Norway may develop at any time in the rest of the north Atlantic salmon farming industry, and represents a threat to the development of the industry (Aaen, Helgesen, Bakke, Kaur, & Horsberg, 2015 ). There are a number of ways to maintain effective medical treatments using currently approved and available medicines; these include increasing the dosage level and using the drugs off‐label and use of combination treatments of two or more therapeutants. In reality, both approaches have been practised. Both these approaches must be considered problematic from a risk assessment standpoint, as the authorities must approve of combinations of therapeutants, through an assessment of the associated environmental and animal welfare implications. In Norway, as is the case in Ireland and other EU member states, each individual chemotherapeutant must have an authorization approved by the Medical Products Agency. Similarly, if two or more treatments are to be used in combination, new approval is required. Whilst there are many studies investigating the effect of commonly used chemotherapeutants, effects on non‐target species of drugs used in combinations are at best poorly understood.

2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015
Azamethiphos 66 1,884 3,346 2,437 4,059 3,037 4,630 3,904
Cypermethrin 55 45 49 30 32 88 107 48 232 211 162 85
Deltamethrin 17 16 23 29 39 62 61 54 121 136 158 115
Diflubenzuron 1,413 1,839 704 1,611 3,264 5,016 5,896
Emamectin 32 39 60 73 81 41 22 105 36 51 172 259
Teflubenzuron 2,028 1,080 26 751 1,704 2,674 2,509
Total 104 100 132 132 218 5,516 6,455 3,374 6,810 8,403 12,812 12,768
Hydrogen peroxide (tons) 308 3,071 3,144 2,538 8,262 31,577 43,246

Since 2007, there has been a focus on using a combination of azamethiphos and pyrethroids (deltamethrin/cypermethrin), which has been shown to have an increased effect on salmon lice, as compared to being used separately. The toxicity of a combination solution of azamethiphos and deltamethrin was assessed in a laboratory study on two species of crustaceans; the chameleon shrimp (Praunus flexuosus) and grass prawn (Palaemon elegans; Brokke, 2015 ). This is the first toxicity study where azamethiphos and deltamethrin were tested in combination. The survey was based on a concentration ratio between azamethiphos and deltamethrin of 60:1 when the two drugs were used together. Both species were exposed for 1 and 24 hr and further observed for an additional 24 hr. Both the common prawn and purse shrimp were significantly more sensitive to the drugs when used in combination. This study showed that non‐target species may experience increased mortality from combination treatments as compared to treatments where therapeutants are used separately.

The current situation in respect of reduced efficacy to key therapeutants in certain farming areas has spurred the development, and use of a number of non‐medical treatments. These include physical removal, cleaner fish, and use of freshwater or other high tech solutions including the use of lasers and “snorkel barriers” (Stien et al., 2016 ). Sletmoen ( 2016 ) gives a useful review of some of the more recent initiatives currently under development. However, at present, the salmon farming industry still depends largely on use of pharmaceuticals as the basis for delousing strategies. There are ongoing efforts to develop effective ways to work around problems related to resistance or increased tolerance. These include integrated pest management approaches combining husbandry techniques with the use of cleaner fish and reduced use of treatments. The incentive to gain organic status for produce and the associated price premium has helped to encourage and facilitate these developments. Early efforts to use cleaner fish centred on the use of wild caught wrasse and were initially very successful (Deady et al., 1995 ; Sayer, Treasurer, & Costello, 1996 ) but difficulties with over winter survival of wrasse, biosecurity fears in terms of transfer of pathogens from the wild caught wrasse and a lack of well‐developed husbandry practices for wrasse led to the discontinuation of their use. In the last decade there has been a resurgence in the use of cleaner fish with both wrasse and lump suckers (Cyclopterus lumpus) being utilized very successfully (Ottesen, Treasurer, FitzGerald, Maguire, & Rebours, 2012 ). There have been developments in the culture of both wrasse (D’Arcy et al., 2012 ) and lump sucker (Bolton‐Warberg, M. Pers. Comm.) and the husbandry and animal welfare protocols for cleaner fish use are much better developed. The effective use of such biological control methods as a means of reducing reliance on treatments is a vital component of an integrated pest management strategy.

3 RISK ANALYSIS TOWARDS AN ECOSYSTEM‐BASED MANAGEMENT APPROACH TO SEA LICE INFESTATION

Both the aquaculture industry and its regulators must make decisions which could potentially have major consequences based on incomplete knowledge and with varying degrees of uncertainty. Use of risk analysis in aquaculture development and management is relatively new (Bondad‐Reantaso, Arthur, & Subasinghe, 2008 ). Nevertheless, several protocols exist for estimating environmental risks arising from aquaculture. NOAA developed guidelines for ecological risk assessment of marine fish aquaculture in 2005 (Nash, Burbridge, & Volkman, 2005 ). This work was further developed by FAO (Nash, Burbridge, & Volkman, 2008 ) who presented broader guidelines for understanding and applying risk analysis in aquaculture in 2008. The same year GESAMP (Anon., 2008 ) provided guidelines on environmental risk assessment and communication in coastal aquaculture. The challenge posed in respect of the management of sea lice infestation is the balancing of one potential hazard, the risk of adverse environmental impacts from the increased use of chemical therapeutants for treatment of sea lice in marine salmon aquaculture, against multiple other risks including, animal welfare, the potential for impacts on wild fish stocks and the requirement to conserve the efficacy of available active compounds by limiting resistance development.

These frameworks for risk analysis have been described under the general heading of Driver, Pressure, State, Impact, Response (DPSIR) and are used by many as a methodological approach to assessing complex risk matrices. Phillips and Subasinghe ( 2008 ) identifies three different approaches to identification of hazards; single site—range of risks; single hazard—several exposure scenarios; and multiple hazards—multiple risks. Gregory, Atkins, Burdon, and Elliot ( 2013 ) propose the use of the DPSIR approach as a suitable modelling framework for problem structuring for marine management. The DPSIR approach seeks to balance pressures and to manage outcomes and therefore advocates a flexible system of decision‐making, guided by individual risk assessments of the cost/benefit of action/inaction. The tool was developed to analyse environmental problems arising from human activities with an objective to assist in achieving sustainable development (Gari, Newton, & Icely, 2015 ). The European Environment Agency also advocates the use of DPSIR as an appropriate decision‐making aid (European Environment Agency, 2014 ). The DPSIR approach (Figure 2.) provides a framework to assess the causes, consequences and responses to change in a complex adaptive system in a systematic way. It also provides a basis for linking policy objectives, environmental risks, benefits and local needs in a decision‐making tool. Krause et al. ( 2015 ) argue for a similar type of iterative feedback process to address the information and knowledge exchange gap which exists between policy makers and the other stakeholders in the aquaculture sector at a global level. Such a tool would provide an objective framework within which to develop risk assessments such as the current model employed in Norway (Taranger et al., 2015 ). This concept is set out graphically in Figure 3. The key elements of this framework in respect of sea lice management are as follows: fish health and welfare (Bergqvist & Gunnarsson, 2013 ; Huntingford et al., 2006 ; Stien, 2013 ; Turnbull, Bell, Adams, Bron, & Huntingford, 2005 ), environmental impacts of therapeutants (Haya, Burridge, & Chang, 2001 ), protection of wild stocks (Taranger et al., 2015 ), avoidance of resistance development (Aaen et al., 2015 ), production standards including organic status. In this scenario fish, health and welfare and production standards are Drivers. Impacts on the environment and interactions with wild stocks are Pressures with possible impacts. The Impacts and State parameters would include resistance development, impacts on non‐target organisms and damage to wild stocks. Assessment of the relative risks under each of these headings will inform the appropriate Response in each case. This framework allows for a more holistic and case‐specific approach to the management of this very important parasite and is an appropriate framework for implementing an integrated sea lice management (ISLM) strategy. An integrated sea lice management strategy where husbandry and technological solutions are the main tools to control the parasite and medicines and chemotherapeutants are only employed when required and in limited circumstances is the objective. To be successful, this will require a more nuanced approach to management in terms of the timing of control activities, the thresholds at which controls are applied and the responses to difficulties in maintaining control.

4 CONCLUSIONS

To underpin any integrated sea lice management (ISLM) approach on a medium to long‐term basis, it is necessary to balance pressures and outcomes and ensure a holistic approach to determining appropriate responses when managing sea lice infestations on farmed stock. We argue that a robust and sustainable approach to sea lice management which will be fit for purpose for a global salmon farming sector requires a problem structuring methodology such a DPSIR. The development of a DPSIR tool to underpin specific ISLM programmes for individual bays or control areas can provide an objective and evidence‐based decision‐making process capable of assessing competing pressures and impacts and providing balanced advice to managers and decision‐makers. Such a framework will be fundamental to advancing aquaculture production, to promote policies and good practices for farming of fish, shellfish and aquatic plants in a responsible and sustainable manner as set out in the FAO Blue Growth Initiative (FAO, 2016 ) and addressing the key challenges identified by the European Union to the sustainable development of European aquaculture (Lane, Hough, & Bostock, 2014 ).

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