Effects of Harmful Algal Blooms on Fish: Insights from Prymnesium parvum

Abstract

Blooms of the planktonic alga Prymnesium parvum pose a global threat, causing fish kills worldwide. Early studies on the exposure of fish to P. parvum indicate that toxic effects are related to gill damage. The more strictly defined concept of adverse outcome pathways has been suggested as a replacement for the mode of action in toxicology studies. In this study, rainbow trout (Onchorhyncus mykiss) were exposed to P. parvum. During exposure, oxygen consumption was determined by respirometry, and ventilation and coughing rate were determined via video surveillance. Per breath oxygen consumption was calculated to assess the ventilation effort to obtain a unit of oxygen. A second experiment monitored fish behavior to assess recovery. The results indicated that oxygen consumption initially increased, but on average fell below the standard oxygen consumption at 70% relative exposure. Being a function of ventilation frequency and oxygen consumption, the per breath oxygen consumption decreased throughout exposure. Behavioral results determined that short-term P. parvum exposure subsequently caused the exposed fish to seek flow refuge immediately and to a greater extent than unexposed fish. The adverse outcome pathway of P. parvum on rainbow trout is that P. parvum acts as a gill irritant resulting in non-recoverable respiratory failure.

Respiratory Physiology of European Plaice (Pleuronectes platessa) Exposed to Prymnesium parvum

“…However, since the reduction in oxygen extraction can be seen minutes post exposure, the cause of suffocation is likely a combined effect of gill epithelial destruction and mucus acting as a barrier for oxygen transport. We have previously [9] observed fish mortality at relatively high algal concentrations without any mucus being located at the gills; this could indicate that, at relatively high P. parvum concentrations, fish suffocate due to gill destruction and, the fish has prolonged lifetime and suffocate due to mucus production at lower concentrations. Thus, the mode of action is likely a balance of the level of exposure.…”

“…This approach allows for the fish to acclimate to the setup over long periods and thereby eliminate most of the stress-related discrepancies in the measurements [30,31]. During the present study, measurements on oxygen extraction and ventilatory flow were desired and a different approach had to be implemented since these cannot be directly measured in a regular respirometer [9]. Therefore, a modified setup and approach used earlier [32], which allowed ventilatory flow and oxygen extraction measurements of undisturbed flatfish, was used.…”

“…Therefore, blooms of P. parvum will have ecological effects on fish stocks, as well as being a threat to aquaculture [7]. The current understanding of the ichthyotoxic mode of action of P. parvum is partly reversible osmotic lysis of the gill membranes [8,9]. It has been hypothesized that exposure to P. parvum causes the suffocation of the fishes [9].…”

“…The current understanding of the ichthyotoxic mode of action of P. parvum is partly reversible osmotic lysis of the gill membranes [8,9]. 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].…”

“…Further, the production of mucus on the gills [7,[22][23][24] can also cause a decreased oxygen extraction [22] due to an increased boundary layer and longer diffusion distance. This study aimed to test how exposure to ecological relevant concentrations of P. parvum affects physiological respiratory responses, such as oxygen extraction (E O 2 ), ventilatory flow (V f ), ventilation frequency (V freq ), and oxygen consumption (ṀO 2 ) of European plaice (Pleuronectes platessa) as compared to fish in normoxia and fish that are exposed to severe hypoxia under the assumption that the gills are the primary target for aquatic toxins in fishes [9,[25][26][27]. The respiratory parameters were measured using a novel device in order to acquire measurements from non-stressed fish, which utilizes the natural behaviour of flatfish.…”