European grayling recruitment in the River Wylye

We surveyed juvenile grayling and trout at six long-term 200 metre sites on the River Wylye. © Dick Hawkes

BACKGROUND As part of the ongoing Wylye Grayling Study (WGS), GWCT, together with Natural Resources Wales and the Piscatorial Society, have monitored European grayling abundance on the River Wylye, a tributary of the Hampshire Avon since 1996. During that period, numbers of grayling have been declining, albeit episodically, and observations from anglers and scientists suggest the decline could be due to recruitment failures (years when few eggs survived to become juveniles that will develop into sexually mature adults). Since 2009, the WGS was expanded to include brown trout. River conditions, including water temperature and discharge rates, have been monitored for the duration of the WGS. We surveyed juvenile grayling and trout at six long-term 200 metre sites on the River Wylye each October from 2009 to 2016. During the survey, the sites were isolated by upstream and downstream nets, and juvenile fish were captured using between three and five electric fishing passes, until the number of grayling captured was less than half the number caught in the previous pass. All captured grayling and trout were lightly anaesthetised, measured, weighed, marked with a uniquely identifying tag and returned to their site of capture.

With financial support from the Grayling Research Trust, GWCT employed a post-doctoral research scientist to develop statistical models exploring associations between observations on grayling recruitment and river conditions. The models served to test the hypotheses that grayling recruitment was associated with temperatures during egg development and post-emergence, a measure of the intensity of drought post-emergence, the number of juvenile grayling sharing limited food and shelter, and the number of trout also sharing those limited resources. The statistical models assumed that the data from the six sites were representative of the whole river, and hence that the findings were relevant to the River Wylye fisheries managers.

Results and implications Grayling recruitment was found to be negatively related to the number of other juvenile grayling during their development, suggesting that there was limited food or shelter to share among young juveniles. Recruitment was also positively associated with summer temperature, unless it exceeded 13.5°C, beyond which the association became negative. It was also positively associated with spring temperature, and negatively associated with low flows during the summer. The same conditions appeared to favour trout recruitment, despite long-standing speculation that the presence of one species would be detrimental to the other.

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Moderate temperatures Few summer droughts

Food limitation River Wylye Juvenile trout

Juvenile grayling

Figure 1

Effects of environmental variables and food limitation by juvenile trout and grayling on the recruitment of grayling measured at six sites on the River Wylye over eight years

Our findings suggest that grayling benefit from moderate river conditions during egg and early juvenile development, but are susceptible to high temperature and drought conditions during summer. Considered against the background of ongoing salmonid population declines, our findings emphasise the importance of management interventions that seek to increase population resilience via restoring natural discharge regimes. These would include sympathetic water use by people and their activities, and limiting future temperature increases, perhaps by protecting riverine habitats.

KEY FINDINGS Factors affecting juvenile grayling recruitment were explored, including river water temperature, discharge, and numbers of other juvenile salmonids sharing limited food. Juvenile grayling recruitment was positively associated with temperatures, until they exceeded 13.5°C, and negatively associated with drought (conditions that also favoured juvenile trout recruitment). Grayling, and perhaps trout, population persistence depends on maintaining natural discharge, requiring sympathetic water use by people and their activities, such as abstraction, particularly under worsening climate change and industrial and human development in the chalk stream catchments.

Stephen Gregory