Hydroacoustic assessment of the fish stock in Theodorushaven, a small Dutch harbour

Assessment of the fish stock in the Theodorushaven was conducted with a dual-beam hydroacoustic system. Species composition determined by a seine and trawl fishery showed large concentrations of fish in the harbour during winter. The hydroacoustic survey showed that the distribution of fish in the harbour area was stratified with fish near the bottom. The overall density in the harbour area was 0.67 fish m^–3. The density in the associated channel area was estimated at 0.1 fish m^–3. The coefficient of variation for the estimated total fish abundance was >25%.Fish >25 cm were not caught in the Theodorushaven fishery, although analysis of the sonar data indicated their presence. Fishery data from the adjacent Volkerak-Zoommeer showed that bream was the dominant species. Therefore, the biomass estimate of the stock >25 cm in the Theodorushaven was based on the standard length–weight relationship of bream. This resulted in maximum biomass estimates of 5800 kg ha^–1 for the harbour area and 1100 kg ha^–1 in the channel area. The minimum biomass estimate, based on the assumption that the more slender zander was the dominant species >25 cm, gave values of 4100 kg ha^–1 in the harbour area and 780 kg ha^–1 in the channel area.