Fish numbers and behaviour in Shetland

To understand fully the relationship between otters and their food, one would have to watch actual otter-fish interactions in every detail. The best possibilities for this occur in clear, coastal waters (such as those off Shetland), and with otters active by day. However, try as I might whilst scuba-diving along the Shetland coasts in a dry suit, I was never able to see wild otters find and take even one single fish. Quite simply, I just could not keep up with the otters in the dense forests of huge algae. I did see fish, many of them, and beautiful they were—but even in that I was nowhere near as efficient as the otters. In places where it took a Eurasian otter 10-20 seconds to emerge with a fat rockling, I might be rummaging around for half an hour before I finally found one. The otters' performance was totally superior to mine.

If direct observations of the interaction between predator and prey were impossible, both in the sea and in freshwater areas, the next best thing was various indirect methods of deducing what happened. By trapping fish, and also observing them in an aquarium in Shetland, we were able to obtain data relating to most of the problems mentioned above, except on fish productivity. This we could estimate only in rivers and streams of study areas in north-east Scotland (Kruuk etal. 1993a), not in the sea.

It is useful to describe some of the fish observations, just to 'get the feel' of one of the otters' prey communities. Of course, otter populations elsewhere will be dealing with different prey situations, and what we were seeing in our Shetland and Scottish study areas has immediate relevance only for the Eurasian otters there. But at least our data give an idea of the kind of relationship between otters as predators and their food species, and it suggested an approach that could be useful elsewhere.

One problem initially was the embarras de richesse: there were many species of fish, all fascinating in themselves. Even after more than 3 years of regular fish-trapping along the coasts of our study area in Lunna, Shetland, we kept coming up with new species: we caught more than 30, as well as various crabs and lobsters, and little was known about any of them.

The most common fish around the Shetland shores is the abundant saithe Pollachius virens, which used to be staple diet for the Shetlanders. A characteristic member of the cod family, numbers of them swim around the rocks and in open water. We often caught smaller ones in our fish-traps, but only in winter, when they move between the subtidal algae. In summer the saithe are still close to the coast but in more open water, and otters catch them only in winter, when the smaller saithe are 'available'.

The most common benthic fish along the coast Shetland, both in our fish-traps and as a prey for our otters, was the eelpout Zoarces viviparus (Fig. 8.1), an interesting northern species with various peculiarities. It is highly camouflaged with its subtle pattern of green, brown and orange; it has a body like an eel and a face like a frog, with bulging eyes and thick lips. Its bones, as found in an otter spraint, are sea-green. A typical specimen is less than 20 cm long, weighing 10-20 g; the largest we ever caught was 28 cm, although it can reach 50 cm (Muus and Dahlstrom 1974; Wheeler 1978). The eelpout produces live young, but very little is known of its behaviour otherwise. In daytime it is found under stones, but in an aquarium, or when diving at night, one notices that eelpout do swim around in mid water like other fish, although almost only in darkness; they are clearly nocturnal (Westin and Aneer 1987). In winter they live in somewhat deeper waters, but later they come closely inshore to mate and reproduce.

Figure 8.1 Bottom-dwelling, nocturnal eelpout Zoarces vivparus, the most common prey of Eurasian otters in Shetland.

Figure 8.2 The five-bearded rockling Ciliata mustela, Figure 8.3 Butterfish Pholis gunnellus at night-frequent prey nocturnal and bottom dwelling, is the prey with greatest of Eurasian otters along all Scottish coasts.

volume in the diet of Eurasian otters in Shetland.

Figure 8.2 The five-bearded rockling Ciliata mustela, Figure 8.3 Butterfish Pholis gunnellus at night-frequent prey nocturnal and bottom dwelling, is the prey with greatest of Eurasian otters along all Scottish coasts.

volume in the diet of Eurasian otters in Shetland.

Figure 8.4 Sea scorpion Taurulus bubalis, a little active, diurnal and benthic species, often taken by coastal Eurasian otters.

The other main actor on the fish scene was the five-bearded rockling Ciliata mustela (Fig. 8.2), also a common prey for the otters elsewhere. This fish shows similarities with the eelpout: somewhat eel shaped, it is also found under rocks and is of comparable size (20-30 g, but up to 200 g in weight). It is common everywhere along the European Atlantic coasts; the fry are pelagic and their huge summer swarms are a frequent food for smaller sea birds. It is strictly a species of the night. There were three other species of rockling, but the five-bearded was by far the most abundant.

The butterfish Pholis gunnellus and the sea scorpion Taurulus bubalis were very common (Figs 8.3 and 8.4). These are the species seen most often when diving or snorkeling, apart from the very small (and for our purposes irrelevant) gobies. The butterfish are beautiful, with their sharply defined black spots along the back and snake-like movements. They occur intertidally in large numbers, in shallow waters between the rocks, stones and algae, but they also go deeper, well out of reach of the otters (Wheeler 1978). They may be up to 20 cm long, but are light (5-15 g); there is not much flesh on a but-terfish, even on a big specimen. They are curiously flattened sideways. They feel slimy, and also spiny because of the hard sharp rays in the dorsal fin—not much of a prize for a predator. Butterfish are mainly nocturnal (Westin and Aneer 1987), but not so exclusively as rocklings.

In contrast, the sea scorpion is active mostly in daytime—if active is the right word, as most of its time is spent lying still on or between the rocks and weeds, waiting for its prey to move. Short and broad, 5-50 g, dark brown and with its head and gill-covers covered with bony knobs and bumps and very sharp spines, the sea scorpion should be well camouflaged and protected against onslaught from an otter. Nevertheless one sees them quite easily when scuba diving, and otters do not seem to be deterred by the spiny protection. They handle them rather carefully, though, often taking even small specimens ashore to eat. There is also a larger relative of the sea scorpion that is taken quite often by otters: the bullrout Myxocephalus scorpius. It weighs 60-120 g and is adorned with a strikingly bright red or orange belly, with white polka dots. It is a thrilling sight to see an otter carrying one ashore with the bright flash of red pointing ahead, and the large pectoral fins standing out, sometimes obscuring the otter's forward view. This species is known to be nocturnal in summer and diurnal in winter (Westin and Aneer 1987).

These were the common Shetland fishes that seemed to play the most important roles in the life of an otter, but I could have included several more, such as the ordinary stickleback Gasterosteus aculeatus and larger species such as lumpsuckers Cyclopterus lumpus and dogfish Scyliorhinus canicula; the latter two are quite common in slightly deeper parts of the coastal zone, and popular with otters. Everywhere along those Shetland shores there were thousands of crabs, almost all shore crabs Carcinus maenas, in sizes of up to 8 cm in carapax width. Otters had no problems catching and eating them (see Fig. 7.3), but energetically it was hardly worth their while.

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