Bregje Beyst

Early postlarval fish in the hyperbenthos of the Dutch Delta (south-west Netherlands)

Early (post)larval fish constitute a significant part of the temporary hyperbenthos, i.e. of the fauna living in the lower reaches of the water column close to the substratum. Information on the densities and spatial and temporal variations of these stages, as well as on their lengths at the moment of occurrence, can be an important contribution to the knowledge of their dispersion, migrations and to the identification of possible bottlenecks in their recruitment. Therefore the hyperbenthos of the Voordelta, a shallow coastal zone in front of the Dutch Delta, as well as that of the subtidal channels and the intertidal brackish marsh creeks of the Westerschelde Estuary, was sampled monthly during one year by means of a sledge and a stow net. A total of 37 taxa were recorded from 410 samples. Densities often exceeded 400 ind 1000 m −2 in the Voordelta and 300 ind 1000 m −2 in the Westerschelde, including the tidal marshes of Waarde and Saeftinghe. The dominant taxa were clupeid larvae (predominantly herring Clupea harengus and sprat Sprattus sprattus ), Pomatoschistus spp. (a mixture of sand goby P. minutus and Lozanos goby P. lozanoi ), common goby Pomatoschistus microps , Syngnathus spp. (probably almost exclusively Nilssons pipefish S. rostellatus ), plaice Pleuronectes platessa , sole Solea solea and flounder Pleuronectes flesus . Most species were found at a larger size in the Westerschelde (and in the tidal marshes) suggesting migration from the shallow coastal area into the estuary. Several species utilize the tidal marshes in the brackish reaches of the Westerschelde. The nursery value of these areas however, is restricted to specific early life history stages of a few species, especially flounder, bass Dicentrarchus labrax and the common goby. The estuary itself seems to function as a nursery for other species like sole and plaice. The hyperbenthic sledge was found to sample specific life history stages of postlarval flatfish (early settlement stages at the onset of asymmetry), which are not easy to sample with other types of sampling gear.

Spatial patterns of the surf zone hyperbenthic fauna of Valdivia Bay (Ecuador)

AbstractThe present research is a first attempt to study the hyperbenthic fauna of the surf zone of Valdivia Bay (South East Pacific--Ecuador), an exposed low tide terrace--rip beach. The aims of the study were to contribute to the inventory of the surf zone fauna of the tropical southeast Pacific coast, to investigate spatial distribution patterns along the bay and to search for possible tidal effects on the hyperbenthic composition. Samples were taken during daylight with a hyperbenthic sledge on 6 consecutive days in August 1999 at high tide, low tide and mid tide. The hyperbenthic community was characterized by quantifying the higher taxa, their density and the relative composition of the dominant groups. No tidal effect could be observed, but two main geographically distinct hyperbenthic assemblages were distinguished along the bay, related to hydrodynamic factors (local current velocity, intertidal beach slope and suspended particulate matter). In the turbulent northern part of the bay the highest densities were encountered and hyperbenthos was, next to the dominant mysid Metamysidopsissp., mainly composed of planktonic organisms (calanoid copepods, chaetognaths and fish eggs). In the calmer southern part of the bay Metamysidopsissp. dominated as well, but the community was completed with other mysid species, and early life stages of Brachyura and fish. Most striking was the general dominance of the mysid Metamysidopsis sp. in all the samples (97%, max. density of 62000 ind. 100 m-2) and the high amount of unknown species. Penaeid shrimp larvae, important for the local shrimp industry, were only found in very low densities during the sampling period (≈1%). Compared to other studies on the whole surf zone hyperbenthic community of sandy beaches, the average densities encountered in Valdivia Bay can be considered to be relatively high (2500--6000 ind. 100 m-2). Although limited in time and performed with a limited taxonomic resolution, the results indicate a high contribution of early life stages of invertebrate and fish species to the biodiversity of the intertidal hyperbenthic community in comparison with temperate regions.