Because the giant otter is diurnal, its diet has been studied by direct observation of its foraging activities, as well as by faecal analysis. This species is a fish eater, with very little else in its diet, although Nicole Duplaix (1980) found crab remains in some spraints. In the different study areas, most of the food is made up by only a few species of fish, selected from an astonishingly numerous fish fauna. Duplaix mentions that near her Surinam study area 'in 1912, Eigenmann collected 70 to 90 species in one haul of a seine net. . . , and another 60 species in a small creek a few hours later' (Duplaix 1980, p 516). Nevertheless, her giant otters captured only 11 species; most of these were trahiras or wolf fish (the characoid Hoplias malabaricus), a fairly large, slow, aggressive predator that lies still in shallow waters, between leaves and branches, and is easy to catch. Other favourites were the similarly slow siluroids or catfish, and perch-like fishes, especially cichlids.
In other parts of its geographical range, for example Guyana (Laidler 1984) and Brazilian Amazonia (Rosas et al. 1999; see also the review by Carter and Rosas 1997), the giant otter also specializes in Hoplias, siluroids and cichlids. Christof Schenck (1997) analysed 60 faecal samples from 'campsites' in Peru; inevitably, each sample contained contributions from several otters, all mixed together. Identifying fish scales in these samples, Schenck concluded that almost half of the remains came from the small cichlid Satanoperca jurupari ('earth eater'), a quarter from the large Prochilodus caudifasciatus (a somewhat roach-like fish), and a large number of rarer species. The giant otters did not take (or rarely took) the abundant piranha species. Many of these giant otter prey are either rather slow fishes, such as the cichlids, or they are nocturnal, bottom-living fishes from shallow waters, easy to catch in daytime (like the prey of the Eurasian otters in marine habitats such as Shetland).
Giant otter diet also includes some rather curious items, such as tapir dung and invertebrates extracted from mud. On rare occasions they take a small mammal or amphibian (Duplaix 1980), a bird or a reptile such as anaconda or other snake, and small caimans and turtles. In Brasilia Zoo, giant otters often catch herons that scavenge in their enclosure on fish remains (Carter and Rosas 1997).
The only study in which giant otters were found to eat substantial numbers of crabs was that of Nicole Duplaix in Surinam. Curiously, she never saw crabs being taken, against many fishes, but found quite large quantities of crab remains in 40% of otters' spraints in areas well away from the core areas in the normal, territorial ranges. Possibly, the giant otters' core areas were located where there were more fish; another explanation could be that non-territorial animals, just like the Eurasian otters in Shetland, were more inclined to take substandard prey.
The sizes of fish taken by giant otters are 10-40 cm (review and summary in Carter and Rosas 1997), dependent on species. Duplaix (1980) cited mean lengths for caught Hoplias as around 20 cm, for cichlids 10-15 cm, but also incidental captures of tiger catfish of 50-60 cm and peacock bass Cichla ocellaris of 35-40 cm. Carter and Rosas (1997) referred to some 'enormous catfish' being taken, with lengths of over 1 m. The main food species in Schenck's (1997) study, Satanoperca, is small, on average about 60 g, but the second most common one, Prochilodus, had a mean weight of around 450 g.
In captivity adult giant otters consume the equivalent of 7-10% of their bodyweight (i.e. about 3 kg of fish) per day, sub-adults about 13% (Carter and Rosas 1997). In the wild, Nicole Duplaix (1980) estimated that adult otters ate about 3 kg daily. Staib (2002) calculated that the giant otters in her study in the Peruvian Amazon consumed 3.2-4.0 kg of fish per otter per day. It appears evident that, weight for weight, giant otters eat less than, for example, Eurasian otters (see above); this may be a function of their larger body size or of their warmer environment.
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