Publication Abstract

Non-invasive monitoring of steroids in fishes

T. Ellis, M.B. Sanders and A.P. Scott

The scientific reasons for monitoring steroids in fishes are the same as for higher vertebrates. However, the impetus to apply non-invasive methods to fishes should be even stronger because the traditional method of blood sampling is arguably more stressful, injurious and problematic than in higher vertebrates. Over the last 15 years, a number of studies have illustrated the potential for non-invasive monitoring of reproductive and stress steroids in a diverse range of fish species living in both fresh- and sea-water. Just as in higher vertebrates, non-invasive monitoring in fishes relies upon passage of steroids from the blood, or excretion (with or without further metabolism), into alternative matrices - mucus, water, faeces and urine - which bear (respective) parallels with the sweat, saliva, faecal and urinary methods used in higher vertebrates. All methods show promise, but differ in the extent of intrusion and various other factors that influence their suitability for specific applications. The bulk of studies applying non-invasive methods to fishes have used the water matrix. We illustrate the value of the water procedure in enabling repeat sampling by presenting some original time series data on cortisol release by fed and unfed fish. Although the water matrix has been proven under controlled conditions, it cannot yet be applied to free-ranging fish due to the uncontrolled dispersion and dilution of the steroid once it has entered the water. The only solution is to find a ‘normaliser’ (i.e. another compound that is released by the fish at a constant rate; and against which the concentration of the steroid can then be expressed as a ratio).  We present new data on the measurement and release pattern of creatinine and melatonin into the water and discuss their potential as normalisers for steroids in rainbow trout.  Creatinine (used as a normaliser for steroids in human urine) is unsuitable as it leaches out from food and shows large differences between fed and unfed fish.  Melatonin also shows a two-fold difference between fed and unfed fish and has the disadvantage that it is only released under night-time conditions. Our data illustrates food as an exogenous source of measured excretory products, the lability of potential normalisers, potential effects of water chemistry on metabolite recovery, and human skin as a source of steroid contamination. We emphasise the need for extensive validation of non-invasive assays and the value of background information to support interpretation of results.

Reference:

T. Ellis, M.B. Sanders and A.P. Scott (2013) Non-invasive monitoring of steroids in fishes. Veterinary Medecine Austria, 100: 255-269

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