Roseate tern diet

Protecting foraging areas

All wild birds are protected by law in some form or another, but some species are afforded additional protection during their breeding season, including their nests, eggs and dependent young (Schedule 1 species of the UK's Wildlife and Countryside Act 1981). Sites supporting important colonies of breeding seabirds, have been designated as European Special Protection Areas (SPA).

Till recently, only the colonies have been designated, but not the surrounding foraging areas. To remedy this situation, conservationists have been interested in the range and pattern of foraging around the breeding colonies. Boat tracking has been used to follow the terns and identify key areas and the distance they travel from their colony. Using these data, conservationists have proposed marine extensions to the existing coastal SPAs to include the important tern foraging areas.

Summary map of the Northumberland Marine SPA (now designated) which has included foraging areas around Coquet Island SPA extending the protection for breeding terns (Natural England, 2017)

Fisheries

The lesser sandeel plays a pivotal role in the marine ecosystem as a food source for a number of predators, including commercially important fish like cod, whiting and haddock. However, sandeels also face considerable pressure from fisheries. In the North Sea, sandeels are used in fish meal and fish oil for agricultural and aquaculture feeds.

It has been the largest single-species industrial fishery in the region and by 1989 the sandeel catch had exceeded 1 million tonnes. Between 1994 and 2003, the average catch per year was 880,000 tonnes. In 2000, in an attempt to tackle the impact on sandeels, the EU created an exclusion zone off the coasts of North east Scotland and Northumberland and the annual catch was reduced further to an average of 313,000 tonnes (between 2003-2010). The closure appeared to have caused an immediate recovery of sandeels within the area, however, by 2007, the spawning stock size had declined to levels similar to when the fishery was still operating. This means there are additional factors impacting the sandeel population.

Figure 1. Total catches (tonnes) of sandeels in the North Sea from 1983 - 2016.

ICES. 2017a. Herring Assessment Working Group for the Area South of 62 deg N (HAWG), 14-22 March 2017, ICES HQ, Copenhagen, Denmark. ICES CM 2017/ACOM:07. 856 pp.

Sea temperatures are predicted to increase, which will most likely deepen sandeel declines. Thus, conservationists are concerned about the current exploitation level of sandeels in North Sea. As a result, they are appealing the UK government to take precautionary management measure to minimise the pressure on the populations of important seabird and marine mammals dependent on sandeels.

Danish trawler fishing for sandeels

Chris Gomersall (rspb-images.com)

Tern Foraging and Diet

Terns use three techniques when foraging: (1) plunge diving (in which they dive up to one metre into the water), (2) diving-to-surface (catching prey from just below the surface with a shallow dive) and (3) dipping (snatching prey from the surface without touching the water).

Terns predominantly feed on fish, but their diet includes crustaceans, squid and other invertebrates, especially when they are foraging for themselves. Chicks are largely fed with nutritious fish to speed up their growth, as they are more vulnerable to predators and adverse weather before fledging.

Most fish taken belong to the following three families:

• Ammodytidae (sandeels)
• Clupeidae (sprat and herring)
• Gadidae (cods)

Outside the breeding season, terns can forage without restrictions, however to feed chicks, the adults need the optimal fish available near the colony. Most tern species have relatively short foraging areas of less than 10km as longer foraging trips away from the nest not only affect chick growth, but also leave the chicks vulnerable to predators. As a result they usually forage closer to the colony and have a provisioning rate between 1.09 – 1.72 deliveries per chick per hour. However, surveys have shown that roseate terns will travel much further when searching for food resulting in a maximum foraging range of 30km.

Roseate tern with sprat

Photo by Brian Burke

Roseate tern foraging

Photo by Wesley Davies

Monitoring of Chick Provisioning

Seabirds diet is an important driver of the breeding success as a plentiful supply of fish can improve the growth and therefore survival of chicks. Terns are specialist predators, feeding predominantly on specific fish species, mainly sandeels and sprats. This means that they are particularly sensitive to any bottom up changes within the food chain. For example, the amount of plankton available influences the sandeel population, which in turn affects the larger predators like piscivorous fish, mammals and seabirds. Terns are vulnerable to these changes as they are primarily surface feeders and dive only to less than a meter depth.

Availability of fish can be detected through the monitoring of top predators’ diet. Terns bring fish to their chicks in beaks, which makes it easy to record prey species, its size and frequency of feeding. Additionally, wardens regularly take biometric measurements of chicks, such as weight and wing length, to monitor their growth rate throughout the breeding season. In some years, it is possible to link poor feeding conditions with lower breeding success.

Roseate Tern chick feeding and flying on Rockabill

Roseate tern chick

Photo by Brian Burke (photo taken under NPWS license)

Roseate tern fledgling

Photo by Brian Burke (photo taken under NPWS license)

Clupeids

The fish family Clupeidae includes pelagic species, such as sprat and herring, which are important food source for seabirds. Similarly to sandeels, sprat and herring feed on copepods, however while herring prefer to feed on larger cold-water copepods like Calanus finmarchicus (the same species sandeels feed on), sprat demonstrates a preference for smaller warm-water zooplankton species such as Temora longicaudata. This makes sprat less vulnerable to rising sea temperatures and an important alternative prey in areas where sandeels have declined.

In areas such as the Shetlands, where sprat is scarce and the herring larvae from the Orkney-South Shetland spawning ground migrates south, away from the islands; the crash of Arctic tern and kittiwake colonies has been attributed to the loss of sandeels and lack of alternative prey.

Roseate tern with several sprat in its bill

Photo by Oran O’Sullivan

Roseate tern chick being fed a clupeid on Coquet

Lesser sandeel

The lesser sandeel Ammodytes marinus is a small pelagic fish of the sand lance family Ammodytidae. Sandeels are a high-energy prey with a long, thin body shape, which makes them easy to ingest. They are an important food source not only to seabirds, but also marine mammals and piscivorous fish.

Lesser sandeels usually spawn in December and January, with the eggs hatching from February to March. The planktonic larvae drift in the ocean currents becoming juvenile fish and begin to search for suitable habitat. During this period, they have a high mortality rate. Once settled, juvenile and adult lesser sandeels are largely resident and rarely disperse over large distances. Sandeels burrow in the sand to avoid predators and to hibernate during the winter. They emerge in spring and summer to feed on the copepods. This is when they are available to terns near the surface.

Lesser sandeels are associated with shallow offshore waters with relatively smooth sea bottom of gravelly sand. These areas are high in oxygen with a low silt content. Due to these specific habitat requirements, they have distinct spawning grounds in the North Sea and Irish Sea. This, combined with the limited dispersal capacity of adults, make sandeels particularly vulnerable to changes in availability of zooplankton caused by climate change.

Courting roseate terns with a sandeel

Photo by Laura Glenister

Roseate tern with a sandeel

Photo by Brian Burke

Not all fish are good for terns

Tern chicks rely on prey with a high energy content for growth. The energy content can vary between different fish species and it is influenced by environmental conditions in the sea. When terns are unable to find their optimal fish species, like sandeels or sprat, they will substitute their diet with other prey species available in the sea. Nonetheless, the alternative food sources may not be of the same nutritional value when compared to their preferred food source. Low energy prey protracts chicks’ development, resulting in higher mortality from adverse weather conditions or predators. In years when sandeel availability is poor, terns feed their chicks with pipefish or shrimp. In addition to the low nutritional value, pipefish are rigid, bony and extremely hard for chicks to ingest, causing some to choke to death. If the lack of ideal prey species persists, it can lead to poor breeding success.

Arctic tern with pipefish

Photo by Chantal Macleod-Nolan

Competition for Sandeels

Seabirds consume an estimated 200,000 tonnes of sandeel every year in the North Sea alone, however it is also an important prey source for other predators, including those with a more generalist diet such as piscivorous fish (whiting, mackerel and haddock) and marine mammals (porpoises, minke whales and seals). Harbour seals that haul out in south-east Scotland are particularly dependant on sandeels.

In addition to natural predators, sandeels are valuable resource to humans. North Sea sandeel fisheries has been operating since the 1950s and for a time would take more that 1 million tonnes of sandeel from the sea. This has been linked to the collapse of sandeel stock in many areas and subsequently closures of fisheries or reduction of fishing quotas.

To read more about fisheries, please click on the fishing boat...

Grey seal adult female and pup

Photo by RSPB (rspb-images.com)

Puffin with a beak full of sandeels

Photo by Wesley Davies

Copepods

Sandeels and clupeids (sprats and herrings) feed on zooplankton known as copepods. Copepods are a group of small crustaceans that are found in the sea column, commonly known as plankton. Calanus finmarchicus is a large cold-water copepod with their egg production overlapping with the hatching period and the early life stages of lesser sandeels. However, since the mid-1980s, the abundance of C. finmarchicus in certain areas of the North Sea has declined. These declines have been associated with the rising sea temperature caused by climate change. At the same time, the warm-water copepod Calanus helgolandicus has increased. Unfortunately, their egg production does not overlap with the hatching period and early life stage of sandeels, meaning it is not an alternative prey source for this species.

Figure 1. Mean annual Calanus abundance in the central North Sea (54N to 58N, 3W to 11E) using data from the Continuous Plankton Recorder provided by SAHFOS. Blue line = C. finmarchicus, red line = C. helgolandicus, grey line = total Calanus abundance, dashed green line = total Calanus trend.

Sandeels have strict habitat and environmental preferences, which means they are unlikely to shift their distribution, despite of the diminishing food resources. Furthermore, the rising temperature reduces the amount of oxygen in the sea substrate, affecting the overwintering habitat conditions and reducing the reproductive potential of sandeels. Overall, these factors affect the stock size and energetic content of sandeel available to top predators such as seabirds.

In contrast, sprat have a preference for smaller, warm-water zooplankton such as Temora longicaudata and Calanus helgolandicus. We can therefore assume that, while sandeel populations will continue to diminish, sprats will become even more important food source for terns in the future.