Applied ecology and management of a European barbel Barbus barbus population of a lowland river.

Abstract

Freshwaters represent some of the most degraded ecosystems in the world, with approximately 56% of the European rivers being sufficiently altered by human activities to modify the composition of their biological communities. River fish communities are often used to indicate this altered status due to their ecological, recreational and economic value. In lowland rivers, habitat alterations include impoundments and activities such as channel straightening, impacting aspects of fish behaviour and lifecycle completion. Species such as European barbel B. barbus are particularly affected due to their propensity for long-distance migrations and requirements of high quality gravels for spawning. Consequently, B. barbus populations throughout Europe are increasingly threatened. Barbus barbus is indigenous to eastern flowing rivers in England, including the River Great Ouse that has been historically subjected to multiple alterations in channel morphology for flood defence and impoundments for land drainage. The river’s B. barbus population is now restricted to the upper reaches where they represent a key resource for angling, yet temporal and spatial data on their populations suggest relatively low abundances in recent years. Over the last 30 years, the regulatory authority responsible for their management (Environment Agency) have managed the population through a combination of enhancement stocking using hatchery-reared fish and habitat improvement schemes, especially gravel jetting of spawning substrates. There is, however, little knowledge on the effectiveness of these. Consequently, this research investigates B. barbus in rivers in England generally and the Great Ouse specifically by assessing the efficacy of stocking and habitat works to enhance populations. The initial research has focused on using scales from historical surveys on the fish communities of three rivers (including the Great Ouse) to determine the trophic relationships of the fishes using stable isotope analysis. Outputs indicated that scales can be used for this analysis and revealed that rather than sharing food resources with functionally similar species such as chub Squalius cephalus, B. barbus occupied distinct isotopic (trophic) space. Their diet was then assessed using stable isotope analysis on B. barbus scales from four English rivers to determine their major food resources. Results indicated that angling heavily modified B. barbus diet, with introduced bait (as pelletized fishmeal) being the most important dietary component. The next phase of the research built of these outputs of both these studies and assessed the impact and efficacy of enhancement stocking of hatchery-reared B. barbus. In both semi-controlled and wild conditions, analyses suggested that enhancement stocking with B. barbus has minimal detrimental consequences for other fishes such as S. cephalus, with strong patterns of trophic niche partitioning. Nevertheless, the efficacy of enhancement stocking might be limited, with low numbers of recaptured stocked B. barbus recorded in the study, with a concomitant genetic study revealing negligible introgression of stocked B. barbus genes into the population, despite the stocking activities. Given that enhancement stocking has been of limited success to improve B. barbus population abundance in the Great Ouse catchment, their spawning habitats were assessed in the river, including whether the physicochemical properties of the sediments and hyporhic water were limiting. Whilst results indicated good quality of hyporehic water, the subsurface sediment was high in fine content, particularly sand. Gravel jetting, a method to clear spawning gravels of fine content, was shown to only provide short term benefits (e.g. 3 months) in reducing this content of fines, with this benefit only apparent in surface sediments and not in the subsurface. An ex-situ experiment to assess the tolerance of B. barbus eggs and larvae to sand content in spawning substrata indicated no effect of high sand content on egg to emergence survival rates, but it did significantly decrease the timing of larval emergence from gravels. This early emergence of B. barbus larvae from substrates with high sand content could potentially impact their subsequent survival in the wild. Thus, the current management strategies employed in the River Great Ouse to enhance the B. barbus populations appear to have limited success, largely failing to meet their objectives. Thus, more holistic management approaches are outlined and suggested for implementation.