Andrew David Nunn

The foraging ecology of larval and juvenile fishes

Knowledge of the foraging ecology of fishes is fundamental both to understanding the processes that function at the individual, population and community levels, and for the management and conservation of their populations and habitats. Furthermore, the factors that influence the acquisition and assimilation of food can have significant consequences for the condition, growth, survival and recruitment of fishes. The majority of marine and freshwater fish species are planktivorous at the onset of exogenous nutrition and have a limited ability to detect, capture, ingest and digest prey. Improvements in vision, development of fins and associated improvements in swimming performance, increases in gape size and development of the alimentary tract during ontogeny often lead to shifts in diet composition. Prey size, morphology, behaviour and abundance can all influence the prey selection of larval and juvenile fishes. Differences in feeding behaviour between fish species, individuals or during ontogeny can also be important, as can inter- and intraspecific interactions (competition, predation risk). Temporal (diel, seasonal, annual) and spatial (microhabitat, mesohabitat, macrohabitat, regional) variations in prey availability can have important implications for the prey selection, diet composition, growth, survival, condition and, ultimately, recruitment success of fishes. For fish populations to persist, habitat must be available in sufficient quality and quantity for the range of activities undertaken during all periods of development. Habitats that enhance the diversity, size ranges and abundance of zooplankton should ensure that sufficient food resources are available to larval and juvenile fishes.

Do fish go with the flow? The effects of periodic and episodic flow pulses on 0+ fish biomass in a constrained lowland river

The hydrological regime is a significant driver of fish population dynamics in rivers, but there is a dearth of information regarding the mechanisms behind its effects on temperate species, especially non-salmonids. This study investigated the effects of periodic and episodic flow pulses on 0+ fish biomass in a constrained lowland river. De-seasonalized cross-correlation analysis was used to examine time-lagged correlations in episodic signals, in isolation of seasonal periodicity, to identify the responses and response timings of 0+ fish production to abiotic variables, and whether apparent “pulse-depletions” in biomass occur instantaneously (e.g., due to fish displacement during high pulses) or after a time lag. As anticipated, 0+ fish biomass was highest during periods of low discharge and high temperatures in summer, but cross-correlation analysis revealed a negative impact of high pulses on 0+ fish biomass with a lag of 7 months. There was no evidence for an instantaneous pulse-depletion effect of discharge on 0+ fish biomass, suggesting that the indirect effects of high pulses, such as habitat or food-web modifications, are more influential.

Monitoring Sea Lamprey Petromyzon marinus Ammocoetes in SAC Rivers: A Case Study on the River Wye

The sea lamprey (Petromyzon marinus L.) is a designated feature of the River Wye and the Severn Estuary Special Areas of Conservation (SAC). This study assessed the conservation status of sea lamprey in the Welsh reaches of the Wye catchment. Sea lamprey ammocoete (larvae) and transformer (metamorphosing larvae) surveys were conducted at 54 sites following the established UK protocol. The mean density of ammocoetes was 2.20 m–2 and, where sufficient ammocoetes were obtained, at least two size (≈age) classes were present, indicating favourable condition status. It was impossible, however, to identify trends in condition status because of lack of historical data. Condition status should also ideally be linked with information on flow dynamics, habitat requirements and river-channel morphology to understand, and be able to respond to, changes in population dynamics. Modifications to the condition assessment criteria for lampreys should be made as more data become available.

Diel variations in the assemblage structure and foraging ecology of larval and 0+ year juvenile fishes in a man-made floodplain waterbody

This study investigated diel variations in zooplankton composition and abundance, and the species composition, density, size structure, feeding activity, diet composition and prey selection of larval and 0+ year juvenile fishes in the littoral of a man-made floodplain waterbody over five 24 h periods within a 57 day period. There was a significant difference in the species composition of diurnal and nocturnal catches, with most species consistently peaking in abundance either during daylight or at night, reflecting their main activity period. There were no consistent diel patterns in assemblage structure or the abundance of some species, however, most likely, respectively, due to the phenology of fish hatching and ontogenetic shifts in diel behaviour or habitat use. There were few clear diel patterns in the diet composition or prey selection of larval and 0+ year juvenile roach Rutilus rutilus and perch Perca fluviatilis, with most taxa consistently selected or avoided irrespective of the time of day or night, and no obvious shift between planktonic and benthic food sources, but dietary overlap suggested that interspecific interactions were probably strongest at night. It is essential that sampling programmes account for the diel ecology of the target species, as diurnal surveys alone could produce inaccurate assessments of resource use. The relative lack of consistent diel patterns in this study suggests that multiple 24 h surveys are required in late spring and early summer to provide accurate assessments of 0+ year fish assemblage structure and foraging ecology.

Monitoring Fish Populations in River SACs

The EU Habitats Directive (92/43/EEC) on Conservation of Natural Habitats and of Wild Fauna and Flora stipulates that Member states maintain or restore habitats and species in a condition that ensures their favourable conservation status in the community. To comply with this Directive requires assessment of the conservation status of aquatic species in rivers designated as Special Areas of Conservations (SACs) against a predetermined set of objectives. This paper describes the protocols developed to determine the condition status of lampreys, bullheads, shad and Atlantic salmon in SAC rivers in the UK.