Abstract
During the last century, the blooms of the toxic haptophyte Prymnesium parvum have been responsible for massive fish kills in both aquaculture and wild populations. Despite decades of research, the ichthyotoxic properties of P. parvum, and how this alga affects fish, is still debated. Using a novel device to measure the respirometry, ventilation volume, ventilation frequency, oxygen extraction, and oxygen consumption of undisturbed European plaice (Pleuronectes platessa) were acquired during exposure to two algal species as well as hypoxia. Fourteen fish (258 ± 44 g) were initially exposed to severe hypoxia and left to recover for at least 48 h. Half of these fish were then exposed to known harmful concentrations of P. parvum (median ± standard deviation (SD); 2.6 × 105 ± 0.6 × 105 cells mL−1), while the remaining half were exposed to the non-toxic alga Rhodomonas salina (median ± SD; 3.2 × 105 ± 0.7 × 105 cells mL−1). During exposure to severe hypoxia, all of the fish were able to maintain oxygen consumption by increasing the ventilation volume. The results from fish that were exposed to P. parvum showed a significant decrease in oxygen extraction (median ± SD; 52.6 ± 6.9 percentage points) from pre-exposure to the end of the experiment, as opposed to fish exposed to R. salina, which were unaffected. These results indicate that suffocation affects the European plaice when exposed to P. parvum. The observed severe decrease in oxygen extraction can be ascribed to either damage of the gill epithelia or increased mucus secretion on the gills, as both would limit the transfer of oxygen, and both have been observed.
Highlights
The toxic alga Prymnesium parvum Carter has during the last century caused massive fish kills in both aquaculture and wild populations [1,2,3,4,5]
Since the data was not normally distributed, a Wilcoxon signed rank test was used to check for differences between normoxia and severe hypoxia
These differences showed that oxygen extraction significantly decreased by 15.6 ± 9.1 percentage points (median ± standard deviation (SD) (W = 52438, p < 0.0001), while ventilatory flow significantly increased by 12.8 ± 3.9 mL s−1, (W = 1117, p < 0.0001); this is a factor 6.1 increase
Summary
The toxic alga Prymnesium parvum Carter has during the last century caused massive fish kills in both aquaculture and wild populations [1,2,3,4,5]. When a bloom of P. parvum occurs, all of the fish species located in the bloom area are affected [5,6]. Blooms of P. parvum will have ecological effects on fish stocks, as well as being a threat to aquaculture [7]. It has been hypothesized that exposure to P. parvum causes the suffocation of the fishes [9]. Macroscopic investigation of the gill cell epithelia of the minnow Gambusia affinis shows damages during exposure to toxin(s) that were extracted from P. parvum [10]. The causative agents for gill damage are debated [7,11,12,13,14,15,16,17,18,19], and it is unknown whether a single toxin is responsible for the Fishes 2019, 4, 32; doi:10.3390/fishes4020032 www.mdpi.com/journal/fishes
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