Application of an electrified benthic frame trawl for sampling fish in a very large European river (the Danube River) – Is offshore monitoring necessary?

The organization of fish assemblages in offshore, deep channel habitats is poorly known in very largerivers compared with shoreline, littoral areas. We report on the parameters and testing of an electrifiedbenthic frame trawl (EBFT), developed for monitoring the distribution and abundance of benthic fishesin the Danube River, Hungary. We also compare the results of the benthic main channel survey witha shoreline electrofishing (SE) data set. Altogether 33 species were collected offshore during the 175trawling paths (500 m long each). Both sample based and individual based rarefaction showed that nighttime SE detected significantly more species with increasing sampling effort than day time trawling ofoffshore areas. However, offshore surveys detected sterlet Acipenser ruthenus, which could not be detectedby SE, even using extreme high sampling effort. Offshore trawling also proved the common occurrenceand high abundance of the strictly protected endemic Danube streber Zingel streber in the river, whichproved to be extremely rare in SE catches. The EBFT caught larger/older individuals of many species thanSE, and indicated diverse size/age structure for many species offshore. Our survey revealed that offshoreareas are intensively used by a variety of species, which occur relatively even, but with variable abundancein the Danube River. We suggest that even a relatively small (i.e. 2 m wide 1 m high) EBFT can be a veryuseful device for monitoring offshore fish assemblages in very large rivers and provide important datafor bioassessment and conservation purposes

How natural capital delivers ecosystem services: a typology derived from a systematic review

There is no unified evidence base to help decision-makers understand how the multiple components of natural capital interact to deliver ecosystem services. We systematically reviewed 780 papers, recording how natural capital attributes (29 biotic attributes and 11 abiotic factors) affect the delivery of 13 ecosystem services. We develop a simple typology based on the observation that five main attribute groups influence the capacity of natural capital to provide ecosystem services, related to: A) the physical amount of vegetation cover; B) presence of suitable habitat to support species or functional groups that provide a service; C) characteristics of particular species or functional groups; D) physical and biological diversity; and E) abiotic factors that interact with the biotic factors in groups A–D. ‘Bundles’ of services can be identified that are governed by different attribute groups. Management aimed at maximising only one service often has negative impacts on other services and on biological and physical diversity. Sustainable ecosystem management should aim to maintain healthy, diverse and resilient ecosystems that can deliver a wide range of ecosystem services in the long term. This can maximise the synergies and minimise the trade-offs between ecosystem services and is also compatible with the aim of conserving biodiversity

Relationship between horizontal hydroacoustic stock estimates and gillnet catches of surface-oriented fish in shallow Lake Balaton (Hungary)

We examined whether gillnetting and hydroacoustics provide comparable relative fish density and fish size distribution estimates in the uppermost water layer of a shallow turbid lake and whether the inclusion of environmental and stock parameters could improve comparability of data between different gears. According to gillnetting, most fish shorter than 14 cm in total length were Alburnus alburnus, and most fish longer than 14 cm were Pelecus cultratus. Size distributions and median sizes obtained from gillnetting and acoustics differed. Pure acoustics-derived density estimates accounted for only <8.8% of the variation in the gillnet catch by number and no correlation was found in biomass indices. Most variances observed in the gillnet catch-per-unit-effort data were associated with water transparency, none of the other investigated variables had significant explanatory power. We conclude that the feasibility of establishing a powerful model describing the relationship between fish density estimates of gillnetting and acoustics is low in shallow and turbid habitats, and it might be successful only if the crucial sampling and environmental parameters affecting catch efficiency of gillnets and post-processing of acoustic data are considered. This study cautions again that gillnetting should be used carefully to analyse trends of fish abundance