Effects of turbidity on the aerobic physiology and feeding behaviour of juvenile snapper (Pagrus auratus)

Abstract

Turbidity as a result of increased suspended sediment in coastal waters is an environmental stress of worldwide concern. Recent research on fish suggests that detrimental changes to gill structure can occur in turbid waters, with speculation that these alterations diminish fitness variables such as growth and development by negatively impacting the O2 uptake capacity (respiration) of fish. To specifically address this unknown, the impact of turbid water on the gill structure, somatic growth rate and O2 uptake rates of a juvenile sparid species (Pagrus auratus) was addressed following exposure to 5 different turbidity treatments (< 10, 20, 40, 60, 80 NTU) for 30 days. Significant gill structural change was apparent with a progressive increase in turbidity and was quantified as a reduction in lamellae density, as well as increase in basal hyperplasia, epithelial lifting and increased oxygen diffusion distance across the lamellae. The weight of control fish did not change but all fish exposed to turbid waters lost weight, confirming that long term turbidity exposure is detrimental to growth, productivity and fitness. However, the hypothesis that structurally altered gills would impair O2 uptake was not supported due to no measurable difference in the standard metabolic rate (SMR), maximum metabolic rate (MMR), aerobic metabolic scope (AMS) or critical oxygen saturation (Scrit) limit of fish between the 5 NTU treatments. The results therefore suggest that P. auratus may be more resilient to turbidity stress than previously assumed, possibly because they maintain excess gill structure under non-turbid conditions to safeguard O2 supply. To further investigate the reasons behind the observed growth deficit with turbidity exposure, the feeding performance of P. auratus was also examined under the same 5 turbidity treatments. Significant reductions in feeding ability were apparent with a progressive increase in turbidity, and was quantified as a reduction in attack success, foraging bites and attack distance. These results suggest that the ability of P. auratus to feed effectively and efficiently may impact on fish growth, however there is likely to be more than one mechanism at play which, as discussed, provides considerable scope for further research.