Case study: Marine food webs

Marine food webs describe the interconnectedness between different species and habitats. They can be used to illustrate and understand the relationships between predators and prey, the way in which energy flows through marine ecosystems and the way in which ecosystems function. An understanding of food webs can also help inform management relating to ecosystem services and the benefits that society gains from Scotland’s seas (see Natural capital, ecosystem services and the Blue Economy section). Figure 1 illustrates the relationships between different marine species in nearshore waters around Scotland.

Figure 1: Nearshore temperate marine food web illustrating the relationships between different species groups including plankton, fish, birds and cetaceans. © Glynn Gorick/Nature Scot.

This case study brings together the results of the relevant species assessments in Scotland’s Marine Assessment 2020 (SMA2020) to provide a descriptive summary of marine food webs in Scotland’s seas in terms of how productive they are, how diverse they are and the abundance/distribution of selected species. This reveals a mixed picture. Taken as a whole, there is a clear message of change in marine food webs, some of which is likely to be caused by climate change, some the result of broader environmental changes (e.g. that caused by non-native species), and some likely to be a response to the direct use of Scotland’s marine environment and the way in which human activities associated with it are managed (e.g. changes in fisheries management).

How food webs have been described

A good understanding of populations of species, and how they are changing, is needed to enable assessments to be undertaken of whether these populations are sustainable, and whether they are likely to be contributing to the overall long-term sustainability of the marine food webs of which they form part. The results have been grouped to reflect different elements of marine food webs: productivity, diversity and abundance/distribution. Table 1 provides further details, including a summary of the key messages from the assessments for each species/species group and a link to each assessment.

In terms of productivity, grey seal pup production continues to increase, whilst breeding success of seabirds is considered to be stable, albeit at a lower level than previously. Diversity has been described through the three fish assessments, revealing no evident trend in deep-sea fish or fish in transitional waters (although limited data lead to low confidence in these results), and a mixed picture from groundfish surveys on the continental shelf with different indicators revealing increases and decreases in certain species offshore. For abundance, where sufficient data are available (birds, plankton, seals, coastal bottlenose dolphins) there is an overall picture of change. Even though the abundance of breeding seabirds is considered stable overall, this masks significant variation between species.

Discussion of results

Interpreting what this mixed picture means for the status of marine food webs is challenging. For most of these species it has been possible to say whether the populations are increasing, decreasing or stable, however, understanding of the drivers of these changes is variable. What is less certain is what the impacts of these changes in species populations are likely to mean for other species and for the wider marine food web of which they form part. Below are three examples that illustrate current understanding of the complexity of the interrelationships.

• The abundance of breeding seabirds has been relatively stable since 2011, however, numbers are now considerably lower than they were in the mid-1980s. As noted above, surface-feeding seabirds have shown the most significant declines, linked to the lack of availability of sandeels. Other species such as gannets, have increased, and again this is thought to be linked to their feeding behaviour, being deeper diving and able to feed on a wider range of prey species. Foster et al. (2017) showed a link between fishery landings and gull numbers on the Isle of Canna. The lower number of gulls recorded more recently appears to be more typical of numbers recorded on Canna in the 1930s than the peaks recorded in the mid-1980s.  It appears likely therefore that a combination of environmental change, climate change, fish population changes and fisheries management practices may all be contributing to the changes currently seen in Scotland’s seabirds.

• The assessment of plankton has shown that the abundance of some types of plankton is changing. Some of this change correlates with changes in sea surface temperature, which is used in this assessment as a proxy for climate change. The precise implications of these changes in plankton are not known however it is likely that higher trophic levels will be impacted by this change in food resource. The assessment highlights that ‘some of the changes observed include increasing trends in diatoms and meroplankton and declining trends in large and small copepods, crustacean and holoplankton lifeforms’. Copepods are an important source of food for many marine species in Scottish waters including larval and adult stages of commercial fish (cod, herring) as well as larger marine animals such as basking sharks. The decline in small copepods has been reported for the wider North Sea (Bedford et al., 2020) and distribution shifts of large copepods, Calanus helgolandicus (a warm-water species expanding north) and Calanus finmarchicus (an arctic-boreal species retracting north), have been observed in the last decades (Edwards et al., 2020). Changes in climate could be driving changes in some of the species that form the base of marine food webs and in turn affecting some of the largest fish in Scotland’s seas. Furthermore there is an indication that seasonal temperature changes are influencing biological processes which in turn is defining the degree of trophic mismatch. An example is the relationship between the lesser sandeel and its copepod prey (Régnier et al., 2019).

Figure 3: Puffin feeding on sandeels on the Isle of May NNR. © Lorne Gill, NatureScot.

• The assessment of the wider fish community, includes the results of the Large Fish Indicator (LFI). The proportion of large fish in a population is important because they are top predators, are a main target for commercial fisheries and contribute disproportionately to spawning. The LFI shows widespread increases in this indicator (i.e. a greater proportion of larger fish in sample trawls) across the Scottish continental shelf. The increases were ‘most pronounced around the Outer Hebrides and around Shetland and north of Shetland’. This reflects recovery of these populations, related at least in part to changes that have been made to fisheries management for demersal fish. Although the indicator shows increases across most of Scotland, some areas are still showing decreases, as are indicators of species richness or diversity in offshore areas. Overall these fish populations are not considered to be meeting good environmental status. Additional measures may be required where assessment thresholds are not met and evidence of recovery trends is either absent or weak, although it should be noted that the LFI is expected to have a time-lagged response to changes in fishing pressure. However, since assessment thresholds have generally yet to be achieved over most of the area assessed, any relaxation of management would be premature.

Next steps

This is a descriptive case study, intended to highlight the complexity of the situation as well as stimulate discussion and development of a full marine food web assessment for the next SMA. The intention is to move beyond simply describing what is known about the populations of species, to a more quantitative assessment that incorporates elements relating to function, dynamism and the relationships between different species and habitats. Over the next assessment period it is expected that there will be significant scientific developments that will support development of this indicator, for example, relating to energy flows. There are also ongoing discussions through OSPAR and the UK Marine Strategy relating to how to assess the status of marine food webs at these different scales. It is likely that development of a Scottish marine food web indicator will be based on collaborations through these wider fora.