Publication Date:
2024-06-18
Description:
The area of investigations belongs to the subtidal (7-16 m depth) and is mostly covered by fine sand with spots of coarse sand in between. Based on the sediment distribution, two associations can be differentiated: fine and coarse sands, with smooth transitions (mixed and silty sediments). The species belonging to the different associations correspond with those of earlier investigations (ZIEGELMEIER 1963, STRIPP 1969, RACHOR & GERLACH 1978, ZIEGELMEIER 1978, RA CHOR 1980, SALZWEDEL et al. 1985, 1990, NIERMANN 1991 ), except for the new invader, the bivalve Ensis directus. During this study fluctuations in number of species and individuals could be observed. Diversity and evenness decreased in spring 2001 due to strong recruitment of polychaeta, but mainly of Ensis directus. 23 years after the invasion, Ensis directus is an important member of the sandy subtidal areas which are exposed to currents with a mean abundance of 925 m-2. E. directus does not seem to prefer special habitats. Their locomotion capacity above and within the sediment enables this species to react quickly on bottom disturbances and thus to survive in current exposed habitats. The mussel shows extremely high abundances (1972 respectively 5481 m-2) on fine and silty fine sands. During this study mainly one and two year old razor shells were found, some with a maximum age of 3.5 years. E. directus is an opportunistic species with mass development (MOHLENARDT-SIEGEL et al. 1983), high recruitment(〉 25000 m-2 in July 2001) and strong growth (3.6 mm month-1). Additionally, the mass mortalities (mortality of 77 respectively 84 % year-1) were observed, the reasons for that are not known yet. The interaction between low temperature and limited movement could be responsible for the washing out of the individuals. Since mortality takes place especially in springtime, the reasons could be the depletion after spawning aswell. Spisula solida was found regularly in the investigation area (frequency: 64 %), but in low numbers (mean abundance: 5 m-2). In coarse sand locally higher abundances were found (13 m-2). The surf clam is one of three dominant species (11 %) in this area. Just as E. directus, S. solida is a fast burrowing mussel (KOCK 1995) and therefore adapted to exposed sedimentary habitats. Nevertheless, the abundances in 2000 2001 lie well below values of those of the mid 90ies (MEIXNER 1993, 1994, KOCK 1995). This can also be seen in the decreasing landings of surf clams since the middle of the 90ies. Whether the effects of the ice winter 1995/96 or other reasons are responsible for the disappearance of S. solida, remains unclear. In the investigation area mainly small individuals were found (mean length: 9,7 mm). Due to the low figures in the sample neither growth rate nor recruitment or structure of the stock could be studied. The study shows the gap between the stocks of E. directus and S. solida, as well as fishery in the North Sea. Various details of the species biology remains unclear. Therefore supporting
studies are necessary to ensure sustainable fishery.
Type:
Thesis
,
NonPeerReviewed
Format:
text
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