Factors affecting the growth and recruitment of cyprinid populations of the River Wensum, Eastern England, with specific reference to roach Rutilus rutilus (L.).

Abstract

1. The roach Rutilus rutilus population of the River Wensum, Eastern England, has long been the topic of deliberation amongst the angling community due to a perceived decline in their catches since the 1970s. Analysis of fish population survey data collected by the Environment Agency and its predecessor organisations since 1983 revealed that although the roach populations have shown considerable temporal variability around their long-term mean abundances, their estimated abundance in 2009 was not significantly different to that estimated in the 1980s. A significant decline in the abundance of dace Leusicus leusicus (L.) was detected, although the abundance of chub Leuciscus cephalus (L.) has increased. 2. Annual variation in the recruitment strength of 0 group roach contributed to their temporal variability in population abundance. Recruitment was largely driven by climate, specifically water temperatures in the first year of life of year classes. Point abundance electric fishing sampling conducted in 2007 and 2008 revealed that nursery habitat was limited for the larval and juvenile life stages of the roach population, revealed by only 6 % of all points sampled containing 4 at least one roach. The probability of roach capture in a sample point only exceeded 0.80 when depths exceeded 1m and macrophyte cover the sampled area exceeded 60 %. 3. The growth rate of adult roach has declined between the 1970s and the present, with this long-term depressed growth only apparent since the initiation of phosphate stripping in the mid to late 1990s. Prior to phosphate stripping, roach growth was largely dependent on water temperature; post-stripping, it was significantly associated with levels of ortho-phosphate. Thus, whilst this reduction in nutrient input into the river was positive for its chemical and biological water quality, it now prevents individual fish growing rapid to a size considered as a 1Especimen 1F by anglers (>1 kg). It is this depressed growth and reduction in the numbers of 1Especimen 1F roach being produced in the river that is contributing to the perceived declines of roach by the angling fraternity. 4. To prevent flooding in the river catchment, a number of flood prevention works have been regularly completed by authorities, including channel straightening and removal of in-stream woody debris. Whilst these tend to have negative consequences for fish production, the cutting of in-stream macrophytes during the summer months to ensure the channel was sufficiently clear to facilitate flood relief flows was measured as having a significant deleterious impact for juvenile roach. Comparison of pre- and post-weed cutting electric fishing point samples revealed presence and abundance of juvenile roach decreased by approximately 50 % following weed cutting. 5 5. These outputs were used to develop a series of management recommendations to assist the production of roach in the river without compromising other river management perspectives such as flood risk management. A key aspect of this is the creation of in-stream and off-channel refuge and nursery areas for roach that promote their survival and growth across all aspects of their lifecycle.