Living on Fish

A while ago I offered a post on some of the huge number of bird species that live by eating insects; the numbers involved are staggering. At least equally so, and in terms of sheer mass probably more so, are the numbers of animals that live wholly or significantly on fish. Mostly I'll be talking about birds again, but not entirely. 

Pied Kingfisher Ceryle rudis, Queen Elizabeth NP, Uganda.

The fish resource, though severely depleted by human overfishing, is extraordinary. One authoritative source puts the annual human wild fish catch at 90-95 million tonnes. It's hard to imagine that in terms of numbers of fish, and of course many animals take fish smaller than those generally consumed by people.

Moreover fish is a food high in energy and protein, so energy-consuming hunting strategies can be profitably applied. The Pied Kingfisher above could well have caught his lunch (and it is a he, with that broad breast band) by hovering, which burns lots of energy. 

Pied Kingfisher hovering, Amboseli NP, Kenya.

Many birds plunge into the water from above, or swim on the surface then dive, and pursue the fish underwater, a high energy game indeed. Many of these birds hunt in flocks, which are sometimes vast, converging on schools of fish which can number millions of potential prey items. To encounter such a hunt is among the most dramatic spectacles nature can afford us.

Cormorants (several species) converging on a huge fish school, Humboldt Current, northern Chile.
This is only a small part of the vast loose flock that streamed out to sea in response to other
birds already there.

Kelp Gulls Larus dominicanus and Black-browed Albatross Thalassarche melanophris in the
Strait of Magellan; upwelling cold waters provide the richest fisheries.

Blue-footed Boobies Sula nebouxii (again a tiny part of a flock of hundreds) smashing into the water
from tens of metres up, Galápagos. Note how they close their wings just before they hit the
water. They have no nostrils, which would be a serious disadvantage here.

Peruvian Pelicans Pelecanus thagus diving, Isla de Chiloé, southern Chile.

To an Australian, seeing the two big brown American pelicans diving from on high is a real surprise. Australian pelicans (which are bigger), hunt by swimming, then dipping their huge bills with elastic pouch into the water. They too often hunt in flocks however, chasing the fish schools, often accompanied by cormorants.

Australian Pelicans Pelecanus conspicillatus and Pied Cormorants Phalacrocorax varius,
Longreach Waterhole near Elliott, Northern Territory.

Australian Pelicans fishing, Canberra. The water here is shallow so they had to turn their
heads side on to submerge them. They scoop up water in the bill pouch and squeeze it out, retaining
fish and other prey items.     

 

Pelicans waiting for fish - sometimes stunned by the turbulence - to come through the lock
at Blanchetown on the River Murray, South Australia. The resting cormorants are
presumably sated; you can catch enough fish to fill your energy needs in
a relatively short time.

The cormorants benefit from the fish stirred up by the bigger birds, and will even snatch fish from the corners of a pelican's beak. They of course do pursue the fish underwater.

Little Black Cormorants Phalacrocorax sulcirostris, fishing in Kinchega NP,
western New South Wales.

Fish are slippery - they have an antibiotic surface slime and very smooth scales for sliding efficiently through the water - so a specialised bill is required. Cormorant bills are strongly hooked.

Little Black Cormorant, Coffs Harbour, NSW.
You'll see the hook better if you click on the picture to enlarge it.

Penguins and petrels and shearwaters have similar hooks.

Flesh-footed Shearwaters Ardenna carneipes, Lord Howe Island.

However many effective fish-eaters do not. Darters - a group of four diving fishers superficially resembling cormorants - tend to stalk along the bottom and ambush their prey rather than actively pursuing it like a cormorant. They have sharp-tipped bills and tend to stab upwards, surfacing and swallowing the stricken fish head-first.

Female Australian Darter Anhinga novaehollandiae with fish.
The weird kink in the neck is due to a hinge between the eighth and ninth
cervical vertebrae which enables the bird to thrust its neck forward like a spear.
The bill actually has little serrations on the inner edges to assist in swallowing.

This can be seen better in other species, including these Royal Spoonbills Platalea regia.

Royal Spoonbills at Jerrabomberra Wetlands, Canberra. The one on the left is swallowing a small fish.
Below you can see the bill serrations if you click on the photo.

These spoonbills are hunting primarily by touch by means of a 'bill tip organ' comprising many thousands of tightly packed sensors both in the tip of the bill and in the upper and lower jaws. These combine touch and taste and cause the bill to snap shut if it encounters something edible. Many birds which hunt food in mud and muddy water have such sensors.

Wood Storks Mycteria americana, Pantanal, south-eastern Brazil.

The really big storks have such organs too, but often their fish prey is so big that they can rely on their vision.

Jabiru Jabiru mycteria with big fish, Pantanal.
Despite the bird's huge size, it struggled for a long time to subdue and swallow this mighty meal.
However I doubt that it needed another one that day!

Saddle-billed Stork Ephippiorhynchus senegalensis, with big catfish (I think),
Amboseli NP, Kenya.

A slightly different use of the bill-tip organ is applied by the fascinating skimmers, three distinctive species in the gull and tern family. Their bottom mandible is much longer than the upper. They fly along the water surface trailing the tip of the bottom one in the water and when it encounters a fish or other food item the bill snaps shut on it.

Black Skimmers Rynchops niger;
Pantanal above, and Isla de Chiloé, southern Chile, below.

Fish-eating birds of prey - fishing eagles, ospreys and even fishing owls - certainly have an appropriate hooked bill, but in fact they inherited that, ready evolved, from non-fishing ancestors. They catch their fish using powerful clawed feet.

Eastern Osprey Pandion cristatus, Hervey Bay, Queensland.
The two closely-related osprey species are found all over the world; they are the only
day-time birds of prey to be exclusively fish eaters. Their large back-hooked claws
differ from those of most fishing eagles, and are adapted to slippery fish.

White-bellied Sea Eagle Haliaeetus leucogaster carrying fish dinner,
Port Macquarie, NSW.

African Fish Eagle Haliaeetus vocifer, Queen Elizabeth NP, Uganda.
The ten species of fishing eagle, found across the globe, are now
regarded as all being in the one genus. Though the claws aren't hooked,
the power of the feet is evident in this photo.

Grey-headed Fish Eagle Haliaeetus ichthyaetu, Sabah, Malaysian Borneo.
Until recently this and another species were regarded as separate from the other
fish eagles; they are even more fish-reliant than the others and it is no coincidence
that they have oprey-like recurved claws.

Fishing owls are found both in Africa and Asia, with three species each; the two groups are quite separate and the African ones differ in being almost exclusively fish-reliant.

Buffy Fish Owl Ketupa ketupu, Kinabatangan River, Sabah.
The African fishing owls plunge their legs into the water like fishing eagles,
but the Asian ones are more fastidious, not liking to get their feathers wet.

The many species of abundant gulls, all over the world, do catch fish, but they also scavenge many fish carcases; these days of course many of these are due to discards from human fishing.

Pacific Gull Larus pacificus, Esperance, Western Australia.

Silver Gull Chroicocephalus novaehollandiae, Cairns, north Queensland.

Many marine mammals (especially seals, sea lions and toothed whales) and some freshwater ones are fish eaters. Of these some of the best-known and most specialised are the otters, a group of highly specialised aquatic weasels, some of them quite large. 

The magnificent Giant Otter Pteronura brasiliensis, here in the Pantanal of Brazil,
showing its beautifully adapted paddle-shaped tail.

Giant Otters sharing - and squabbling over - fish in the Manu section
of Peruvian Amazonia. These two areas are relative strongholds of this superb animal,
which can grow up to 2.5 metres long and weigh over 30kg. Populations though are
tragically depleted by illegal hunting for its fur. They are strongly gregarious.

Neotropical Otter Lontra longicaudis, Pantanal, in a roadside lagoon.
About half the size of the Giant Otter and solitary, it is not well known.

The Marine Otter Lontra felina, here off the Isla de Chiloé, southern Chile,
is rare and is also poorly understood. It is quite different from the
Sea Otter of North America.

There are fish-eating reptiles, most notably crocodilians, all of which eat a lot of fish; some eat little or nothing else.

Yacaré Caiman Caiman yacare with substantial fish lunch, Pantanal.

And there are even some fish-eating invertebrates, from sea anemones to insects such as giant water bugs and dragonfly larvae, to spiders. The genus of big fishing spiders, Dolomedes, is found across the world. The long-legged spiders run across the water surface where they seize and deliver a venomous bite to fish and invertebrates.

Fishing spiders in Yasuní NP, Ecuadorian Amazonia above,
and southern Sarawak, Malaysian Borneo, below.

If you like fish, you're not alone. Hopefully this post might have offered you a broader perspective on the fish dinner.

Ringed Kingfisher Megaceryle torquata with fish, Pantanal.

NEXT POSTING THURSDAY 15 JULY

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