Are grey seals contributing to the harbour seal decline?

Student Spotlight - Izzy Langley

PhD Research at the Sea Mammal Research Unit, University of St Andrews

Email: il32@st-andrews.ac.uk   Twitter: @LangleyIzzy

Project overview

Around the UK, grey seal population numbers are stable or increasing, whilst harbour seal populations have declined in areas along the north and east of Scotland¹. My PhD project at The Sea Mammal Research Unit (SMRU), in collaboration with The University of Aberdeen and The Scottish Marine Animal Stranding Scheme (SMASS) and funded by the SUPER DTP, is exploring the potential role of grey seals in the observed regional declines of harbour seals. I am currently coming to the end of my first chapter, where I have investigated how the diets of harbour seals and grey seals have changed through time.

The harbour seal decline

Harbour seals (Phoca vitulina) are found throughout the northern hemisphere, comprising three geographically distinct sub-species with different population trends². Northern Europe has around 22% of the global population, with significant populations in the UK³ and in the Wadden Sea⁴. In the UK, both harbour and grey seal populations are protected under national legislation (e.g. The UK Conservation of Seals Act 1970, Marine Scotland Act 2010), European Union regulations (i.e. EU Habitats Directive), and are considered indicators of marine ecosystem health under the Marine Strategy Framework Directive and the OSPAR Convention. For management purposes, the UK is divided into 14 Seal Management Units (SMUs) and harbour seal populations in three of these have experienced marked declines³. These declines have been most pronounced in the North Coast and Orkney SMU, which was historically the largest harbour seal population in the UK but has decreased by 85% in the 20 years prior to the most recent counts in 2016³.

Previous research has found that the decline in harbour seals has not been caused by an increase in the mortality of pups⁵, or by an increase in the exposure of a bacterial pathogen which would affect reproduction⁶. The genetic structure of harbour seal populations in the UK can be clustered into four geographically distinct groups⁷, which suggests little mixing between populations with contrasting trajectories. Mathematical simulations of regional populations have shown that the observed declines in places such as Orkney can only be explained by a decrease in the survival of adults⁸, yet the exact cause of this remains unknown.

Competition with grey seals

One of the ecological drivers behind the observed decline in harbour seal numbers could be competition with grey seals (Halichoerus grypus). Competition can occur between species that overlap in any of their resources; this could be space to haul out on land for example, or competition for their preferred prey. The waters around the UK host both species of seal, and grey seal numbers are doing very well, with populations in the North Sea increasing exponentially⁹. Both grey seals and harbour seals haul out on land for what we believe is to rest and also digest¹⁰, and for extended periods of time to breed and moult. Although these seasons occur at different times of year, with harbour seals breeding in the summer and grey seals breeding in the autumn/winter, the two species can be observed from land hauled out together at mixed species haulout sites throughout the year.

However, as marine mammals, seals spend a large proportion of their time at sea, out of sight of land. For a number of decades, we have been able to track their at-sea movements using satellite telemetry devices which have provided an important insight into where seals go to find food. Grey seals and harbour seals overlap considerably in their at-sea movements^11,12. There has also been extensive research into the diet of seals around the UK, and with slight regional variation, sandeels and cod appear to be important prey for both seal species¹³. It is therefore possible that grey seals and harbour seals interact and compete for resources. If this were the case, and resources such as prey have become limited, we may expect to see a decline in one of the species.

To address this, I am investigating how the diets of harbour seals and grey seals have changed through time. I hypothesise that if harbour seals are being outcompeted for their preferred prey, there would be evidence of this in the breadth of prey species they are eating. When seals eat and digest fish or other marine organisms, all that remains in their faeces is prey hard parts such as the inner ear bones of fish, or the beaks of cephalopods. From these hard part remains, we can identify what the seal has been eating in the previous few hours to days. Through multiple collaborations, I have access to a wealth of diet information for both harbour and grey seals stretching back to the late 1980’s. If you are interested in this work, I am hoping to present some preliminary results from this analysis at the UK & Ireland Regional Student Chapter for the Society for Marine Mammalogy 2021 conference this June (https://synergy.st-andrews.ac.uk/ukrsc/conference-2021/); I hope to see you there!

Grey seal predation

Grey seals not only potentially compete with harbour seals for resources, but recent evidence has found that adult male grey seals in the UK and in European waters kill and eat other marine mammals. Reports have included observations of grey seals predating on grey seal pups¹⁴, juvenile harbour seals¹⁵, and juvenile harbour porpoises¹⁶. There is also evidence from the strandings record of adult harbour seals with fatal wounds likely caused by grey seal predation¹⁷.

So, have you seen a grey seal acting strangely?

We need your help to understand more about such behaviour and its prevalence. In the coming months, we will be launching a new reporting page on the SMRU website, where we encourage you to share any information, including images and videos you may have of potential grey seal predation events.

Please note, if you find a dead seal, please report these directly to the appropriate organisation. In Scotland, this is The Scottish Marine Animal Strandings Scheme (SMASS) and in England & Wales is the UK Cetacean Strandings Investigation Programme (CSIP). These two strandings networks have collected a wealth of data over the years of grey seal inflicted mortality on harbour seals. To compliment this dataset, we are interested in:

a) How many individual grey seals are marine mammal predators?

We can identify individual grey seals using photographic identification of natural markings. This will allow us to estimate the minimum number of perpetrators within a population, and the geographical range of this behaviour.

b) How many grey seal predation events are we missing from the strandings data alone?

Previously, unless a carcass washed up on shore and was reported to the strandings networks, grey seal attempted predation or predation events went unmonitored. We are interested in the photographic and video evidence of aggressive interactions between grey seals and marine mammals, which could lead to, or have already led to a predation event.

Next steps

I am half-way through the second year of my PhD and am coming to the end of my work with the diet data. The next stage of my investigation into competition between harbour seals and grey seals is to dig into the at-sea movement data. As with diet, if harbour seals are being outcompeted for resources, we may expect to see a change in their movement behaviour over time. This will include looking at the time spent foraging, as well as the duration and distance of foraging trips, to give an indication of foraging effort.

And beyond this, I hope that our new reporting page on the SMRU website will start to collect some fascinating accounts of potential grey seal predation... Watch this space!

 

References

1. Special Committee on Seals. (2019). Scientific advice on matters related to the management of seal populations: 2017. Fife, Scotland: SCOS, University of St Andrews. http://www.smru.st-andrews.ac.uk/files/2020/08/SCOS-2019.pdf
2. Bjørge, A., Desportes, G., Waring, G. T., & Rosing-Asvid, A. (2010). Introduction: the harbour seal (Phoca vitulina) - a global perspective. NAMMCO Scientific Publications, 8, 7-11.
3. Thompson, D., Duck, C. D., Morris, C. D. & Russell, D. J. F. (2019). The status of harbour seals (Phoca vitulina) in the UK. Aquatic Conservation: Marine and Freshwater Ecosystems, 29(51): 40-60.
4. Reijnders, P. J., Brasseur, S. M., Tougaard, S., Seibert, U., Borchardt, T., & Stede, M. (2010). Population development and status of harbour seals (Phoca vitulina) in the Wadden Sea. NAMMCO Scientific Publications, 8, 95-105.
5. Hanson, N., Thompson, D., Duck, C., Moss, S. & Lonergan, M. (2013). Pup mortality in a rapidly declining harbour seal (Phoca vitulina) population. PloS one, 8(11): e80727.
6. Kershaw, J. L., Stubberfield, E. J., Foster, G., Brownlow, A., Hall, A. J. & Perrett, L. L. (2017). Exposure of harbour seals Phoca vitulina to Brucella in declining populations across Scotland. Diseases of Aquatic Organisms, 126(1): 13-23.
7. Olsen, M.T., Islas, V., Graves, J.A., Onoufriou, A., Vincent, C., Brasseur, S., Frie, A.K. & Hall, A.J., (2017). Genetic population structure of harbour seals in the United Kingdom and neighbouring waters. Aquatic Conservation: Marine and Freshwater Ecosystems, 27(4): 839-845.
8. Arso Civil, M., Langley, I., Law, A., Hague, E., Jacobson, E., Thomas, L., Smout, S.C., Hewitt, R., Duck, C., Morris, C., Brownlow, A., Davison, N., Doeschate, M., McConnell, B., and Hall, A.J. 2019. Harbour Seal Decline – vital rates and drivers. Report to Scottish Government HSD2. Sea Mammal Research Unit, University of St Andrews, pp. 46. http://www.smru.st-andrews.ac.uk/files/2020/02/HSD2-yr4_annual-rep_web.pdf
9. Thomas, L., Russell, D.J., Duck, C.D., Morris, C.D., Lonergan, M., Empacher, F., Thompson, D. & Harwood, J. (2019). Modelling the population size and dynamics of the British grey seal. Aquatic Conservation: Marine and Freshwater Ecosystems, 29: 6-23.
10. Sparling, C. E., Fedak, M. A. & Thompson, D. (2006). Eat now, pay later? Evidence of deferred food-processing costs in diving seals. Biology Letters, 3(1): 95-99.
11. McConnell, B. J., Fedak, M. A., Lovell, P. & Hammond, P. S. (1999). Movements and foraging areas of grey seals in the North Sea. Journal of Applied Ecology, 36(4): 573-590.
12. Sharples, R. J., Moss, S. E., Patterson, T. A. & Hammond, P. S. (2012). Spatial variation in foraging behaviour of a marine top predator (Phoca vitulina) determined by a large-scale satellite tagging program. PLoS one, 7(5): e37216.
13. Wilson, L. J. & Hammond, P. S. (2019). The diet of harbour and grey seals around Britain: Examining the role of prey as a potential cause of harbour seal declines. Aquatic Conservation: Marine and Freshwater Ecosystems, 29: 71-85.
14. Brownlow, A., Onoufriou, J., Bishop, A., Davison, N. & Thompson, D. (2016). Corkscrew seals: grey seal (Halichoerus grypus) infanticide and cannibalism may indicate the cause of spiral lacerations in seals. PloS one, 11(6): e0156464.
15. van Neer, A., Jensen, L. F. & Siebert, U. (2015). Grey seal (Halichoerus grypus) predation on harbour seals (Phoca vitulina) on the island of Helgoland, Germany. Journal of Sea Research, 97: 1-4.
16. Leopold, M. F., Begeman, L., van Bleijswijk, J. D., IJsseldijk, L. L., Witte, H. J., & Gröne, A. (2015). Exposing the grey seal as a major predator of harbour porpoises. Proceedings of the Royal Society B: Biological Sciences, 282(1798), 20142429.
17. The Scottish Marine Animal Stranding Scheme Annual Report (2019). Marine Scotland, Scottish Government. https://www.strandings.org/smass/publications/reports/SMASS_Annual_Report_2019.pdf