Sensitivity of native freshwater fish and invertebrate species and the role of photoprotectant slime

Levels of ultra-violet (UV) light are naturally high in New Zealand and the exposure of many stream-dwelling aquatic organisms would undoubtedly have increased markedly as a result of widespread deforestation occurring since human colonisation. This extensive landscape modification resulted in reduction of the previously 80-90% forest cover to about 30% remaining today. Additionally, UV levels may be expected to increase in the future as a result of thinning of the stratospheric ozone. The region of influence of the ‘ozone hole’ is presently largely confined to region over Antarctica, however, recent years (particularly 1998) has seen the hole extend widely to include parts of South America. The seasonal nature of this phenomenon may result in particularly high seasonal peaks in UV exposure. Increased levels of UV radiation have been shown to result in a range of adverse effects on aquatic species. Overseas studies have demonstrated significant behavioural and physiological effects on sensitive freshwater fish and macroinvertebrates species at UV levels minimally higher than existing levels (e.g., Hurtubuise et al. 1998). Notably, studies have also demonstrated a wide range of tolerance levels for different species, prompting the need to understand factors that affect sensitivity. A number of New Zealand’s freshwater fish species are potentially highly susceptible for a number of reasons. Firstly, New Zealand’s streams and rivers are largely of high optical clarity that will result in effective UV transmission. Secondly, most of our native fish species are diadromous, with juveniles migrating from the sea to constitute the highly prised ‘whitebait’ catch. These fish are behaviourally and possibly inherently sensitive to UV exposure. Indeed, species such as the inanga (Galaxias maculatus) which lay their eggs on bank-side grasses may be especially susceptible. Understanding the potential effects of UV on freshwater species is particularly relevant to management of river ecosystems. Many factors affect the exposure of organisms to UV, including shade, dissolved organic matter, organism behaviour and natural photoprotectants. These factors must be considered in addition to the organism’s inherent sensitivity to UV. Manipulation of riparian vegetation provides a practical mechanism to markedly reduce UV exposure in river ecosystems and thus mitigate adverse effects. UV radiation may affect organisms both directly and by phototoxicity associated with the presence of chemical contaminants in the environment. The damaging effects are wavelength dependent, with direct effects largely attributable to the UVB wavelengths (280-320 nm), while the longer wavelength UVA (320400 nm) contributes to photoactivation associated with contaminants. This report addresses the inherent sensitivity of selected freshwater fish and invertebrate species to UVB exposure. Specifically we addressed the following questions: 1. How sensitive are freshwater riverine species to UVB exposure? 2. How do the measured sensitivities compare with organisms from other environments? 3. How do these sensitivities compare with potential field exposures?