136
G.C.Cadee
Water jets are also used by the New Zealand eagle ray Myliobatis tenuicaudatus to dig out bivalves they are feeding on (Gregory et al. 1979);
others claim Myliobatis dig holes by flapping their pectoral fins (McGinitie
and McGinitie 1968). The holes produced by three species of stingrays
(Dasyatis) reach sizes of up to 1 m in diameter and up to 30 cm in depth
(Howard et al.1977). VanBlaricom (1982) observed somewhat smaller holes
that had been produced by the round stingray Urolophus halleri and the bat
ray Myliobatis californica off La Jolla, California. He observed a strong
seasonal variation and an average daily disturbance rate of 0.84 % of the
transect. Taking 5 cm as the average depth of the entire excavation made, I
calculate that rays rework annually an amount of sediment equalling a
layer of 15 cm (365 x 0.0084 x 5 cm= 15 cm). Myrick and Flessa (1996) estimated the same two species of rays to rework as much as 1 m of sediment
annually on the lower tidal flats of Bahia la Choya, northern Gulf of California, Mexico.
Very comparable holes to those produced by rays were reported by Cadee
(1990) from the Dutch Wadden Sea (Fig. 6.5). These were, however, made by
shelducks, Tadorna tadorna. Other ducks, such as the eider Somateria mollissima, may make similar holes during food collecting (Wietfield 1980). Blackheaded gulls Larus ridibundus produced up to 3 m long, 15 cm wide and ca.
5 cm deep feeding traces by trampling and walking backwards in shallow
water left on tidal flats (Cadee 1990). By trampling, they fluidise the sediment
in which worms and bivalves then float and thus become an easy prey. These
traces have wrongly been explained as resting traces (e. g. Hertweck 1970; see
Cadee 1990 for discussion). Sediment reworking by these birds showed strong
seasonal fluctuation and equalled a sediment layer of 2.5 cm per year or ca.
10 % of that reworked by deposit feeders on the same tidal flat. Flamingoes
may produce very characteristic feeding holes when feeding on tidal flats:
circular holes, about 1 m in diameter, with a hill in the middle (Fig. 6.6; photograph taken by H. Michaelis on the Banc d' Arguin, Mauritania, and kindly
made available for reproduction here). Treading around and around on mud,
they sieve the sediment with their very characteristically structured bill for
small organisms or even microscopic algae; discarded sediment piles up in
the middle of the ring (Jenkin 1957).
Digging for prey is described for the sea otter Enhydra lutris in Alaskan
waters (Calkins 1977). In shallow water they make holes 15-45 cm across and
up to 50 cm deep, to dig out bivalves. In Monterey Bay, California, Hines and
Loughlin (1980) observed sea otters digging holes even larger (150 X 50 cm
large and up to 50 cm deep) made during successive dives by the same animal.
The crab Cancer pagurus digs holes in subtidal sands in search of food of up
to 50 cm in diameter (Hall et al.1991). Woodin (1978) describes feeding pits of
horse-shoe crabs (Limulus polyphemus) and blue crabs (Callinectes sapidus)
on a tidal flat in Virginia.
G.C.Cadee
Water jets are also used by the New Zealand eagle ray Myliobatis tenuicaudatus to dig out bivalves they are feeding on (Gregory et al. 1979);
others claim Myliobatis dig holes by flapping their pectoral fins (McGinitie
and McGinitie 1968). The holes produced by three species of stingrays
(Dasyatis) reach sizes of up to 1 m in diameter and up to 30 cm in depth
(Howard et al.1977). VanBlaricom (1982) observed somewhat smaller holes
that had been produced by the round stingray Urolophus halleri and the bat
ray Myliobatis californica off La Jolla, California. He observed a strong
seasonal variation and an average daily disturbance rate of 0.84 % of the
transect. Taking 5 cm as the average depth of the entire excavation made, I
calculate that rays rework annually an amount of sediment equalling a
layer of 15 cm (365 x 0.0084 x 5 cm= 15 cm). Myrick and Flessa (1996) estimated the same two species of rays to rework as much as 1 m of sediment
annually on the lower tidal flats of Bahia la Choya, northern Gulf of California, Mexico.
Very comparable holes to those produced by rays were reported by Cadee
(1990) from the Dutch Wadden Sea (Fig. 6.5). These were, however, made by
shelducks, Tadorna tadorna. Other ducks, such as the eider Somateria mollissima, may make similar holes during food collecting (Wietfield 1980). Blackheaded gulls Larus ridibundus produced up to 3 m long, 15 cm wide and ca.
5 cm deep feeding traces by trampling and walking backwards in shallow
water left on tidal flats (Cadee 1990). By trampling, they fluidise the sediment
in which worms and bivalves then float and thus become an easy prey. These
traces have wrongly been explained as resting traces (e. g. Hertweck 1970; see
Cadee 1990 for discussion). Sediment reworking by these birds showed strong
seasonal fluctuation and equalled a sediment layer of 2.5 cm per year or ca.
10 % of that reworked by deposit feeders on the same tidal flat. Flamingoes
may produce very characteristic feeding holes when feeding on tidal flats:
circular holes, about 1 m in diameter, with a hill in the middle (Fig. 6.6; photograph taken by H. Michaelis on the Banc d' Arguin, Mauritania, and kindly
made available for reproduction here). Treading around and around on mud,
they sieve the sediment with their very characteristically structured bill for
small organisms or even microscopic algae; discarded sediment piles up in
the middle of the ring (Jenkin 1957).
Digging for prey is described for the sea otter Enhydra lutris in Alaskan
waters (Calkins 1977). In shallow water they make holes 15-45 cm across and
up to 50 cm deep, to dig out bivalves. In Monterey Bay, California, Hines and
Loughlin (1980) observed sea otters digging holes even larger (150 X 50 cm
large and up to 50 cm deep) made during successive dives by the same animal.
The crab Cancer pagurus digs holes in subtidal sands in search of food of up
to 50 cm in diameter (Hall et al.1991). Woodin (1978) describes feeding pits of
horse-shoe crabs (Limulus polyphemus) and blue crabs (Callinectes sapidus)
on a tidal flat in Virginia.