Fourageermogelijkheden voor zeevogels in de boomkorvisserij: een verkennend onderzoek

Between 28 June and 2 July 1993, 34 discard experiments and counts of associated scavenging seabirds were performed from a commercial 2000 Hp beamtrawler (HD 7 “6 Gebroeders”) to the west of Helgoland and north of the Dutch Waddensea islands (fig. 1). The ship aimed at catching Sole and the landed fraction of the catch mainly included Plaice (40%), Sole (25%), Dab (12.5%) and small quantities of roundfish. The discards-fraction of the catch, i.e. discarded fish and benthic invertebrates, was roughly estimated at 5-10 kg on each landed kg. Discards included mainly Dab, Plaice, Grey Gurnard, Red Gurnard, Whiting, Dragonet and Cod. From the landed quantities and from estimates of the proportion discarded in each haul, it was estimated that 750-1500 kg were discarded each haul, comprising 30-60 kg roundfish, 350-700 kg flatfish, 350-700 kg benthic invertebrates, and 10-12 kg offal. A haul took approximately 1.5 hours and the fish was stripped and gutted immediately (this took approx. 0.5 hours). Discards are produced during towing at a speed of 6-7 knots as a steady trickle (figure 2). Only the 30 discard experiments to the west of Helgoland were analysed in detail. Eight species of seabirds were found associated with the trawler, of which Fulmar, Herring Gull, Lesser Black-backed Gull, Great Black-backed Gull and Kittiwake occurred frequently and in substantial numbers (table 1). Maximum numbers were observed when the catch was sorted and stripped. Similar numbers were observed at nearby beamtrawlers (on average 9 trawlers within 3 miles around the ship). Most Fulmars and Great Black-backed gulls were immatures or non-breeders (judged from plumage characteristics), while most Herring Gulls (86.1%), Lesser Black-backed Gulls (85.1%) and all Kittiwakes were adults. Lesser Black-backed Gulls numerically predominated at the trawl. Herring Gulls were most abundant at the more northerly and northwesterly trawling stations, hence at comparatively great distances from the shore. Offal and roundfish were preferred by scavenging seabirds (consumption rates 93.6% and 85.3% respectively), whereas flatfish was mainly taken when the intensity of feeding was high (mid-day; overall consumption rate 34.1%). Benthic invertebrates were virtually ignored (consumption rate 0.3%). Roundfish discards of all lengths were consumed, although consumption rates fell somewhat below 80% in larger fish (figure 3). Most discarded flatfish of over 18 cm length were rejected. Kittiwake and Herring Gull were most succesful consumers of offal (table 2). Roundfish discards included a large fraction of mainly 16-22 cm long gurnards and another fraction of mainly 24-30 cm long gadids. Median lengths of the commonest roundfish in the discard fraction are listed in table 3. Consumption rates were particularly low for (small) Dragonet (58.3%) and Hooknose (35.7%). Most succesful roundfish consumers were Great Black-backed Gull, Lesser Black-backed Gull and Fulmar (table 4). However, Fulmars were not seen to swallow fish, but pecked small quantities of flesh and intestines. Success indices for Lesser Black-backed Gulls were high only with regard to gurnard consumption (table 5), but rather low with respect to the consumption of gadoid fish (table 6). Herring Gulls seemed to prefer gadoid fish over gurnards. The median length of roundfish consumed by large gulls was rather dif- ferent: 19 cm for Lesser Black-backed Gulls, 26 cm for Herring Gulls, 27 cm for Great Black-backed Gulls. However, these differences were caused by a different selection of species of roundfish (table 7). Flatfish was consumed occasionally by all species and recorded frequencies were as expected from numbers at the trawl (table 10). Median lengths of discarded Dab, Plaice and Sole were 18, 22 and 22 cm respectively (table 8), while consumption rates of Plaice were lowest. Median lengths of consumed flatfish were 17 cm for Lesser Black-backed Gulls and Herring Gulls, and 19 cm for Great Black-backed Gulls (table 9). The feeding flock of birds concentrated near the stern when feeding intensity was high (figure 4), with Kittiwakes very near the ship and Great Black-backed Gulls mainly in the rear and of the flock. Of the large gulls, Lesser Black-backed Gulls suffered most losses of fish through robbery, while Great Black-backed Gulls were the most successful pirates (table 11). Scavenging numbers at the trawl and consumption rates peaked during mid-day (figure 5). The entire fleet of on average 9 beamtrawlers within 3 miles around the observation platform (HD 7) was estimated to be associated with over 1600 scavenging seabirds. Of these, 68.0% were Lesser Black-backed Gulls and 21.7% were Herring Gulls. The number of Herring Gulls was high considering the distance from the nearest colony (earlier studies showed that most breeding Herring Gulls stay within 10 km from land), while the number of Lesser Black-backed Gulls was extraordinary considering the local breeding population of only ca. 2100 pairs. It is concluded that, although most of these gulls were not moulting and in adult summer plumage, their status as breeding birds is doubtful. The Kittiwakes could originate from breeding colonies on Helgoland. Herring Gulls were found to be dominant over Lesser Black-backed Gulls at the trawl. More fish were lost through robbery by the latter, and Herring Gulls were more successful when feeding on offal, which has a comparatively high calorific value, and gadoids, which were big and easy to handle fish. The observations did not confirm earlier findings that Lesser Black-backed Gulls were more manoeuvrable birds at the trawl than Herring Gulls. Considering the estimated quantities of discards and consumption rates at the trawl, it is estimated that the discards-stream of a single haul could support 500 Herring Gulls or Lesser Black-backed Gulls a whole day; 9 trawlers at 8 hauls during daylight some 36,000 large gulls. Since on average 544 gulls were attending the vessel, this would indicate a turnover of nearly 100%, which was obviously not the case since birds were seen to move from ship to ship. The major sources of error in these estimates are (1) the estimate of the discard fraction is incorrect, (2) the consumption rates found in discard experiments are too high, (3) the estimate of the associated numbers are incorrect. Future studies will focus on these problems.