North American Journal of Aquaculture | 2019
Comment: Practices for Drawing Blood Samples from Teleost Fish
Abstract
We read with interest a recent paper by Duman et al. (2019), which, based on the title, presumably was intended to serve as an overview of practices for drawing blood samples from teleosts. Given that the “go-to” reference for blood sampling of fish is Houston (1990), an update certainly seemed in order. However, as organismal biologists and stress physiologists, we were disappointed at the emphasis on the sampling of dead fish, with very limited discussion of live-sampling methods, and the focus on blood collection solely for assessing disease states. Indeed, the purpose of blood sampling is largely irrelevant in the context of a methods-oriented paper like this (e.g., the purpose could be to assess health, stress, genetics, oxygen carrying capacity, endocrine function, maturation status, etc.) except in cases where the collection method itself may perturb the parameter of interest (e.g., as in cortisol, glucose, and lactate titers; Lawrence et al. 2018). Even while recognizing that the outlet for the work was an aquaculture journal, we were also disappointed by the seemingly narrow focus on fish in aquaria or culture facilities, especially given the broad title of the paper. Duman et al. (2019) draw upon a range of literature (including studies on wild fish) in creating their narrative, but they fail to make the connection between what they present and the broader application of blood sampling techniques in understanding general fish biology. This contrasts with what was presented by Houston (1990). Indeed, aside from methods like cannulation, which requires a more substantial surgical procedure (e.g., Clark et al. 2011), blood sampling can occur in a multitude of settings (e.g., riverbank, boat, aquaculture facility, reef crest, clinic, etc.) such that the source of the fish and the sampling location are largely irrelevant. There are several aspects of Duman et al. (2019) that warrant specific critique. The first is the statement that the heart of a fish has “two ventricles”: that is simply incorrect (e.g., Farrell and Smith 2017). This may seem trivial, but inexperienced phlebotomists attempting to obtain blood from cardiac puncture should recognize that there is indeed only a single ventricle. Relatedly, the authors repeatedly mention drawing blood from “veins.” The reality is that at most locations in a fish s body (especially the caudal peduncle), the likelihood of obtaining a sample consisting entirely of venous blood (i.e., not a mixed sample with arterial blood) would be rather low (e.g., O Neill et al. 1998; Mandelman and Skomal 2009; Esbaugh et al. 2016). On the surface, this may seem like a minor detail, but if one is measuring blood gases, for example, the knowledge of whether a blood sample is drawn from a vein or an artery is critically important (hence why in most studies one refers to sampling blood from the caudal vasculature). The authors note that blood sampling is used infrequently on fish (especially ones in aquaria) because (1) the veins are not visible under the skin, (2) many fish species are small (<20 g), (3) there may be inadequate blood volume for sampling in aquarium fish, and (4) there may be a low chance of the fish surviving after blood is drawn. Duman et al. (2019) go on further to suggest that it is also not practical or feasible to conduct an average of two to three fish health examinations using blood withdrawal. As mentioned above, blood can be drawn from veins and arteries, but there are very few instances in which the source matters (e.g., work on blood gases). Beyond that, there are common anatomical landmarks that enable one to draw blood irrespective of whether the vasculature is visible. Duman et al. (2019) suggest that to obtain blood from the caudal vasculature, it is important to avoid the fish s internal organs (e.g., gonads, intestines, air sac, etc.). In Figure 8, they show their proposed location for sampling the caudal vasculature—a location that we strongly advise against unless one has euthanized the fish prior to sampling. In the text, they suggest that the sampling should be done posterior to the anus, but Figure 8 shows otherwise (anterior). Blood collection in the location depicted on Figure 8 would be very difficult if not impossible without causing unnecessary damage to vital tissues and organs, which contrasts with the recommendations of the authors in the text. To our knowledge, blood is very rarely drawn from that location in a live fish, and the proposed sampling location was not discussed in the previous review by Houston (1990). Instead, sampling from the caudal vasculature is typically conducted quite posterior to the anus (see Figure 1; i.e., the caudal peduncle) and usually posterior to the anal fin (recognizing interspecific variation), which minimizes the risks of causing damage to vital organs. In one passage, Duman et al. (2019) suggest that the needle should be inserted 4–5 cm anterior to the tail, but this is poor guidance, as the distance will vary with body size and morphology. On some species (like North American Journal of Aquaculture 81:424–426, 2019 © 2019 American Fisheries Society ISSN: 1522-2055 print / 1548-8454 online DOI: 10.1002/naaq.10115