There are many published papers regarding the natural science of Atlantic salmon from both sides of the Atlantic, some of which are listed below. So many papers and such interest in this species illustrates the interest in Atlantic salmon and its importance as a socio-economic resource. Like other diadromous species their survival needs straddle freshwater and marine ecosystems.

Nominal data and statistics

Protecting Atlantic salmon and reversing the decline of wild Atlantic salmon is a challenging task. Many organisations have tasked themselves with reversing the known declines, including NASCO, AST, local government bodies such as IFI, DAERA, AFBI in Ireland and other groups such as Salmon Watch Ireland and local angling clubs and community groups.  Good decision-making needs scientific data and an understanding of real time issues to produce policy and constructive actions. Fishery data will always be imperfect, and its collection is bound by resource and cost restraints. Quantitative sampling protocols are scientifically established to estimate reality. Nominal data, whether collected from fishermen or scientific survey (Fishery dependent or fishery independent) refers only to the available data. Statistical probability-based reality is key in ascertaining reality of stock dynamics.

Realistic plans can only be guided by what we know/ what we think we know on the balance of probabilities. Perhaps the best we can hope for is an assessment which shows improvement or decline and how a decline can be turned round to and improvement.  It is clear from many reports that many rivers are failing to meet their conservation limits which is likely to compound the downward population trend for salmon as fewer juveniles will be available for growth. Importantly individual rivers will have individual bottlenecks in the life cycle constraints from ova, fry , parr, smolt relative to the availability of spawning habitat in relation to nursery habitat and connectivity between the two.

Current Models

Current fishery management protocols are generally based on spawning stock biomass (SSB) and maximum sustainable yield (MSY).  Salmon unlike other ocean pelagic fish (e.g. mackerel, herring, blue whiting ) are anadromous, (Ocean and freshwater for life cycle) and carry a comparatively small number of eggs to be nurtured by deposition in river nursery habitat for conception and juvenile growth. Juvenile output from individual rivers is limited by a finite fluvial area to initiate and complete a circular life cycle from ova deposition to adult survival.

Forecasting and modelling approaches are used to provide management protocols based on the available nominal data. Whilst it is clear that there a specific knowledge gaps in understanding the population dynamics of Atlantic salmon a useful target is the conservation limit.  The conservation limit management protocol is used to manage Wild Atlantic salmon stocks which is based on nominal fishery data. The term limit refers to the minimum number of ova (converted to mature fish) required to complete the life cycle from conception in rivers to river migration of adults fish after time at sea growing to maturity. Attainment of the conservation limit for rivers is defined probabilistically on the estimated number of ova required to achieve MSY over a modelled estimate utilising nominal data. Fluvial area is an imperfect though convenient measure of juvenile salmon productivity in rivers. Fluvial area (river area in normal flows) is a limiting parameter. In other words, a river with a small fluvial area is unlikely to be as productive as a river with a large fluvial area. However, that assumption is not straight forward as many as individual rivers will have individual water quality and hydro-geomorphological characteristics. These characteristics include catchment profile, water chemistry, pH, drainage dynamics and riverbed structure determining a HGM profile.

Whilst preventing pollution and degradation of water quality is vitally important for healthy salmon rivers, the hydro-geomorphological (HGM) profile of individual rivers is also a vital consideration and neglected metric. Where water quality is optimum the quantity and spatial proximity of spawning nursery and holding habitat will determine productivity output of juvenile smolts. For example, fry and older parr preferentially select different habitat structures for optimum growth and survival.

Natural Living Assets

Natural Living Assets suggests that the CL would be more accurate if the hydro-geomorphology (HGM) profile of individual rivers were utilized as a base line measure of smolt output to specifically account for spawning habitat and nursery habitat and their proximity to each other . Rivers therefore need to be assessed in terms of quantities of specific life cycle unit habitats and spatial proximity of these to each other. For example, nursery and spawning habitat (see my research paper).

It is suggested that rivers with equivalent fluvial areas would produce a smolt output dependent on the bottlenecks defined by the HGM profile. For example, a river may contain relative limiting quantities of spawning habitat and or nursery habitat with variable connectivity between the various habitats. The HGM profile accounts for quantities, connectivity and spatial proximity of spawning and nursery habitats. Other factors such as water quality pH, hardness will also have an impact on juvenile quality.

Challenges to habitat protection and HGM profile

Competing or Inappropriate agendas for catchment management and drainage which alters riverbed structure impact spawning, nursery and holding habitat with significant negative impacts on productivity by limiting useful spawning areas and nursery areas. There are many examples of drainage channels excised which have had negative impacts on salmon habitat spawning, nursery areas and pools. The river glide, riffle, pool structure is a useful measure of these impacts which need to be understood by hydrologists and hydro geo river engineers if salmon habitat is to be protected (Life Cycle Unit Analysis). Thus, rather than promoting a HGM approach within the context of salmon management plans there may also be benefit in communicating the value of wild Atlantic salmon to those involved with infrastructure and competing water management priorities.

I argue that the HGM profile of a river would indicate where riverbed habitat provides an opportunity for juvenile productivity as spawning or nursery habitat for juvenile salmon. It will therefore be critical to ensure that suitable spawning and nursery habitat is protected in individual streams and enhanced where possible. It also needs to be stated that any HGM profile protections requires excellent or good water quality to facilitate fluvial optimal productivity.

Organisations such as the North Atlantic Salmon Conservation Organisation (NASCO), the Atlantic salmon Trust (AST), Inland Fisheries Ireland (IFI) and DAERA in Northern Ireland provide guidance on fishery management options and apply resources to develop and implement salmon development plans. They are responsible to acquire and accumulate scientific data to assist with decision making for policy.

Spawning Site Audits

Spawning site habitat can be verified by a winter redd survey.  The quality of nursery habitat can be determined by electro-fishing studies.

Redd Survey

I have been conducting redd surveys for many rivers by casual observation ever since my school days. After completing an MSC in Environmental Management with GIS in 2015 I have conducted annual surveys on two river systems with comparable catchments. Each of these catchments can be described in terms of riffle, glide, pool structure (Life cycle Unit Analysis) which can be related to potential opportunities for natural salmon production reflected by their unique water chemistry and HGM profiles.

I am of the view that Salmon management strategies should aim to protect the integrity of the Hydro- Geomorphological (HGM) profiles of individual rivers to support Atlantic salmon,….in addition to maintaining chemical water quality. A good place to start would be to audit current habitat status for the various life cycle preferred habitats across many rivers. Habitat status can be defined hydro geomorphologically as the mix of spawning, nursery and pool habitats. Nursery habitat profile can be calibrated with juvenile electro-fishing for each individual river. Most rivers throughout Ireland and NI are electro-fished regularly (WFD requirement Fish Classification Scheme, FCS 2 Protocol) and annually (semi-quantitatively) as part of the local salmon management plan to assess salmonid stocks in individual rivers. There is therefore a huge amount of data catalogued across many of Irelands’ rivers north and south ( IFI, TEGOS, AFBI, DAERA ). Spatial locations of juvenile salmon (fry and parr) counts with spatial locations are identified. Analysing the spatial location of electro-fishing sites aligned with spatial habitat profiling would provide a meaningful habitat analysis and provide a basis for a salmon habitat management and improvement plan.  

The main factors affecting the HGM profile of any river include those listed below. The WFD “programmes of measures” aim to address many of these. However, it will be necessary to fully consider the requirement of Atlantic salmon as a vital component of the aimed for good ecological status, including those rivers classed as heavily modified or not. The socio-economic value of salmon to local communities should not be underestimated.

• altitude  source
• rainfall patterns
• riparian zone land use
• catchment geology

• drainage rate
• substrate conditions / classifications
• erosion and deposition quality of sediments in relation to salmon needs.
• gradient profile
• Meander profile
• rate of spate rise and fall, in relation to weather patterns
• spate flood plain
• pH variables
• biological oxygen demand fluctuations ( seasonal, sporadic)
• temperature fluctuations

Spawning Habitat verified by redds

Redds are found in shallow gravel riffles and glides where there are gravel deposits of the correct size and suitable water velocities. Observations of salmon redds can be reliably verified through careful observation and measurement over the duration of a spawning season, if water levels and weather conditions are suitable. Verification includes observing, measuring redd dimensions:  width of the redd across the stream, length downstream, tailspill length, depth of the pot, and depth at the tail spill. I have been monitoring salmon redds since 1990 and through extensive experience, have deployed a methodology to identify and verify salmon spawning habitats. This involves systematically identifying potential spawning areas by walkover survey (A drone survey would be useful). The hydrology flows of the many small spate salmon rivers in Ireland, generally return to non-spate levels after a few days without rainfall, facilitating redd observations in spawning season. More specific measures of flow rates would provide useful data. It is important to monitor the weather closely and take advantage of periods of favourable weather to conduct assessments.

Conducting a redd survey

Practical experience shows that salmon typically spawn and cut redds within clearly defined hydro-geomorphological zones where gravel and flows are optimal—many of which have suffered historic degradation. Observations confirm that stable hydro-geomorphological conditions foster consistent annual salmon redds in specific sites. Therefore, prioritising the protection of these spawning areas and restoring historically significant sites are essential actions. Whether restoration efforts can compensate for evolutionary processes spanning hundreds or thousands of years remains an urgent question amid the ongoing Atlantic salmon crisis. Immediate action is required to safeguard remaining habitats from poorly conceived drainage measures and other widespread pressures affecting river catchments.

A redd map provides a useful aid to communication by clearly identifying redd locations spatially. A geographic location is taken using a GIS Easting and Northing reading (Garmin) recorded and displayed to produce a redd map using ARCGIS or QGIS.   Some local knowledge will help with any survey, perhaps with the support of the local angling club or community group. In many smaller spate rivers in Ireland, three consecutive rain-free days in December are typically sufficient to undertake an effective redd count. Care is always taken not to disturb fish that are in the act of spawning and measurements are only taken of vacated redds. When fish are still working the redds, they are observed at a distance and counted with estimated measurements. A spell of cold frosty weather seems to encourage spawning from late November through December and occasionally into January.

From my experience Redds are generally completed over a few days and female fish will reside in the completed redd for a few days until disturbed. Some locations are well used with many fish utilising the same spatial area, where suitable gravel and flows exist. Male fish can be seen competing and fighting around a redd area where there are good numbers of these though there are many riffles and glides with just one or two redds.  At the height of the spawning activity fish will lose their fear and will be quite visible in the shallow riffles and glides. The rivers assessed by me are all small spate rivers which rise and fall relatively quickly after rainfall. Redds are easily visible when freshly dug due to the cleaned and shiny nature of the gravel. A more experienced eye is required when the redds are less fresh and less visible due to naturally accumulated algae and darkening back to river bed normal, though the dimensions can be identified within the stream. 

Utilizing GIS mapping is a great working and communication tool. Gaining local knowledge and hydro-geomorphological (HGM) profiling facilitates accurate spatial location of spawning sites. Spawning sites are notably observed in stable gravel zones, often spatially fixed year after year.  Some redds sites will shift position due natural erosion, large spates, and deposition processes or bankside engineering interventions. Bankside and engineering operations are often carried out to protect bankside erosion and undermining of banks or to facilitate increased run off often with negative consequences for redd sites. Unfortunately, numerous Irish rivers have experienced compromised HGM profiles because of extensive drainage and riparian engineering works. Such modifications, along with broader catchment management practices, can adversely affect the integrity of spawning habitats; indeed, it is widely recognised that nearly all rivers in Ireland have undergone channel alterations, impacting the necessary hydro-geomorphological characteristics for Atlantic salmon.

The equipment required for a redd survey includes, a GIS recorder ( GARMIN), notebook, meter stick and a map and or known knowledge of main spawning areas. A close watch of the weather forecast is also vital. a pair of wellies is generally sufficient as these rivers are quite shallow after the spate run-off. From my experience, the first frost stimulates spawning activity, and surveys can be conducted after 2/3 days without rainfall in small spate streams. Although redds can be washed away in severe flooding events, many spawning sites are stable and visible for a few weeks after construction. The appropriate substrate flows are found in the same spatial areas year after year.

 Drainage operations disturb the riverbed often causing excessive erosion and an unstable riverbed with gravel movement, particularly after spates, rendering the area unsuitable of for redd creation by adult fish. In some cases, the replenishing supplies of gravel from upstream works are affected resulting in diminished gravel flows or in some cases increased gravel flows with insufficient depth.  

Table 1Necessary recording columns for data collection

Easting	Nothing	Width across stream	Length of pot downstream	Length of obvious tailspill	Depth of Pot	Depth of Tailspill

Supporting Study

The Norwegian research paper below provides robust scientific support for the critical importance of HGM profiles in sustaining viable salmon habitats

Sebastian, U.P.R.J.L., Espedal, S.E.O., Gaute, S.E.G.T.W., Hauer, V.C. and Barlaup, B.O.D.B.T., 2021. Long-term effects and cost-benefit analysis of eight spawning gravel augmentations for Atlantic salmon and Brown trout in Norway.

Supporting statements taken from this Norwegian paper ref below supporting the need for habitat controls in relation to salmon

• “Salmonids are gravel bed spawners
• “Atlantic salmon are nest builders that dig redds to lay eggs in gravel substrate with fertilization and burial of eggs in gravel substrate.
• “Characteristic spawning habitats are found in glides, often between runs or pools, and riffles on gravel sediments (grain size range 5–100 mm).
• “Water depth at these sites during median discharge is usually 0.1–1 m and water velocity 0.1 to 0.6 m s-1 for both Atlantic salmon and Brown trout (Klemetsen et al., 2003; Barlaup et al., 2008; Louhi et al., 2008).
• “As interstitial spawners (Pulg, 2009), the species depend on loose and clean gravel to provide sufficient interstitial water supply.
• “The eggs usually need 2 to 5 months to develop in the interstitial spaces, depending on temperature (Barlaup et al., 2008; Louhi et al., 2008; Pulg, 2009).
• “Eggs require average interstitial O2 concentrations (IO2) larger than 6.7 mg l-1, average grain size diameters (Dg) larger than 5.7 mm, and percentage fine sediment (PF,\1 mm grain size) lower than 18.5%. High egg-to-fry survival (50–100%) is correlated with IO2 of at least 10.4 mg l-1, Dg of at least 12.9 mm and PF of
• “Lack of spawning habitats in regulated rivers is considered a main driver of the decline of gravel bed spawning fishes (Sear & DeVries, 2008a; Hauer et al., 2018a, b).
• “River regulation may cause reduced gravel supply by blocking gravel transport from upstream reaches with physical barriers, but also by increasing transport capacity (scouring) or in the opposite, by reducing flood dynamics (reduced turnover and lack ing renewal of substrate, Barlaup et al., 2008; Pulg et al., 2013).
• “Bank stabilization, riprap, development of infrastructure in the floodplain, and other measures will reduce lateral gravel input that replaces sediments transported downstream by floods (Hauer et al., 2018a), resulting in net losses of spawning habitat and shrinking of the area suitable for gravel bed.
• “Surplus of fine sediments (grain size\1 mm) infiltrating the gravel and clogging pore space often cause a degradation of spawning habitats. Water and oxygen supply of eggs and alevins are interrupted, leading to high mortality (Soulsby et al., 2001; Greig et al., 2007; Pulg et al., 2013). Increased fine sediment accumulation can be caused by river regulation, e.g. dams and impoundments. It is accelerated by erosive land use and input of fines
• “The occurrence and distribution of salmonid spawning habitat is rather dependent on gravel supply, gravel stability, and scouring rate (Hauer et al., 2020). Negative impacts of river regulation on spawning gravel supply and dynamics can be offset by gravel augmentation. Spawning gravel augmentation has become widely spread in Europe and North America as a mitigation measure in regulated rivers (Wheaton et al., 2004; Sear & DeVries, 2008a; Barlaup et al., 2008; Pulg et al., 2019; Hauer et al., 2020; Staentzel et al., 2020). However, gravel augmentations often do not address the mechanisms underlying the decline and the availability of high-quality spawning habitats, such as dams and input of fines. Augmentations therefore have a limited life span depending on fine sediment load (Pulg et al., 2013), flood frequency, and magnitude (Merz et al., 2006)
• “The results rather show that well-designed gravel augmentations are not necessarily short lived as experienced in several studies in highly impacted rivers (Barlaup et al., 2008; Pander et al., 2015; Roni, 2019), but may function over decades depending on the river type, geomorphology, sediment supply, and river regulation. Our results indicate that low suspended loads and little discharge of fines have contributed to a long life span of spawning gravel augmentations. Other explanations for the observed longevity are the cleaning activity of spawning fish as well as a sufficient choice of gravel”
• “The results may contribute to a more precise planning and maintenance of gravel bed spawning habitats, which is especially relevant for the implantation of the Water Framework Directive in the Europe, for mitigation measures in regulated rivers, as in the environmental design approach, but also for works in the context of the UN decade of ecosystem restoration and the many other attempts to improve habitat for fish in rivers. Spawning gravel augmentations may not only be considered as habitat enhancement, but may also function as a tool for process-based river restoration in some cases (Beechie et al., 2010; Pulg et al., 2013), for example, at lake outlets where gravel augmentations restore historic glaciofluvial gravel banks and can provide enduring spawning habitats. We recommend further investigations and data collections of such measures including costs to improve the knowledge base and to monitor the measures’ total life span.”

Some of the anticipated signposting directions form above study are detailed below.

• Individual rivers will differ in their relative abundances of the various habitats.
• Life cycle modelling around habitat is also critical since alevins, growing into fry and fry growing into to parr will have preferred survival requirements. Thus, a salmon population will be reflected in the hydro geomorphological profile of an individual river given good water quality
• Wider catchment factors will also be important as these will reflect peak to trough drainage, erosion sediments profile.
• Climate change rainfall patterns and droughts will have an impact.
• The impact on water quality also needs careful consideration and the conservation target for a sustainable ova deposition from adult that ensures long term sustainability.
• Ultimately habitat will limit smolt output from ova deposition, alevin, fry, parr survival.
• Audit and quantify spawning habitat and other nursery habitat 
• Assess how this relates to nursery habitat and secure residential habitat for returning adults.

Verification of redd habitat

My pilot study measured redds to the nearest 5 cms for width across stream, length of pot downstream, length of tailspill, depth of , and depth of tailspill. Initial analysis suggests that up to 1 meter cubed of gravel is moved in the creation of an average redd

Table 2Average dimensions of redds cms 2018 – 2025 from Carey Glenshesk and Glen Donegal from appox 100 /year

Year	Width	Length	Depth Well
2018	61	31	15
2019	78	32	16
2020	79	71	32
2021	65	60
2022	71	48
2023	72	63	27
2024	72	74	25
2025	75	64	29
Avg	72	55	24

Table 3 Estimated weight of gravel moved by a salmon to produce a Redd utilising various gravel weight estimates

Breadth across stream cms	Length Downstream cms	Well Depth cms	Measured by meter stick and GPS spatial location
72cms	55cms	24cms	95080 M 3 approx  I M Cubed
Estimated weight of dry weight gravel	Gravel Calculator | aCalculator.co.uk	0.04 tonnes	40 KG estimate
Source	Gravel Calculator – Calculate how much gravel you need	352 lbs	160 Kg estimate
Source	RIVER ROCK CALCULATOR [How Much River Rock do I Need?]	135	135 KG estimate
Assumes square dimensions	Redds probably have circular sides	Doesn’t account for tail spill length	ie a larger tail spill would suggest more gravel moved

.

DAERA NIEA has a river habitat assessment RHAT which useful to asses river habitat needs to be modified to take more account of spawning and nursery habitat for Wild Atlantic salmon as a salmon management tool.

Assessing invertebrate communities in such habitats would also be beneficial.  

Sea Survival of smolts

It is also a concern that sea survival of salmon smolts to returning adults is under stress in the ocean. A recent paper reported that the trophic level energy flows within the ocean are altered and apparently reduced. (seeTyldesley, E., Banas, N.S., Diack, G., Kennedy, R., Gillson, J., Johns, D.G. and Bull, C., 2024). Patterns of declining zooplankton energy in the northeast Atlantic as an indicator for marine survival of Atlantic salmon. ICES Journal of Marine Science, 81(6), pp.1164-1184. ( zoo plankton Energy!)

Should salmon be considered as a sea species spawning in freshwater or a freshwater species going to sea to grow and develop? Would selecting one or other option aid management of the species for maximum sustainable yield (MSY).

It will be a challenge to determine a process for protecting salmon at sea. It is arguably more straight forward to protect river habitat to ensure that rivers continue to be productive habitats for Atlantic salmon.

Survival of emigrating smolts at sea is arguably density independent (spatially unlimited growth relative to river) given the ample ecological niche space for greater survival of juveniles into the ocean space. Predator prey relationships within the ecosystem food chain will have a role in determining survival of smolts to adults. 

Traditional pelagic ocean fishery management principles utilise spawning stock biomass as a key management parameter as a monitoring measure to achieve maximum sustainable yield. NASCO and ICES refer to pre-fishery abundance as a control measure which is a potential measure of surplus stock that can be harvested without doing damage to future catches and MSY. That treats salmon as an ocean pelagic fish ( cf herring Mackerel which shed their millions of eggs into the open ocean). Salmon on the other hand have around 500 eggs per kilo of weight and have evolved to be deposited securely in gravel rivers. Thus the anadromous nature of theses pelagic fish requires a river habitat to nurture their development and an ocean habitat that functions to meet their growth and survival needs.

The anadromous nature of salmon differentiates wild Atlantic salmon from other pelagic species. Many studies show that juvenile salmon are territorially competitive during the fry to parr growth period in rivers and require and individual territory to survive within a nursery area. The conservation limit constraint suggests that juvenile densities above a threshold will not result in further production of parr and ultimately smolts. Smolts and returning adults lose their territorial necessity as they are non-feeding and are often seen in migrating shoals. Ultimately productivity is limited by freshwater nursery habitats, and if the conservation limit theory is real it must also relate to habitat defined by fluvial area or a subset of fluvial area. E.g. areas of spawning habitat, for secure ova deposition, nursery habitat for growth from fry to parr and deeper holding water for adult spawning fish awaiting Autumn spawning Habitat Guidelines Brochure. Many studies and monitoring programmes assess fry recruitment by electro-fishing rivers. There are fewer studies relating to the requirements of fry growth to parr and many monitoring programmes utilize fry counts as a measure of survival.

I am of the view that the Hydro Geomorphological( HGM) profile can used to determine the number of adult salmon required to fully stock a river. Large populations require a large habitat area though the HGM profile also influence the output.

Where and when salmon spawning and nursery habitat is degraded, the population is bound to decrease compounded by poor sea survival and conservation limit reduced. That needs to be reversed by increasing the habitat and therefore upping the conservation limit. A multi-scale analysis and classification of the hydrogeomorphological characteristics of Irish headwater streams – NERC Open Research Archive,  Whilst the dynamic relationship between ova deposition, hatching, fry and parr can be complex and difficult to quantify, it none the less provides a baseline for population densities and survival of different age classes of juveniles.