Multi-use of the sea: from research to practice

. The increasing demand for ocean resources exerts an increasing pressure on the use of ocean space across all European Sea Basins. This underlines issues of compatibility (or conflicts) between different maritime uses as well as between economic activities and environmental protection. The idea of multi-use (MU), as a guiding concept for efficient allocation of compatible activities in the same marine space, can increase spatial efficiency and at the same time provide socio-economic and environmental benefits. However, its transition from a concept to real-world development is facing several barriers. Based on analysis of five European sea basins done under the Horizon 2020 MUSES project (Multi-Use in European Seas), this paper aims to clarify the concept of MU by discussing: 1) the definition in the literature and practice so far, and; 2) how existing regulatory and planning regimes are supporting and challenging the development of several MUs (considered as the most promising). The analytical methodology developed for the MUSES project relied on data collected via desk research and semi structured interviews with key stakeholders (e.g. industry, regulators), over the period of seven months. The semi-quantitative analysis of data conducted, identified the commonalities and differences among countries in respect to each of the analyzed MUs. The paper points out priorities for the MU development in different sea basins and recommends initial steps to overcome existing barriers, whilst maximizing local benefits. This paper is a starting point towards a broader scientific debate on: (i) what could be the role of management policies (like for instance maritime spatial planning - MSP) in supporting and fostering MU concept development, (ii) what are technical and technological challenges for technically advanced MUs, (iii) how added values of MUs concept (e.g. benefits for local economies, positive impacts on environment) could be enhanced.


Introduction
Multi-use (MU) at the sea is a relatively new research topic that has emerged from three distinctive sources.The first one is research and innovation, that prompts out development of new technologies offering novel ways of exploitation of sea resources and improvement of its conservation measures.Several research projects [1] have been conducted e.g. to investigate the possibility of establishing multi-purpose off-shore platforms, serving needs of off-shore energy production, mariculture or sea tourism and even regular navigation.Some combinations have been researched even more extensively such as off-shore wind energy and aquaculture and substantial documentation exists in this field (e.g.[2], [3]).The second source is business itself, e.g.tourism in the Mediterranean region considering new opportunities such as pescatourism [4] or underwater culture heritage.This provides possibilities for combinations such as tourism and fishery or tourism and protection of underwater artifacts.The third source is scarcity of space.The emergence of maritime spatial planning [5][6][7][8][9][10][11][12][13] made it evident that marine space is not abundant and it should be treated as a scarce resource [14].Therefore, MSP urges for sparing use of the sea space [15] and MU is among the considered solutions.Some scholars see MU as an analogue to the economies of scales that drive terrestrial spatial development [16].

The essence of Multi-Use
The discussions and developments of MU of marine resources in the political and academic arena have generated a variety of terms to describe the context.Each nomenclature is trying to capture and convey important information about the particularities of their investigated scenario.As a result many differing terms have emerged during the last 15 years for the same concept idea: co-and translocation, multi-and multi-functional use, co-use, secondary and additional use and coexistence to name a few.
The information conveyed by these terms can cover every dimension from legal and business relationships of users to even temporal and physical aspects of the multi-use relationship.
According to a definition elaborated within the MUSES project, 'multi use' (MU) is considered as a "joint use of resources in close geographic proximity".The term is an umbrella term covering a multitude of use combinations in the marine realm and representing a radical change from the concept of exclusive resource rights to the inclusive sharing of resources by one or more users.This can involve either a single user or multiple users.The use means distinct and intentional activity through which a direct (e.g.profit) or indirect (e.g.nature conservation) benefit is drawn by one or more users.The user means individual, entity or group that intentionally benefits from a given resource, and the resource is understood as a good or service that represents a value to one or more users (e.g.biotic, such as fish stocks; or abiotic, such as ocean space) and can be exploited through either direct (e.g.fishing) or indirect (e.g.nature conservation) uses [1].
It is often difficult to differentiate between genuine MU and the mere coexistence of several uses.For example, ships and fish use the same seawaters.However, this should not be considered as a MU, even though the condition of lack of exclusivity is fulfilled.It's doubtful whether this joint use of resources is intentional (rather than coincidental) and is beneficial to both parties.Also diving in wreck sites should not be considered as a MU.However, diving in the intentionally prepared and maintained underwater sanctuaries is considered as a MU, because it is based on conscious decisions and provides benefits to both users.The first type means that multi-use of marine resources refers mainly to the geographical connection of resource uses to create benefits for society and single actors.An example of such a multi-use is the combination of offshore wind and tourism through boat tours viewing the offshore wind farm [17].
The second type means even closer (functionally and geographically) integration of uses to create even more added value than a side-by-side scenario.This closer integration looks for synergies in integrating the operations and implementation of offshore activities and can start by e.g. the simple sharing of the use of offshore supply vessels to reduce individual operations costs.The synergistic integration of activities culminates in multi-use platforms.MU offshore platforms are engineering solutions, designed to incorporate modules of other compatible activities (e.g.TROPOS Project).A fully integrated multi-component and multi-purpose offshore platform serves as a main infrastructure shared by two or more ocean uses (e.g.H2Ocean project designed a platform coupling renewable energy harvesting + hydrogen generation + aquaculture + environmental monitoring) [18].
In terms of sequence in which the development occurs, two scenarios of MU creation are considered as presented in Figure 1.

Joint development of uses
MU where two (or more) combined uses (from the blue growth sector i.e. aquaculture or offshore wind) are applying for licenses at the same time

Staggered development of uses
One existing (traditional) use is already in place and the new (emerging) one is coming in MU where one sector is already in place (e.g.underwater heritage protection) and is being combined with the new use (e.g.tourism)

Multi Use as research subject
Several types of MU combinations have been researched in a wide variety of possible MU combinations, all of them at different stages of their maturity and feasibility.The list of combinations (Tab.1) was compiled after identifying combinations that have been analysed by past projects.A total of 26 case studies analysed in past projects (e.g.MARIBE, MERMAID, H2Ocean and TROPOS) have resulted in 11 uses considered as MU.Source: own elaboration by SUBMARINER and AWI Figure 2. illustrates that combinations may differ in terms of their potential/feasibility and time of appearance.Some of them are very probable in the near future, some may be possible in several years' time, and others are not likely to occur at all.However, this matrix is indicative of the complexity of the MU research.

The researched sea basins
It is evident that MUs might differ in the EU sea basins due to their specific features facilitating development of some uses and hindering others.Five distinctive sea basins are defined in the EU sea waters if the outermost regions are not included: the North-Eastern Atlantic (EA), the North Sea (NS), the Baltic Sea (BSR), the Mediterranean Sea (Med) and the Black Sea (BS) (Fig 3).Each of these sea basins is characterized by different physical conditions resulting in different uses of sea resources.However, despite obvious differences, several common trends important for MU development are observed: 1) sectors dominating in the given sea basin seem to strongly influence development of MU, 2) environmental assets tend to have a more important role in allocation of the sea space to particular uses, 3) local and regional economic development is a driving force for local MU initiatives.

Fig. 3. Sea basins analysed under MUSES project (drawing on [1])
A brief comparison between sea basins is provided in Table 2. Evidence is clear that some physical conditions (wind potential) support multi-use based on wind farms in the North Sea and Eastern Atlantic, whereas high temperature signifies an importance of tourism and possibility to combine tourism with other activities in the Black Sea and Mediterranean Sea.Low salinity hinders mussels aquaculture in the Black Sea and the Baltic.Source: own elaboration by MIG Availability of space is also a relevant factor influencing the development of MU.In small sea areas where space is scarce, MU might be seen as an opportunity to use space in a more efficient way.In the oceans and other deep sea areas, MU might be driven mainly by the economic benefits of such an approach (e.g.offshore MU platforms) rather than spatial efficiency.

The most relevant multi-use combinations for each sea basin
Analyses were conducted at three geographical scales: -Scale 1 -Intra-country scale: within single country; -Scale 2 -Basin/sub-basin scale: sum of findings from all countries within a basin or sub-basin; -Scale 3 -Trans-boundary scale: two or more countries.The analyses were conducted with the use of various research methods including desk-based review of relevant regulations (international to local levels), project reports and case studies, scientific reports, workshops and interviews with stakeholders associated with marine planning in general and MU in particular.Relevant data was collected at country-level and results aggregated and analysed at sea basin level.
The stakeholders' preferences for individual MUs were revealed in the course of in depth interviews.The stakeholders' opinions were confronted with the previous desk research findings and related to the sectoral experience with MU development and to some extent also with the policy will in promotion of MU.As the result, the combinations were prioritised taking into account that at least one sector has been already existent (and preferably demonstrated some MU experience) and some policy will was in place (e.g. for tourism, fishery and aquaculture in the Mediterranean sea basin).The top three combinations per sea basin are presented in Table 3.
Altogether, in all five sea basins, 14 MU combinations have been identified as existing or having potential.Six of them have been selected as the most relevant in the sea basins at least in one of the sea basins (Table 3).
Table3.The most relevant MUs selected in the sea basins analysis Note: blue number indicates the number of countries within the sea basin in which the given MU exists, orange number indicates the number of countries in which the given MU has potential as one use is already in place.
MU The fourth MU in terms of practical deployment is Offshore Wind and Aquaculture, that has been tested or exists in six countries (though in some cases energy is mainly a supplement to existing aquaculture with no ambition to produce energy for sale).
However, in terms of future development, the picture looks quite different.The biggest expectations are formulated by stakeholders towards Offshore Wind and Aquaculture as well as Underwater Cultural Heritage and Tourism and Environmental Protection.Both MU combinations exist or have development potential (with one use already in place) in 13 and 16 countries respectively.Both of these MU combinations have been prioritised in four sea basins, however the first type of MU is not so prominent for the Black Sea and the second one for the North Sea basin.This can be explained at least partially by physical characteristics of these sea basins and their policy specificities in terms of blue growth.Wind energy is not a priority in the Black Sea whereas in the North Sea underwater cultural heritage is not regarded as a development driver (i.e. the following sectors take a lead in blue economy: commercial fisheries, oil and gas production, shipping and maritime transport, tourism and offshore renewable energy development).

Conclusions for further research
Selection of the most important MUs for each sea basin seems only a top of an iceberg.There is a need for further research in order to make the MU concept operational.To summarize the findings from this paper, the following topics need further detailed research: 1. Researching MU in the context of resilience of marine ecosystems, since so far the economic and planning perspective prevails in researching MU. 2. Establish the economic value of different combinations which might be challenging due to important externalities related to them.Such research can change opinions of stakeholders and give space for new priorities in relation to MU deployment.3. Better understanding of stakeholders' opinions with regard to MU.For instance in the Mediterranean sea basin, combinations related to tourism were prioritised as the most relevant due to their prevalence and importance for almost all countries in the given sea basin.Whereas combinations related to offshore wind scored high due to high probability of France to increase investment in multitrophic aquaculture combined with floating wind turbines that might offer EU breakthrough for this MU.Those peculiarities deserve more in depth analysis.4. Analysing possible deployment paths of the most promising MUs in the sea basins, in particular the combinations related to offshore energy and tourism as driving sectors.The assistance should be tailored to the maturity level of the supported combinations and the size of barriers hindering their development.Also sea basin specificities must be taken into consideration.The support must be adjusted to the macro regional needs.Casting support for MU development requires prior understanding of the reasons behind prioritisation of some uses by macro regional experts.
All of the aspects mentioned above call for further research.In order to support the MU approach in a conscious way -i.e. to move from research to practice -a different approach seems necessary.Previous research has been focusing on the technical aspects of MU deployment.To complement this research, the social aspects need to be further investigated, using behavioral economics, business anthropology and other fields of social science.Applied research covering stakeholders' motivations and attitudes, describing drivers and barriers and identifying feasible policy solutions is essential for successful MU deployment in the future.
Recognizing the multitude of possible multi-use scenarios in European seas, two essential types of MUs are defined: a) Multi-use of geographical, human, biological resources b) Multi-use of technical resources (marine infrastructure & platforms)

Fig. 1 .
Fig. 1.Two possible scenarios for the sequence of multi-use developments.Source: own elaboration SUBMARINER and AWI

Fig. 2 .
Fig. 2. Feasibility of multi-uses combining two uses.Source: own elaboration by AWI and SUBMARINER

Table 1 .
MU combinations identified in the international projects Offshore RenewablesAquaculture (biomass and fish), Monitoring of the sea environment (marine mammals, fish and bird life) TROPOS (Mediterranean, Tropic, Sub-tropic, Lead: PLOCAN) Project ID 288192 Maritime transport (offshore port and base Fisheries (service station, storage), Aquaculture (fish), Energy (solar and ocean wave),

Table 2 .
Overview of prevailing physical conditions in the analysed sea basins Source: own elaboration by Maritime Institute in GdańskOut of eight in-depth analysed MUs, the most frequent (in terms of appearance as the existing in EU countries, see table3) have been the three related to tourism (MU2, MU4 & MU6).The combination of Fisheries and Tourism and Environmental Protection has been tested or established in ten countries and within three sea basins in which tourism is a driving force for blue growth.Also, the combination of Underwater Cultural Heritage and Tourism and Environmental Protection has often occurred (ten countries) in four out of five sea basins.The third most frequent MU is Aquaculture and Tourism -existing (according to the MUSES categorisation)| in six member states of Southern Europe located in the Mediterranean and East Atlantic sea basins.