Ardea
Official journal of the Netherlands Ornithologists' Union

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van Dobben W.H. (1952) The food of the Cormorant in the Netherlands. ARDEA 40 (1-2): 1-63
The food of the Cormorant, Phalacrocorax carbo sinensis (Shaw and Nodder) in the Netherlands. The investigation was carried out in a large rookery (about 2,000 nests, situated on trees), near the former Zuiderzee (fig. 1). The birds take their food for 94% from the north-eastern part of the IJsselmeer, generally within a distance of 25 km from home. The fishing is restricted to the daytime. The movements to and from the rookery (fig. 3) are discussed in chapters 4 and 6. Social fishing was observed only once. There are many indications that the birds dislike the (7 km) journeys over land to the lake and try to limit them to a minimum. Brooding birds go as a rule once a day, birds with nestlings once or twice. The number of feedings is accordingly low (chapter 12). This is made possible by the large capacity of the stomach (fig. 4) which may contain a maximum of 750 grams of fish. In a rookery situated near the hunting grounds the birds seem to fish more frequently. When a rookery is disturbed, many fishes are ejected, which makes it easy to obtain samples of the food. In the years 1938, 1939 and 1940, 13,000 fishes were examined. Hours after taking its meal a Cormorant can vomit undamaged parts of fishes and even whole fishes, while the digestion is rather slow and limited to the deeper part of the stomach (fig. 4). From some of the vomits it could be established that they were complete. When such vomits are collected early in the morning from brooding birds they generally represent a daily ration. Two series of complete vomits averaged resp. 340 and 285 g. (tables II, III). Another means to form an opinion on the food and the daily ration was found in the analysis of pellets (table I). Although the gastric juice of a Cormorant proved to be very acid (pH 0.9) the fish bones are generally removed undigested by vomiting, often in the form of pellets. In the latter case the pituitary lining of the gizzard is pushed off and envelops the bones and scales remaining from a meal. It could not be established whether pellets are formed after each meal, but it is probable that a pellet include~ the remains of one fishing tour. It was established experimentally that a large young Cormorant develops normally one daily ration of 300 g, but remains meagre. 100 g. more give it an excellent appearance. The calculations about the total requirement of the rookery are based on an average daily ration of 400 g (about 20% of the body weight) for adults and large young. The total requirement of a rookery with 4,000 adults, 2,600 non-breeding birds and 5,000 young during one season is estimated at 740,000 kg. The fish population of the IJsselmeer is characterised by the fact that only few species are numerous (chapter 15). Eel, Pike-Perch, Smelt and Ruffe occur in the whole lake; Roach, Bream and Perch are mainly restricted to the coastal zone. These species are captured by the Cormorants in large quantities, except the Smelt (figs. 5, 6). In addition many Tench are taken, often together with fish showing the typical light colour of all inhabitants of the IJsselmeer. Their origin, however, is not quite clear. The share of several species in the food of the Cormorant is presumably determined by the preference towards large fishes and their accessibility. The latter quality depends on living habits and swimming speed. Specimens under a weight of 6 g are rarely taken. The largest fishes recorded are: Pike, Awaite-Shad (both measuring 40 cm), Pike-Perch (38 cm), Roach, Perch, Tench (all about 30 cm) and Eel (65 cm). Apart from the Eel, such specimens represent a weight of about 500 gr. The composition of the food shows many .fluctuations (fig. 5-8). Differences between two successive years indicate changes in the fish population. Seasonal changes are due to alterations in size or behaviour of the fishes. Daily fluctuations are mainly connected with the weather and ruled by the accessibility of the Eel. In chapter 18-24 the most important fish species are discussed separately. The Eel. In the early spring no Eels are landed. The capture begins with sunny days in May and shows a clear connection with rises in temperature (figs. 7, 8). The majority of the Eel shows an injury at the nose. It is supposed that they are surprised by the Cormorant when they, in warm weather, lift the head out of the mud. The length frequency distribution of the captured Eel is recorded in fig. 9. It is calculated that the rookery took 7.5 million Eel from the IJsselmeer, while fishermen landed 55 million specimens. The actual damage is estimated at 10%. The Pike-Perch. In 1938 and 1939 a large majority of the Pike-Perches in the IJsselmeer belonged to the 1937 born generation. Their growth is recorded in fig. 10. In 1938 few specimens were captured from this generation, although they were numerous and sufficiently large. Other numerous fish from the same size may have been easier to capture. In 1939, when the generation from 1937 excelled by its size, increasing quantities were taken from March till June (fig. 6), when the larger part measured more than 36 cm and could no more be managed by the birds. In total 620,000 specimens were taken in 1939. While a large majority of the whole stock is taken by human fishery after reaching the legally established measurement (40 cm) it is certain that some damage is caused by the Cormorants. This damage is estimated at 10% of the total yields of the Pike-Perch fishery. Length frequency distributions of the Ruffe, the Roach, the Bream, the Perch and the Tench, landed by the Cormorants are recorded in figs. 11-15. The relative high proportion of elder generations reflects the preference toward large fishes. The changing share of age groups can be explained by numerical differences between successive generations. A large majority of the victims is 1 or 2 years old. An exception is formed by the Tench, which is taken mainly at an age of 3 years and more. This may be explained by the fact that young specimens live hidden between the waterweeds, while mature Tenches feed at the bottom in open places. The length frequency distribution of the Bream indicates that this fish becomes less accessible towards a length of 17-18 cm but is taken at an increasing scale at a larger size. This coincides with a change in behaviour and diet, after which the Bream more or less accepts the habits of a large Tench. The pike is taken in small numbers mainly at a length of 30-40 cm in the spring. There may be a connection with the spawning season. Smelt and Stickleback are taken occasionally only, obviously because of their small size. Chapter 31 gives some data from Cormorant rookeries near the large rivers. In Lekkerkerk (fig. 2) a small number of real saltwater fishes are recorded. During June large mature Awaite-Shads are landed occasionally. Birds from a rookery near Hook of Holland fish in the North Sea as well as in inland waters. Large Shrimps were recorded as a prey. In chapter 32 the parasites are discussed. Almost every Cormorant stomach is infested with the little nematode Contracaecum, which seems to do no harm. It could be established that from the large Roaches taken by the birds from the IJsselmeer, 30% was infested with the large cestod Ligula intestinalis L., while in the fish water the rate was only 6.5%. It can be assumed that the parasite increases the accessibility of the fish by hindering its movements. It could not be proved that the Cormorant is a suitable final host for the tapeworm, so it remains uncertain whether the bird plays a role in the spreading of the disease it seems, at first sight, to suppress.


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