Effects of watercress farming on fish populations

  • Asa Benjamin White

    Student thesis: Doctoral Thesis


    Watercress (Nasturtium officianale) is a salad crop grown commercially in watercress beds irrigated
    with water from chalk aquifers. The effluent from irrigation, and in some instances, salad washing
    processes, are discharged into adjacent chalk streams. There is concern that macroinvertebrate
    assemblages downstream of discharges reflect organic pollution, which has been attributed to
    siltation and more recently the release of phenethyl isothiocyanate (PEITC). An antiherbivore
    metabolite produced by watercress in response to physical damage, PEITC has the potential to be
    released into chalk streams via two pathways: (i) via irrigation water emanating from watercress
    beds following disturbances such as harvesting and other crop-damaging activities; (ii) via the
    discharge of salad wash effluent from the rinsing of watercress, a process carried out on small
    number farms. While PEITC toxicity to macroinvertebrates is well-studied, the impact that PEITC
    may be having on fish populations has received little attention. Watercress farms discharge into
    chalk stream headwaters where fish embryos are incubated, so there is a potential for this sensitive
    early life stage to be directly exposed to PEITC.

    To determine the impact watercress farm discharges are having on habitat, macroinvertebrate and
    fish, three watercress farms each were surveyed biannually. Each farm varied in its utilisation of
    salad washing, with the aim of the study to investigate whether any changes in physicochemistry,
    habitat, macroinvertebrate prey availability from watercress bed irrigation and salad wash effluent
    rendered sites suboptimal to support fish populations. Organic pollution stress was assessed
    through macroinvertebrate assemblages using the Walley Hawkes Paisley Trigg (WHPT) biotic
    index, which provides a score based on the relative abundances of pollution sensitive and pollution
    tolerant taxa. Sites receiving salad wash effluent had significantly lower WHPT scores than expected
    and higher total macroinvertebrate abundance, while sites receiving watercress bed irrigation
    water only scored higher than predicted. Fish species other than brown trout (Salmo trutta) were
    found at higher densities below discharges. However, S. trutta found at lower densities were in
    better condition, possibly due to decreased intraspecific competition and greater
    macroinvertebrate prey abundances. In contrast to sites that received just watercress bed irrigation
    discharge, sites below salad wash effluent had densities of young-of-year S. trutta that were lower
    than expected, suggesting that PEITC release from salad washing may reduce embryo survival.

    To assess embryotoxicity of PEITC, a series of laboratory trials exposed S. trutta, common carp
    (Cyprinus carpio) and zebrafish (Danio rerio) embryos to 0.01, 0.1 and 1µg/L PEITC during embryonic development. In all three species, exposure to 1µg/L resulted in complete mortality between 1-3 dosing days, while embryos exposed to 0.1 µg/L PEITC suffered higher mortality rates, significantly delayed hatching, higher incidence of spinal deformations and significantly altered behaviour compared to controls. These levels of exposure were orders of magnitude below estimates of PEITC discharges from salad washing, suggesting that salad wash effluent may have been a factor in the low densities of young-of-year S. trutta downstream of salad wash effluent discharge.
    Date of AwardSept 2020
    Original languageEnglish
    Awarding Institution
    • University of Brighton
    SupervisorNeil Crooks (Supervisor), Angelo Pernetta (Supervisor) & Christopher Joyce (Supervisor)

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