Lost at sea? The Atlantic salmon’s ocean odyssey

The golden eagle, red deer, red squirrel, otter, harbour seal and Atlantic salmon are iconic images of Scotland’s wildlife. The salmon’s life cycle and migrations have fascinated mankind for generations, but now something has gone wrong. Salmon are dying at sea; rivers all around the North Atlantic have been affected but, until recently, little was known about the migration patterns and distribution of salmon at sea and the factors affecting them. New research has provided fascinating insights into why salmon are being lost at sea. This discussion forum, attended by nearly 200 participants, focused on the challenges, new and old, faced by Atlantic salmon during their ocean odyssey. The event was supported by the Cromarty Firth Fisheries Trust, the Fishmongers’ Company, the North Atlantic Salmon Conservation Organization (NASCO) and Scottish Natural Heritage (SNH).

Introduction

The President of the RSE, Sir John Arbuthnott MRIA FMedSci, welcomed delegates to the discussion forum. He indicated that the RSE had been established in 1783, by the order of King George III, with the objective of discovering and developing new knowledge and its useful application. Today, the RSE covers all disciplines, including law, theology, medicine and ecology. He highlighted the remarkable life-cycle of the Atlantic salmon and the mystery surrounding how a small fish can leave fresh water, adapt to oceanic conditions and then migrate thousands of miles through the North Atlantic before returning precisely to its river of birth. The discussion forum would provide an opportunity to present the findings from recent research, aimed at unravelling some of the secrets of this remarkable journey.

Professor Michael Usher OBE FRSE, Chairman of the discussion forum, welcomed participants on behalf of the Steering Committee (Dr John Baxter, Dr Peter Hutchinson and Lord David Nickson). He noted that the salmon is an iconic part of Scotland’s wildlife and is one of the few species living in the wild with a direct economic value. He referred to the salmon catch statistics published by the Scottish Government for the period from 1952 to 2012. These show that the annual rod catch during this period has remained relatively constant (approximately 60,000 salmon) but the net catch has declined twenty fold (from approximately 400,000 to 20,000). He raised a question – ‘where have all the salmon gone’?

Dr John Baxter, SNH, and Dr Peter Hutchinson, NASCO

Dr Hutchinson indicated that the Atlantic salmon is a small resource compared to other marine fish species. At its peak, the total catch for the entire North Atlantic was only approximately 12,500 tonnes and today is about one tenth of that level. However, he stressed that it is a highly-prized and valuable resource with many facets to its value. Expenditure by anglers in Scotland is approximately £73 million each year but, additionally, people care about the species, an indicator of a healthy aquatic environment, and they are willing to pay to conserve it even if they have no interest in fishing for it. Through international negotiations run from Edinburgh, harvests of salmon in the distant-water fisheries, far from the salmons’ natal rivers (for example, off West Greenland and around the Faroe Islands), have been dramatically reduced and there have been major reductions in fishing effort in homewaters all around the North Atlantic, particularly in sea fisheries. Despite all these sacrifices, scientists have estimated that the abundance of salmon in the ocean, prior to any fisheries, has halved from around seven million fish in the mid-1980s to around 3.5 million today. This decline has been most marked for salmon in southern areas and for those fish spending more than one winter at sea. In some rivers in the southern parts of its range, the Atlantic salmon faces extinction. Whilst production in freshwater is relatively stable, there has been a marked increase in mortality at sea. He reported that a recently completed international research programme, deploying innovative new technologies, has started to unravel some of the mysteries of the salmon’s ocean odyssey and has highlighted the consequences of the changing climate for salmon at sea. Management options in the ocean are limited, so there is a need to ensure that man’s activities in fresh and coastal waters do not exacerbate the situation. Recent research has provided new tools to support conservation; for example, in identifying stocks contributing to mixed-stock fisheries and in quantifying bycatch of salmon in large-scale fisheries for pelagic fish. He concluded that, as one salmon conservationist has noted, ‘Mother Nature has thrown down the gauntlet; she must not be allowed to throw in the towel’.

Professor Ken Whelan, Atlantic Salmon Trust

Professor Whelan noted that, over the past three decades, an increasing proportion of Atlantic salmon have been dying at sea during their oceanic feeding migration and that, arguably, the greatest challenge in salmon conservation is to gain insight into the spatial and ecological use of the marine environment by different regional and river stocks. The decline in abundance is particularly marked for southern stocks on both sides of the Atlantic. In response, NASCO’s International Atlantic Salmon Research Board (IASRB) has concluded that only by better understanding where the salmon are at sea, and how they get there, will it be possible to identify the factors influencing their survival. The Board has, therefore, developed an international programme of cooperative research, the SALSEA Programme. He indicated that this is the most ambitious Atlantic salmon research programme ever undertaken and it seeks to increase understanding of how Atlantic salmon use the ocean; where they go; how they use ocean currents and the ocean’s food resources; and what factors influence migration and their distribution at sea. It has been possible because of the development of bespoke sampling tools designed to tackle the breadth and scale of the challenges faced by researchers. These include sophisticated genetic techniques to assign salmon sampled at sea to their region or river of origin and novel trawl gear and tagging devices. In the northeast Atlantic, the SALSEA Merge project involves a consortium of 20 organisations, and some of the main findings are as follows:

• the distribution pattern of specific populations of salmon has been spatially mapped at different genetic assignment levels and the likely migration routes for some individual river stocks e.g. the Loire Allier (France) and the River Bann (Northern Ireland), have been mapped;
• the distribution of post-smolts is closely linked to ocean currents, and inter-annual variation in wind fields, and thus the surface currents, alter the migration pathways. Several key areas in the migration routes, where shifts in the migration direction may occur due to climate change, have been identified;
• marine growth rates vary from year to year, but were higher in 2002 and 2003 than in 2008 and 2009. Growth rates during the first period at sea are lowest for salmon of southernmost origin;
• the increased mortality at sea is linked to ocean warming and changes in the food supply.

Professor Whelan concluded that the SALSEA Programme has provided valuable new insights into the distribution and migration of salmon at sea. This research has highlighted the overlap between salmon and large-scale fisheries for pelagic species, such as mackerel, and the potential for bycatch and, consequently, protected migration corridors might be needed to safeguard the salmon at certain times of the year in certain locations. Through NASCO, a new research programme is being considered, based on tracking technology, to attempt to partition marine mortality. Additionally, regular monitoring of post-smolts is required, and the opportunities to integrate such studies into ongoing surveys for pelagic species should be explored so as to solve the conundrum of why Atlantic salmon are dying at sea.

Professor Christopher Todd, Scottish Oceans Institute, University of St Andrews

Professor Todd described the findings from research, conducted in collaboration with Marine Scotland Science, on long-term changes in the size and quality of salmon returning to Scottish rivers over the past five decades. The theme of his presentation was The importance of a date for salmon at sea. He indicated that knowledge of salmon at sea has been derived from research vessel surveys, high seas commercial fisheries and also data storage tags that can be attached to individual fish. The latter can provide detailed information, but the logistical and practical difficulties in successfully retrieving tags in order to get the saved information are a constraint on their utility. An alternative strategy is to interrogate the fish themselves for archival information that they store regarding their time at sea.

By analysing the spacing of growth rings that are incrementally deposited on salmon scales, he indicated that it has been possible to visualise growth history, and it is clear that many individual salmon are currently encountering extended periods at sea, when their growth is markedly constrained. He noted that marine growth checks have not been observed prior to the year 2000. Importantly, a simple method has been developed for estimating a date for each growth ring on a salmon’s scales, and this allows the timing of growth hiatuses to be determined. Furthermore, his research has been able to confirm, for one sea-winter salmon returning to Scottish rivers from the Norwegian Sea, the period during the marine migration that determines the condition factor (a measure of ‘skinniness’) of individual salmon at their return from the sea. Some fish are currently up to 26% underweight for their length. This decline in condition appears to be strongly linked to a warming ocean climate, not as a direct effect of temperature but rather of changes in feeding opportunities. He concluded that these insights offer a better understanding of the growth experience of individual fish, but it is important to stress that in tracking changes in populations of returning adult salmon, information is only derived from survivors, and there is still a lack of information on the vast majority of salmon that fail (for whatever reason) to return.

Dr John Armstrong, Marine Scotland Science Freshwater Laboratory

Dr Armstrong explored the theme of Coastal migration: challenges for salmon; challenges for scientists. He noted that Atlantic salmon migrate through the Scottish coastal zone as emigrating smolts and then as returning adults, having increased dramatically in size. The outward salmon migration requires general orientation towards northerly feeding grounds, whereas the return journey requires precise location of their natal river in which they will subsequently spawn. The precise behavioural mechanisms by which salmon achieve these goals, and the routes that they follow, are not fully understood, but probably involve various senses including hearing, sight and smell, and the ability to detect the Earth’s magnetic field.

He noted that the coastal zone is a rapidly changing habitat and a powerhouse of development, as Scotland expands its aquaculture and marine renewable energy economies. Changes in the environment associated with global climate, fluctuations in fisheries and the harvesting of coastal resources can be expected to interact with salmon migrations. Assessing the extent of such interactions, the impacts, if any, and the scope for mitigation, is challenging scientists to develop an improved understanding of the biology of salmon. With regard to marine renewable energy, the potential impacting factors include: electromagnetic fields from cabling; strike and near-strike damage; noise during construction and operations; and other factors such as disorientation and increased predation risk.

He indicated that a strategy for moving forward includes integration of locational tracking technology, genetics, oceanographic modelling, and evaluation of sensory physiology to create an understanding of how salmon overcome the challenges of coastal migration. Recent experiments on Scottish homing adult salmon have involved studies of their depth distribution using satellite pop-up tags, and investigating the salmon’s response to AC electromagnetic fields. He reported that homing salmon predominantly inhabit the upper five metres of the water column, but there is variation within and/or amongst individuals; all individuals dive and the maximum dive depths imply that the full water column is used. He noted that salmon are most likely to interact with surface renewable energy devices, but could interact with devices at any depth. He concluded by describing some options for the further development of science strategies.

Discussion

During the Discussion session a number of themes were explored.

The role of disease in affecting marine mortality was discussed. Marine Scotland monitors for diseases in wild salmon and, in response to an outbreak, would take appropriate action. In recent years, red vent syndrome has been detected in wild salmon populations. The disease is caused by a marine nematode parasite, Anisakis simplex. Although unsightly, it is not believed that this parasite causes increased mortality in wild salmon.

It was noted that the decline in condition factor of salmon returning to Scottish rivers suggests that there is a marine feeding problem and clarification was sought as to whether sampling of plankton has been undertaken in the Northern Norwegian Sea. Reference was made to recent scientific studies presented at the NASCO/ICES ‘Salmon Summit’, including work based on the findings from the Continuous Plankton Recorder surveys run by the Sir Alastair Hardy Foundation for Ocean Science. There has been a northward shift in plankton, resulting in changes in species composition in areas frequented by salmon, together with changes in the size and nutritional status of plankton and of some fish species that are important prey of the salmon, such as capelin. Furthermore, the question arose as to whether there could be interactions between salmon and mackerel and herring, which are currently very abundant. It was noted that there may be competition between salmon and these species, but that whilst there is a negative correlation between salmon abundance and that of these pelagic species, proving cause and effect would be challenging and time consuming.

There was also a discussion of the importance of distinguishing between natural cycles in ocean productivity and those associated with anthropogenic effects. It was noted that there appear to be some differences in ocean productivity between the northwest and northeast Atlantic. Links have been made between the North Atlantic Oscillation (NAO) index and salmon abundance, but this index is a broad-scale summary index of complex atmospheric and oceanographic effects, and some of the indices used have been rather unconventional. When the standard accepted NAO index is used, no significant correlation between the index and salmon abundance is detected.

There was a discussion of plans for future research on salmon at sea and how this could be funded, given the significant expense involved and current pressure on research budgets. The need to better integrate future salmon research in programmes for other marine species was highlighted. It was noted that the SALSEA Programme has been attractive to funders because it has been developed by an inter-governmental organisation and involves a partnership between public and private sectors. NASCO is now considering a further programme of research intended to quantify mortality at different points along the salmon’s migration route, using acoustic tracking technology. It will be important to the success of this programme to include other species, as has been done in the Pacific Ocean, and to draw on existing tracking programmes such as the Ocean Tracking Network.

It was noted that the recent research has led to improvements in understanding the migration routes of salmon, and the question arose as to whether this could lead to increased exploitation in both legal and illegal fisheries, in the absence of marine protected areas (MPAs) for salmon. There is an ongoing initiative to identify MPAs in Scotland. These are not yet being considered for salmon and more detailed information on coastal migrations would be required before these could be defined. It was noted, however, that the NASCO Convention prohibits all fishing for salmon beyond 12 nautical miles in most parts of the North Atlantic, creating a protected area free from directed salmon fishing. Whilst there has been some fishing by vessels re-flagged to avoid the provisions of the Convention, this was successfully addressed by NASCO through diplomatic initiatives in the early 1990s and there have been no sightings since 1993.

Reference was made to different trends in abundance between rivers on the east and west coasts of Scotland, and the possible reasons for this. It was noted that it is not straightforward to investigate the causes of these different patterns. For example, sea trout are performing well in the Tweed, but very poorly in the Moray Firth and west coast regions of Scotland. Recent experimental studies involving controlled releases of batches of smolts, treated and untreated against sea lice, have provided some very valuable findings to inform management.

The SALSEA research has included development of genetic baseline data and new analytical tools that are becoming cheaper to use and will be very valuable in support of salmon management in the future. It was noted that studies of the condition factor of salmon returning to Scottish rivers have been based on samples from a mixed-stock fishery, but that the same trends have been detected in fish returning to the North Esk, indicating that the phenomenon is widespread amongst grilse (one sea-winter salmon).