The fish fauna of the Oosterschelde, a decade after completion of the engineering works

• Main Authors: Hostens, K., Hummel, H., Mees, J., Vincx, M., Heip, C.H.R.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2003
• Subjects: Estuaries; Juveniles; Nursery grounds; ANE; Netherlands; Oosterschelde; Anguilla anguilla
• Online Access: http://www.vliz.be/nl/open-marien-archief?module=ref&refid=33843

In this study we investigate whether long-term changes in diversity, density and/or biomass of the juvenile fish fauna of the Oosterschelde can be detected. Quarterly beam trawl data from the period 1999-2001 are compared with quarterly data from 1987-89. Also, a brief comparison infrequency of occurrence was made with data from 1960-76. In 1999-2001, a total of 42 fish species was recorded; which is comparable to the period 1960-76. Diadromous species (e.g. Dismerus eperlanus and Anguilla anguilla ) showed a reduced frequency of occurrence. Some species (e.g. Gobius niger ) still entered the Oosterschelde through sluices; others (e. g. Gasterosteus aculeatus ) could not withstand the high salinities in the Dutch Delta area. The number of species per season was 21-28 in 1987-89 and25-34 in 1999-2001, while the number of species per station was 15-22 in 1987-89 and 17-31 in 1999-.2001. This increase could mainly be attributed to rare species. The commonest species in both periods were Pomatoschistus minutus , Pleuronectes platessa , Limanda limanda and Trisopterus luscus . A 45% increase was noted in total average fish density in 1999-2001, which could mainly be attributed to P. minutus . The total average biomass decreased with 35%, but by excluding P. platessa and L. limanda , the decrease was limited to the eastern part in 1999-2001. Both on temporal and spatial scales, density and biomass were significant I y different for more than 10 species. Multivariate analyses revealed seasonal (autumn-winter versus spring-summer) and spatial differences (western-central-eastern subareas), with a separation of both periods. For most species the long-term spatio-temporal differences did not reflect 'real' changes in the nursery function of the Oosterschelde. Either they reflected natural population variability due to a delayed appearance of strong yearclasses after cold winters (e.g. high densities of P. platessa and L. limanda in 1987), or changes were obscured through sampling bias (e. g. an increased net-efficiency through clogging of the nets for Sygnathus rostellatus and P. minutus in 1999-2001, or an unexplained reduced efficiency for gobies in 1987-89). F or all species with strong yearclasses (also Myoxocephalus scorpius in 1987 and Sprattus sprattus in 1999), the averaged densities rapidly declined to a 'normal' state of homeostas is within the successive years and sea- sons of both periods. The highest densities were generally recorded in the western and northern part of the Oosterschelde in both periods. Many species (28) showed an increase in the central part. Still, for most species the loss at one station was compensated with an increase at another station in most subareas. This may be related to the location of food resources (e.g. mussels for flatfish in the western part, shrimps for Callionymus lyra in the central part, and high macro- and hyperbenthic biomasses in the northern part). Also, a correlation between substratum type (> 50% silt concentration, mainly in the western and northern parts) and high species abundance was observed. For the estuarine resident and marine juvenile species (highest densities of O- and 1- group individuals in summer and autumn) the Oosterschelde can still be considered to be an important nursery area. For several marine seasonal species the ecosystem is still functioning as an important feeding area. Estuarine ecosystems like the Oosterschelde are able to absorb and adjust to changes, but they can also be very vulnerable. One could speak of 'homeostasis' in the fish population in most parts of the Oosterschelde, except in the eastern part where at least 16 species- and especially all flatfish and gadoid species- drastically declined in 1999-2001