Gilthead seabream (Sparus aurata) is a key aquaculture species in the Mediterranean. Recently, a group of researchers from Portugal, Spain, and France published a study in Aquaculture Reports investigating the effect of environmental enrichment (EE) on the stress levels of the fish, using heart rate as a proxy during husbandry procedures such as feeding, netting, cleaning, and a formaldehyde bath as a prophylactic measure against parasites. 

Heart rate measured in 18 fish during husbandry procedures
The researchers implanted 18 fish (785 ± 76 g) with Star-Oddi’s DST milli-HRT physio-loggers and separated them into six tanks - three with EE and three without EE. Seven untagged fish were added to each tank. After a five-day recovery period, the fish underwent a series of husbandry procedures: feeding, chasing, and cleaning on days six, seven, and eight. On day nine, an anti-parasite treatment was conducted, where the fish were bathed in formaldehyde. For this procedure, the water level was lowered to one-third of the tank, 150 ppm formalin was added, and the fish were left undisturbed for one hour - a common practice in Portuguese aquaculture.

ECG data processed using Star-Oddi's HRT Analyzer software
Following the four days of husbandry procedures, the fish remained undisturbed in the tank for eight days while recordings continued or until the loggers' memory was full, at which point the devices turned off. The fish were then kept in the tanks for six months before being euthanized and the loggers retrieved. Finally, the Star-Oddi HRT Analyzer software was used to verify heart rate based on 50,148 ECG records and to calculate the amplitude of the QRS waveform.

Environmental enrichment affected the heart rate
Environmental enrichment had a significant impact on lowering heart rate and led to a sharper reduction in heart rate to baseline after husbandry procedures. However, EE did not affect heart rate during post-surgical recovery. The study also reported an increase in QRS amplitude associated with increased heart rate during husbandry procedures but not during surgical recovery. 

Further investigations are needed to determine whether these responses have a causal relationship, but the authors theorize that this might be due to increases in stroke volume.

The paper was published in Aquaculture Reports and can be found here

Photo: © Canal Mar Menor