The dynamics of larval and 0+ juvenile fish population in lowland rivers

Nunn, Andrew David

Abstract

Early development represents a critical period in the life cycle of fishes, with a wide range of biotic and abiotic factors influencing eventual recruitment to the adult populations. Despite their key importance in the sustainability of fish populations, however, there is a dearth of knowledge regarding the ecology of 0+ fish communities in lowland rivers. Thus, the factors affecting 0+ fish populations were studied in three lowland river catchments, namely the River Trent, the Warwickshire Avon and the Yorkshire Ouse, representing a range of topographical and biotopical characteristics. The 0+ fish populations were sampled monthly between May 1999 and October 2004 inclusive by micromesh seine netting and point abundance sampling by electric fishing. Captured fish were used to obtain information on 0+ fish community structure, growth, spawning periodicity, and the food and feeding relationships of larval and 0+ juvenile fish.

In the Trent and, particularly, the Avon, rheophilic fish species were heavily outnumbered by eurytopic species due to a lack of suitable spawning and nursery habitat for rheophilic species. Strictly rheophilic species were most abundant in the Ouse catchment where the habitat was characterised by fast-flowing water and a sand or gravel substratum. Almost all fish species used floodplain waterbodies at some point in their life history. For some species this use was transitional (rheophilic species) or facultative (eurytopic species), whereas for others it may be obligate (lirnnophilic species). Floodplain waterbodies also served as refugia for both 0+ and >0+ fish in winter, especially during floods.

Floodplain waterbodies provided enhanced feeding conditions for 0+ fish, with zooplankton diversity, size ranges and density being greater than in main river channels. For all fish species, ontogenetic shifts in resource use were identified. Young larvae had narrow diet spectra and the greatest interspecific diet overlap, with feeding diversity and the importance of species-specific traits increasing with development. There were significant, positive relationships between prey size and gape height for larvae, but not for juveniles. The majority of fish, however, consumed prey substantially smaller than the maximum theoretically possible inferred from their gape. Some taxa were consistently selected over others, irrespective of size, suggesting that taxa-specific, as well as size-specific prey characteristics, may be important in the selection process.

Chub and bleak, and to a lesser extent bream, gudgeon and minnow, adopted fractional or protracted spawning strategies, while dace, roach and perch spawned only once per season. Multiple spawning events are important for recruitment success as they disperse the mortality risk of the early developmental stages. In some years, therefore, relatively strong year classes may result, despite high mortality in early summer and smaller than average mean lengths, providing overwinter conditions are not severe.

Growth of 0+ fish was positively correlated with water temperature and negatively correlated with river discharge, while year-class strength (YCS) was positively correlated with 0+ fish growth and with the position of the Gulf Stream. River discharge (rather than water temperature) may be the key factor in determining YCS, however, either directly or indirectly. For example, a poor year class may be the result of elevated river flows causing high mortality during a critical period in the life history of a particular fish species, such as the larval period, or it may equally be the result of poor growth caused by a reduction in water temperature, discharge-associated increases in energy expenditure or a lack of suitable food. In most years it is likely that indirect effects will prevail over direct effects due to non-synchronicity of periods of elevated discharge and fish hatching, although this will differ between rivers. It could be that, in effect, water temperature determines potential YCS while discharge determines realised YCS.

Citation

Nunn, A. D. (2005). The dynamics of larval and 0+ juvenile fish population in lowland rivers. (Thesis). University of Hull. Retrieved from https://hull-repository.worktribe.com/output/4219167

• Thesis Type: Thesis
• Deposit Date: May 9, 2017
• Publicly Available Date: Feb 23, 2023
• Keywords: Biological sciences
• Public URL: https://hull-repository.worktribe.com/output/4219167
• Additional Information: Department of Biological Sciences, The University of Hull
• Award Date: Aug 1, 2005