Time and energy constraints of fishing behaviour in breeding Cormorants <i>Phalacrocorax carbo sinensis</i> at lake IJsselmeer, The Netherlands

TWO Cormorant colonies in The Netherlands (Naardermeer and Oostvaardersplassen), exploiting the same water bodies but situated at different distances from them, were compared with respect to daily variations in exact fishing sites and corresponding variations in time budget and fish consumption. Mean flying distances between colony and fishing site were estimated at 22 and 15 km respectively. Birds from the Naardermeer colony carried out less trips but of a longer duration than birds from Oostvaardersplassen, most markedly so in the chick rearing period (2 trips per day of 185 min vs. 3 trips of 165 min). Daily fluctuations in time spent away from the colony were clearly dependent on daily shifts in main fishing sites. On days when fishing was concentrated at larger distances, individual foraging trips lasted longer, due to the increase in flying time. Neither net fishing time nor daily fish consumption, as estimated by pellet analyses, compensated for the increment in time and energy expenditure on those days. It was estimated that the average daily energy expenditure would amount to about 2.8 . BMR (basal metabolic rate) in birds from Naardermeer and to about 2.7 . BMR in birds from Oostvaardersplassen. Fish consumption estimates based on pellet analyses led to an estimated DME (daily metabolisable energy) of 2 . BMR for both colonies. Thus, an overall negative energy balance became apparent, resulting in estimated mass losses throughout the breeding season of on average 980 and 860 g for Naardermeer and Oostvaardersplassen birds, respectively. Mass losses are likely to be higher with increasing travelling distances, indicating that travelling distance may influence reproductive output. This could be one of the factors causing consistently lower reproductive outputs at the Naardermeer throughout the years.</p

Mass fishing by Cormorants <i>Phalacrocorax carbo sinensis</i> at lake IJsselmeer, The Netherlands:A recent and successful adaptation to a turbid environment

The habit of mass fishing by Cormorants at lake IJsselmeer, The Netherlands, is a recent phenomenon. During the first half of the 1970s the birds changed behaviour probably as a result of the deteriorating under water visibility in the lake (3-4 m water depth). The behavioural switch coincided with years of high numbers of Smelt Osmerus eperlanus and Ruffe Gymnocephalus cernuus present in the southeastern part of lake Markermeer, the birds' main fishing area at that time. Social fishing by Cormorants is directed towards the catch of relatively small, pelagically dwelling fish. It is argued that for a large water system where social fishing is the rule, a minimum colony size of c. 1000 pairs is required. Typically each colony had one socially fishing group (4000-5000 birds) that slowly changed position through the course of the day. Depending on the direction of the wind the flock's position could greatly change between days. Hunting speed was measured and coincided with maximum swimming speed of medium sized fish prey (15-25 cm). Hunting speed increased during the season probably as a result of the greater swimming speeds of the fish at higher temperatures. Intake rate was closely linked to the birds' position within the flock indicating local depletion of the fished water layer. Mass fishing was especially rewarding at intermediate light intensities under water (50-80 cm Secchi depth, or 300-500 mu E.m(-2)s(-1) at 40 cm depth). The habit of pushing up the fish against the light back-ground of the clear top water layer was only possible when wind caused no greater turbidity than 40 cm Secchi depth (100 mu E.m(-2)s(-1)) which is considered a breakpoint for this kind of behaviour. Adapting the habit of mass fishing effectively enabled the birds to exploit the turbid, rapidly changing environment which resulted in the extension of the foraging range thus maximising colony size relative to the resources available.</p

Solitary foraging in sand pits by breeding Cormorants <i>Phalacrocorax carbo sinensis</i>:Does specialised knowledge about fishing sites and fish behaviour pay off?

Though most Cormorants from the colonies exploiting the lake IJsselmeer area in The Netherlands nowadays have taken up the habit of fishing in large groups, some individuals are still observed fishing solitarily. This phenomenon occurs predominantly during the coldest months at well-defined patches, often located in sand pit regions. A case study in a sand pit in the early spring of 1982 revealed that Cormorants from Oostvaardersplassen took advantage of the fish located at their winter refuges inside the pit by specifically diving along the pit's steepest slopes. Thus, they were able to achieve a more than average daily ration of about 475 g of fish within a mere 15 minutes of foraging in the second half of March. Later on, the site became less profitable and the birds gradually left. The importance of high intake rates in early spring is stressed, in view of time consuming activities like nest site occupation, pair formation and accumulation of body reserves for egg-laying, incubation and chick feeding. It is suggested that only the 'higher quality' birds may have the experience and skill required to use solitary fishing techniques as a profitable alternative to social fishing in early spring.</p

Competitive food exploitation of smelt Osmerus eperlanus by great crested grebes Podiceps cristatus and perch Perca fluviatilis at Lake IJsselmeer, The Netherlands

The impact of predation by an avian predator (great crested grebe Podiceps cristatus) and a predatory fish (perch Perca fluviatilis) on a common resource, the small planktivorous fish smelt (Osmerus eperlanus) was studied in Lake IJsselmeer, The Netherlands, in July-October 1985-1988. At this time of the year the grebes are in simultaneous wing-moult and therefore temporarily flightless. Four factors limit the predation pressure by grebes: (1) an underwater visibility threshold of c. 40 cm Secchi depth is the lower limit at which large numbers attend the moulting site, (2) a lower density threshold of exploitable smelt biomass of c. 30 kg/ha determines the actual fishing areas, (3) a fish size threshold of 6.5 cm total length further limits prey availability, and (4) the vertical movements of the prey impose a strong (diel) time constraint on the birds, allowing only crepuscular foraging. Perch, the other main predator in this system, is less restricted in its foraging. Perch were found to be in direct competition with the grebes, preying heavily on the juvenile smelt. Over the 4 years of study 90% of all predation from 15 July to 15 October (72 233 kg/ha) was due to perch. The high level of predation by birds and fish was only possible due to a continuous immigration of smelt into the area (1.45 kg/ha per day). In three of the four years, however, the combined predation by fish and birds surpassed the immigration rate of the prey, which led to a strong reduction in smelt stocks in the study area. A conceptual model is developed to describe the different sets of constraints on the grebes' foraging. First, the state of eutrophication in relation to the weather condition determines the degree of algal blooms, and thus underwater visibility, in late summer. This is the major factor governing the numbers of grebes on the moulting area. Second, the size of the population of predatory fish determines the overall food availability (biomass and size distribution of smelt). These factors are partly interconnected and related to human action (pollution, fisheries). It is suggested that, despite deteriorating visibility conditions, the largest moulting site for grebes in Europe at Lake IJsselmeer exists because the stocks of predatory fish are kept low by overfishing

Riding on the crest of the wave:Possibilities and limitations for a thriving population of migratory Cormorants <i>Phalacrocorax carbo</i> in man-dominated wetlands

This paper summarises the general findings of the scientific contributions to this special issue of Ardea on Cormorant biology in Europe. After a brief historical introduction setting the main traditional questions that arise whenever man and a species of a free-living animal compete for the same resources in the same habitat, an overview is given of how Cormorant populations have recovered after protective measures were taken over most of Europe. This recovery, particularly spectacular in the continental population, has brought the species once again into direct conflict with human interests. Research has been aimed at unravelling the principal natural factors limiting population growth. Surface area, productivity and accessability of fishing grounds for breeding birds seem to set the extent to which the population may increase but the end of the population growth seems not yet to be at hand. Because of their migratory behaviour birds spread all over western Europe outside the breeding season, showing often site tenacity to stop-over sites as well as predictable migration patterns. Factors influencing the non-breeding distribution include age, sex and probably individual experience of the birds concerned. Local conflicts between human interests and breeding as well as non-breeding Cormorants seem impossible to cope with satisfactorily on a local level, without consideration of the population as a whole. Not every possible impact of Cormorants on fish stocks is bound to be treated as negative: recent studies tentatively indicate that the birds may serve as an 'aid' to fisheries management, keeping eutrophic waters free from dense Bream stocks. Finally, the position of top-predator that Cormorants occupy in aquatic ecosystems, makes them vulnerable to the accumulation of micropollutants. These may hamper breeding success in various ways and, in larger concentrations, may even cause death. Monitoring breeding success as well as pollutant contents in adult and juvenile Cormorants as well as in their eggs may serve as a means to check on trends in environmental quality of aquatic ecosystems.</p