WI - Cormorant Research Group The Bulletin - No. 4, June 2000 Original papers

THE USE OF CHEWING PADS AND OTOLITHS OF CYPRINIDS FOR
ESTIMATING FOOD INTAKE OF CORMORANTS Phalacrocorax carbo

Jeroen Nienhuis

Zoological Laboratory, University of Groningen, P.O. Box 14, NL-9750 AA Haren, The Netherlands
Institute for Inland Water Management and Wastewater Treatment RIZA, P.O. Box 17, NL-8200 AA Lelystad, The Netherlands
Present address: Lokveenweg 18, NL-9751 CJ Haren, The Netherlands

Food choice is one of the better known aspects of Cormarant ecology. In fresh water cyprinids are one of the most common family of prey species (Dirksen et al. 1995, Melin & Martyniak 1991, Suter 1997, Veldkamp 1995b). In studies concerning pellet analysis the proportion of cyprinids not identified to species level can be high. This is the consequence of two things. The difference between the otoliths of the different species are sometimes very small and the species specific pharyncheal bones are very vulnerable to gastric juices. Veldkamp (1995a) found that chewing pads combine the good qualities of both otoliths and pharyncheal bones. They are species specific and less vulnerable to gastric juices. In many cases it is possible to identify a fish species with otoliths as well as with chewing pads. The question rises which of these two fish remains is most resistant to gastric acid.

In the 1993 breeding season food choice of Cormorants of the Oostvaardersplassen colony in the Netherlands was studied. A total of 384 pellets were analysed. These contained 90 Bream Abramis brama otoliths (asterisci) and 52 chewing pads of the same species as well as 594 chewing pads and 1092 otoliths (asterisci) of Roach Rutilis rutilis. The latter value also includes unidentified cyprinids since 90 percent of all identified cyprinids did belong to this species.

The length of these fish remains are used to calculate the length of the fishes they originated from. The formulae used for these calculations can be found in table 1. The fish lengths calculated from otolith length and from chewing pad length are compared in order to find out which of these lengths results in the longest fishes.

Table 1. Formulas to calculate the fish length from the length of fish remains. All measurements are in mm. FL=fork length, TL=total length, OL=otolith length, CP=chewing pad length, OW=otolith width (width perpendicular to the straight line describing the maximum length)
species   range source
Bream FL = 21.86 + 63.257 * OL 1.0-5.0 Veldkamp 1994b
  FL = 7.826 + 41.805 * CP 1.0-6.5 Veldkamp 1994a
Roach lnTL = 4.061 + 1.237 * lnOW 1.0-4.0 Nienhuis 1995
  lnTL = 3.897 + 0.734 * lnCP 1.5-13 Nienhuis 1995

In total there were remains of 61 Bream (table 2). The actual number of fishes eaten will be somewhat higher, since it is unknown of how many fishes neither chewing pads nor otoliths were found. With the data from table 2 the real number of Bream eaten can be calculated. Both otoliths of a fish were absent in 13.5% of all fishes (n=52). No chewing pad was found in 16.6% of the cases (n=54). The chance that chewing pads as well as both otoliths were absent is 0.135*0.166=0.022 (2.2%). This figure leads to a total number of 62 Bream consumed by the Cormorants. The average number of otoliths found in pellets is 1.45±0.72 per fish. For chewing pads this is 0.84±0.37 per fish. In fishes these values are 2 and 1 respectively.

For 28 Bream the fish length calculated with chewing pads and with both otoliths could be compared directly. The average fish length calculated from otolith length is significant longer than the length calculated from chewing pad length (largest otolith within a pair: z=3.78, p=0.0001, smallest otolith: z=2.65, p=0.0040). The average differences in fish length for the largest and smallest otolith are 30±27 mm and 18±30 mm respectively. This means that otoliths of Bream are less vulnerable to gastric juices than their chewing pads.

Table 2. The number of Bream of which chewing pads and/or otoliths were found.
    Chewing pad  
    present absent
Otoliths present 45 9
  absent 7 ?

The average length of the Roach calculated with otolith width (118±52 mm) is much lower than the length calculated with chewing pad length (162±47 mm, figure 1). For Roach chewing pads are more resistant to gastric juices than otoliths.

The use of chewing pads has clear advantages to pharyncheal bones since they are less vulnerable to gastric juices (Veldkamp 1995a). The advantage of chewing pads to otoliths is less clear. Roach chewing pads are less affected by gastric juices than otoliths resulting in a better estimation of the fish length. Compared with Roach, Bream has a relatively small and thin chewing pads (Veldkamp 1995a) which are affected more by gastic acid than otoliths of the same species.

Figure 1. Length-frequency distribution of Roach as calculated from otolith length and chewing pad length.

References

Dirksen, S., T.J. Boudewijn, R. Noordhuis & E.C.L. Marteijn. 1997. Cormorants Palacrocorax carbo sinensis in shallow eutrophic freshwater lakes: prey choice and fish consumption in the non-breeding period and effects of large-scale fish removal. Ardea 85: 167-184.

Melin, M. & A. Martyniak. 1991. Food composition of the Cormorant Phalacrocorax carbo sinensis in north-eastern Poland; preliminary results. In: Van Eerden, M.R. & M. Zijlstra (eds.). Proc. wordshop 1989 on Cormorants Phalacrocorax carbo. Rijkswaterstaat Directorate Flevoland, Lelystad.

Nienhuis, J. 1995. Voedselkeuze van Aalscholvers Phalacorcorax carbo sinensis in de Oostvaardersplassen in 1993 in relatie tot het weer en het reproductief succes. Internal report 1995-17 Lio, Rijkswaterstaat Directie IJsselmeergebied, Lelystad.

Suter, W. 1997. Roach rules: shoaling fish are a constant factor in the diet of Cormorants Palacrocorax carbo in Switzerland. Ardea 85: 9-27.

Veldkamp, R. 1995a. The use of chewing pads for estimating the consumption of cyprinids by Cormorants Palacrocorax carbo. Ardea 83: 135-138.

Veldkamp, R. 1995b. Diet of Cormorants Phalacrocorax carbo sinensis at Wanneperveen, The Netherlands, with special reference to Bream Abramis brama. Ardea 83: 143-155.

Jeroen Nienhuis, Zoological Laboratory, University of Groningen, P.O. Box 14, NL-9750 AA Haren, The Netherlands
Institute for Inland Water Management and Wastewater Treatment RIZA, P.O. Box 17, NL-8200 AA Lelystad, The Netherlands
Present address: Lokveenweg 18, NL-9751 CJ Haren, The Netherlands