Kelp forests dominate the rocky coasts of temperate Australia and are the foundation of the Great Southern Reef. Much like terrestrial forests, these marine forests create complex habitat for diverse communities of flora and fauna. Kelp forests also support coastal food-webs and valuable fisheries and provide a suite of additional ecosystem services. In many regions of Australia and around the world, kelp forests are in decline due to ocean warming, overgrazing, and pollution. One potential tool in the conservation and management of these important ecosystems is habitat restoration, the science and practice of which is currently undergoing substantial expansion. We summarize the present state of Australian kelp forests and emphasize that consideration of the initial drivers of kelp decline is a critical first step in restoration. With a focus on Australian examples, we review methods, implementation and outcomes of kelp forest restoration, and discuss suitable measures of success and the estimated costs of restoration activities. We propose a workflow and decision system for kelp forest restoration that identifies alternative pathways for implementation and acknowledges that under some circumstances restoration at scale is not possible or feasible. As a case study, we then apply the Society for Ecological Restoration’s 5-star evaluation to Operation Crayweed, Australia’s primary example of kelp forest restoration. Overall, no single method of kelp forest restoration is suitable for all situations, but outcomes can be optimized by ameliorating the driver(s) of kelp decline and achieving ongoing natural recruitment of kelp. Whilst scalability of kelp forest restoration to the seascape-scale remains a considerable challenge, the present review should provide a platform for future restoration efforts. However, it is also crucial to emphasize that the challenges of restoration place a high value on preventative conservation and protection of existing kelp forest ecosystems – prevention is invariably better than cure.

Marine ecosystems and their associated biodiversity sustain life on Earth and hold intrinsic value. Critical marine ecosystem services include maintenance of global oxygen and carbon cycles, production of food and energy, and sustenance of human wellbeing. However marine ecosystems are swiftly being degraded due to the unsustainable use of marine environments and a rapidly changing climate. The fundamental challenge for the future is therefore to safeguard marine ecosystem biodiversity, function, and adaptive capacity whilst continuing to provide vital resources for the global population. Here, we use foresighting/hindcasting to consider two plausible futures towards 2030: a business-as-usual trajectory (i.e. continuation of current trends), and a more sustainable but technically achievable future in line with the UN Sustainable Development Goals. We identify key drivers that differentiate these alternative futures and use these to develop an action pathway towards the desirable, more sustainable future. Key to achieving the more sustainable future will be establishing integrative (i.e. across jurisdictions and sectors), adaptive management that supports equitable and sustainable stewardship of marine environments. Conserving marine ecosystems will require recalibrating our social, financial, and industrial relationships with the marine environment. While a sustainable future requires long-term planning and commitment beyond 2030, immediate action is needed to avoid tipping points and avert trajectories of ecosystem decline. By acting now to optimise management and protection of marine ecosystems, building upon existing technologies, and conserving the remaining biodiversity, we can create the best opportunity for a sustainable future in 2030 and beyond.

Habitat forming ‘ecosystem engineers’ such as kelp species create complex habitats that support biodiverse and productive communities. Studies of the resilience and stability of ecosystem engineers have typically focussed on the role of external factors such as disturbance. However, their population dynamics are also likely to be influenced by internal processes, such that the environmental modifications caused by engineer species feedback to affect their own demography (e.g. recruitment, survivorship). In numerous regions globally, kelp forests are declining and experiencing reductions in patch size and kelp density. To explore how resilience and stability of kelp habitats is influenced by this habitat degradation, we created an array of patch reefs of various sizes and supporting adult Ecklonia radiata kelp transplanted at different densities. This enabled testing of how sub-canopy abiotic conditions change with reductions in patch size and adult kelp density, and how this influenced demographic processes of microscopic and macroscopic juvenile kelp. We found that ecosystem engineering by adult E. radiata modified the environment to reduce sub-canopy water flow, sedimentation, and irradiance. However, the capacity of adult kelp canopy to engineer abiotic change was dependent on patch size, and to a lesser extent, kelp density. Reductions in patch size and kelp density also impaired the recruitment, growth and survivorship of microscopic and macroscopic juvenile E. radiata, and even after the provisioning of established juveniles, demographic processes were impaired in the absence of sufficient adult kelp. These results are consistent with the hypothesis that ecosystem engineering by adult E. radiata facilitates development of juvenile conspecifics. Habitat degradation seems to impair the ability of E. radiata to engineer abiotic change, causing breakdown of positive intraspecific feedback and collapse of demographic functions, and overall, leading to reductions in ecosystem stability and resilience well before local extirpation.

Ectoparasites can reduce individual fitness by negatively affecting behavioural, morphological and physiological traits. In fishes, there are potential costs if ectoparasites decrease streamlining, thereby directly compromising swimming performance. Few studies have examined the effects of ectoparasites on fish swimming performance and none distinguish between energetic costs imposed by changes in streamlining and effects on host physiology. The bridled monocle bream (Scolopsis bilineatus) is parasitized by an isopod (Anilocra nemipteri),which attaches above the eye. We show that parasitized fish have higher standard metabolic rates (SMRs), poorer aerobic capacities and lower maximum swimming speeds than non-parasitized fish. Adding a model parasite did not affect SMR, but reduced maximum swimming speed and elevated oxygen consumption rates at high speeds to levels observed in naturally parasitized fish. This demonstrates that ectoparasites create drag effects that are important at high speeds. The higher SMR of naturally parasitized fish does, however, reveal an effect of parasitism on host physiology. This effect was easily reversed: fish whose parasite was removed 24 h earlier did not differ from unparasitized fish in any performance metrics. In sum, the main cost of this ectoparasite is probably its direct effect on streamlining, reducing swimming performance at high speeds.

The biodiversity benefits of kelp aquaculture and afforestation are increasingly acclaimed as the industry continues to grow and develop globally, however, whether farmed kelp can provide this ecosystem service remains unclear. Using peer-reviewed literature, we evaluated whether kelp farms provide biodiversity benefits, and identified only 23 studies that discussed the effects of kelp aquaculture on biodiversity, half of which were broad reviews that only assessed the concept of ‘biodiversity’ peripherally (e.g. did not focus on specific responses or taxa). There is also a general lack of experimental research on the topic. Based on the evidence, it seems that kelp farms can create habitat via changes to the local environment, particularly through the provision of structure and changed nutrient cycling. While this can lead to increased abundance and diversity among certain taxa (e.g. fouling organisms), it seems that kelp farms typically create novel habitats that support distinct communities not equivalent to natural kelp forests. Moreover, the potential for kelp farms to support biodiversity depends on a range of operational factors, many of which may be at odds with farming objectives that require the harvest and removal of the habitat that farms provide. While more work needs to be done to address the complexity of comparisons between kelp farms and forests, especially at appropriate experimental scales, it currently seems unlikely that kelp farms will act as kelp forests and deliver meaningful biodiversity outcomes. We should instead recognise farms for providing their own valuable services and support restoration and conservation practices of kelp forests to pursue biodiversity outcomes.

Global seaweed aquaculture production has more than tripled since 2002 and is dominated by Asian countries with farming operations that typically occur in relatively wave‐protected, nearshore areas. To meet future demand, production must move to “non‐traditional” regions and into less contested waters offshore. However, the technological complexities and uncertainties in the performance of seaweed cultivated in high‐energy offshore environments are substantial and must be overcome. Here, we identify knowledge gaps and suggest a research roadmap to inform the advancement of a commercial offshore seaweed aquaculture industry using southern Australian species as case studies: (1) Durvillaea spp. (order Fucales); (2) four kelps (order Laminariales); and (3) the rhodophyte Asparagopsis. These groups lie along a spectrum of commercial viability and readiness for offshore aquaculture, and key knowledge gaps are cultivation technology and the suitability to offshore conditions. Cultivation of Durvillaea is restricted by a low level of biological and technical understanding, but there is high market potential and readiness. For laminarian kelps, commercial production in nearshore conditions is already occurring elsewhere, which make them the most likely candidate for offshore cultivation in the medium term. Asparagopsis is least suited to offshore conditions, with substantial gaps in general cultivation knowledge, and its cultivation is likely to be restricted to land‐based systems or relatively sheltered nearshore waters. The knowledge gaps identified here will inform research and development programs to advance offshore seaweed aquaculture in southern Australia and globally.