Sand eels, seabirds and the food chain nobody sees

Everything below about sand eels and seabirds comes from what actually happens rather than from what is supposed to.

The link is direct

Puffins, kittiwakes, terns and guillemots feed chicks on small forage fish, and sand eels are the dominant prey in much of the north-east Atlantic. Breeding success tracks sand eel abundance and condition closely.

A colony failing is therefore usually reporting something about the fish rather than about the birds. Quality matters as much as quantity, because a chick can only swallow so many items in a day, so a bill-load of thin low-energy fish delivers less than one well-grown sand eel and a colony can fail while the adults are visibly still bringing food in.

Temperature moves the timing

Warming water shifts the timing and species composition of the plankton sand eels feed on. Where the plankton peak no longer coincides with sand eel larval development, the fish are fewer and in poorer condition. Thinner sand eels mean chicks receive less energy per delivery, which is enough to cause widespread failure.

Over a season, the underlying difficulty is that birds and plankton take their cues from different things, so warming shifts the plankton earlier while the birds' laying dates move far less, and the two calendars drift apart.

Industrial fishing adds pressure

Sand eels have been fished industrially for fishmeal and oil rather than for human consumption. That removes biomass from exactly the part of the food web seabirds depend on.

Closures in some areas have been introduced specifically to protect seabird foraging grounds, and are contested by the fishing industry. Quotas are set from surveys carrying wide uncertainty and the fishery takes the same age classes the birds depend on, so the real argument is less about whether fishing matters and more about how much error a shared stock can absorb.

Colonies show it before surveys do

Long-running colony monitoring picks up breeding failure quickly, because it is measured as chicks fledged rather than adults present. Adult numbers lag by years, so a colony can look healthy while failing to reproduce. This is why productivity monitoring matters more than counts.

Worth knowing before you go — productivity is measured by marking individual nests and revisiting them through the season to see how many chicks actually fledge, which is slow, weather-exposed work done largely by wardens and volunteers on cliffs.

Avian influenza compounded it

Recent outbreaks caused substantial mortality in several seabird species already under food-supply pressure. Populations with low productivity recover slowly from adult mortality, because there are few young birds to replace them. The combination is why several species have moved to higher conservation concern categories.

Locally, visitors have a small but real part in that, since the guidance during outbreaks is to keep well back from colonies, not to handle sick or dead birds, to keep dogs off beaches where they wash up, and to report what you find to the relevant agency rather than move it.

Sand eels need a particular kind of seabed

Sand eels spend much of their lives buried, and they will only bury in coarse, well-sorted sand with little silt in it, which confines them to specific banks rather than spreading them across the shelf. That makes those banks a finite and mappable resource, and it is why the same handful of grounds turns up in every argument about closures. Marine aggregate extraction, cable trenching and offshore construction disturb exactly that sediment, and the effect on a buried fish is far harder to observe than the effect on a fishery landing.

It also explains the geography of the colonies: the large ones sit within foraging range of those banks, so a bird that flies fifty kilometres to feed has nowhere else to go when that ground fails.

The takeaway

A failing seabird colony is usually telling you about a small fish nobody sees.

Go slowly enough to notice, and most of this stops being advice and starts being obvious.