Captive pinnipeds are often kept in fresh water in zoological and research institutions. Following the report of cases of hyponatremia in harp seals (Phoca groenlandica) housed in fresh water, it has been proposed that seals without access to salt water should be supplemented with salt tablets.2 This report stressed the fact that interspecies variations in tolerance probably exist and identified the harbour seal (P. vitulina) as a species that is probably more resistant to electrolytic variation.2 In contrast to most pinniped species, harbour seals are known to spend a fair amount of their lives in waters with low salinity (rivers, estuaries, and lakes).6 Therefore, it is likely that this species is able to maintain sodium homeostasis despite a limited salt intake. Consequently, salt supplementation might not be critical for harbour seals that are housed in fresh water, although it is customary in aquaria, zoos, rehabilitation centers, and research institutions. At the University of British Columbia (UBC), harbour seals have been housed in fresh water for several years without any salt supplementation, and clinical signs of hyponatremia have never been observed. Therefore, the objective of this project was to determine if captive harbour seals housed in fresh water should be supplemented with salt.
Experimental Design and Results
This work was conducted on six captive harbour seals kept for different research projects on diving and respiratory physiology at UBC. These animals were collected from the wild as pups and have been in this captive setting for 2 to 4 years. They were housed in covered outdoor enclosures and did not have access to salt water (pools had a flow through fresh water system). Prior to the experiment, the seals had never been supplemented with salt. The diet of these seals was exclusively composed of air-thawed Pacific herring (Clupea harangus) supplemented daily with 50 mg of thiamine and 200 IU of vitamin E per animal. The daily food intake of each seal ranged from 1.6 to 3 Kg of fish depending on time of year. Chemical analysis showed that the herring contained 1.28 ± 0.15 g (mean ± STD) of sodium and 1.58 ± 0.18 g chloride per Kg of fish (method AOAC 968.08).
We first compared the values for sodium (Na), chlorine (Cl), and potassium (K) concentration in blood samples taken throughout the years (1997-2001) on our non-salt supplemented animals with the values from healthy harbour seals at the Marine Mammal Centre (MMC) in Sausalito, California, USA. The animals from the MMC were also housed in fresh water and fed Pacific herring, but were supplemented with 2 g of table salt per Kg of fish. As shown in Figure 1, the distributions of the levels of Na and K for the seals from UBC are shifted to the left compared with the distributions from the MMC seals. These differences were statistically significant for both the Na (p= 0.0041), and the K (p = 0.0001) (Wilcoxon rank sum test).
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Distribution of serum Na, Cl, and K harbour seals housed in fresh water and either supplemented (MMC) or not-supplemented (UBC) with table salt. MMC: 42 samples taken from 42 healthy harbour seals at the Marine Mammals Center, Sausalito, CA, USA. UBC: 37 samples taken from six healthy seals at the University of British Columbia.
Five of the six seals housed at UBC were used to evaluate the effect of salt supplementation on the levels of Na, Cl, and K in serum and on the urinary excretions of these electrolytes. These seals were sedated with Isoflurane via a face mask 3 hours after a regular morning feeding. A blood sample was taken within 10 minutes of the beginning of the handling. Each animal was then confined to a wire bottom cage equipped to collect urine. This procedure was repeated for each animal following 3 weeks of daily supplementation with 6 g of table salt per day (approximately 2 g of salt per Kg of fish). Standard biochemical analyses were performed on the serum and urine samples. Aldosterone levels were measured in serum using a technique previously described for harbour seals.4
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Serum and urinary levels of Na, Cl, and K in five harbour seals housed in fresh water at UBC with and without supplementation with salt (2 g per Kg of fish per day). One line per seal.
As shown in Figure 2, the salt supplementation was associated with an increase in serum levels of Na and K in all five seals, and with an increase in Cl for all but one seal. These increases were statistically significant for the Na (p = 0.03) and the K (p = 0.03), but not for the Cl (p = 0.18) (Friedman rank sum test). The urinary concentrations of Na (p=0.03) and Cl (p=0.03) increased markedly after oral supplementation with salt, whereas urinary levels of K were not significantly different (p= 0.65) (Friedman rank sum test). Values of aldosterone varied from 84 to 1062 pmol/L, and were on average lower after 3 weeks of supplementation with salt [No supplementation = 736 ± 278 pmol/L; following 3 weeks of supplementation = 478 ± 203 pmol/L; (mean ± 2STD)]. However, this difference was not statistically significant (p = 0.65; Friedman rank sum test).
Although Cl levels were similar, values for serum Na and K were on average lower in seals not supplemented with salt (UBC) than in seals supplemented with salt (MMC) (Fig. 1). Even if potential confounding factors could not be ruled out, this finding suggests that the serum levels of Na and K can be affected by the oral salt intake. The results of the experiment done at UBC, where concentrations of Na and K increased following 3 weeks of salt supplementation, are in agreement with this finding (Fig. 2). It is reasonable to believe that the rise in serum levels of Na is a direct consequence of the increase in the oral intake of Na associated with the supplementation. Interestingly, the values of serum K were lower in non-supplemented seals. Even if the levels of K in urine do not support this (Fig. 2), it can be proposed that this finding is due to a difference in the reabsorption of Na within the renal tubules via the Na-K ATPase pump; reabsorption of Na, and therefore excretion of K by the renal tubules, being higher in seals with low Na intake.
The urinary excretion of Na in the non-supplemented seals appeared to be relatively low (between 5 and 51 mmol/L). This suggests that harbour seals have a high capacity to reabsorb Na within their renal tubules. Even if not statistically significant, the aldosterone levels generally decreased following the 3 weeks of supplementation with salt. This supports the role of this hormone in the control of the Na absorptive capacity of the renal tubules. The levels of aldosterone reported here should be interpreted with caution since the stress involved with the blood sampling could have caused high levels following the release of ACTH. Since the limit of this reabsorption capacity is not known, it is not possible to know how close the intake of Na is to the minimal requirement for this species.
It has been proposed that serum levels of Na below 143 mmol/L are critically low and usually associated with clinical signs of hyponatremia while values below 147 mmol/L in seals should be considered as suspect.3 According to these guidelines, four out of six seals housed at UBC have been below the level proposed as suspect.3 However, clinical signs of hyponatremia have never been observed in these seals, and therefore, it seems that their diet of herring is sufficient to maintain electrolyte homeostasis. The distribution of serum Na concentrations in non-supplemented seals (UBC) is similar to reference ranges available for terrestrial carnivores,5 whereas the values of Na for salt-supplemented seals at the MMC are in the range reported for free-living harbour seals.7 Although it is tempting to do so, it would be speculative to conclude that supplementation with salt is beneficial for the seals just because it mimics the situation in the wild.
Until we know the implications of these findings, i.e., whether non-supplemented seals are at risk for hyponatremia, it is probably wise to add salt to the diet of harbour seals when access to salt water is not available. Furthermore, it should be stressed that the way frozen fish are handled can greatly affect their Na content. It has been shown that 25% of the Na is lost when frozen herring are immersed in fresh water.2 Therefore, thawing fish in fresh water or letting air-thawed fish soak in a pool of fresh water will significantly decrease its Na content. The seals at UBC are hand-fed with air-thawed fish, and this practice optimizes the Na and Cl intake.
In summary, our study demonstrates that seals housed in fresh water and hand-fed with air-thawed Pacific herring could maintain their electrolytes homeostasis without additional supplementation of salt. The marked increase in Na and Cl urinary concentrations in seals supplemented with salt suggests that most of the Na and Cl given is excreted directly in the urine. Nevertheless, non-supplemented seals had slightly lower levels of Na and K than supplemented seals. Even if other factors could have accounted for this difference, and if the biological significance of this finding is unclear, we believe that it favors the necessity of supplementing harbour seals, housed in fresh water, with salt. Additional studies will be needed to determine what the minimal requirements in Na and Cl are for this species.
We are grateful to the volunteers that helped us with this project. Special thanks goes to Dr. David Jones' team for logistic support and to Dr. Russ Andrews for useful discussions and assistance. Urinanalyses were performed with the help of Margaret Hendren from the St. Paul's Hospital, Vancouver, Canada. The aldosterone assays were realized by the Animal Health Diagnostic Laboratory, Michigan State University, East Lansing, MI, USA (Dr. Peter A. Graham and Susan Lombardini). Many thanks to Frances M. D. Gulland from the Marine Mammals Center, Sausalito, California, USA for sharing with us her data on Na, K, and Cl in harbour seals. This project was financially supported by the Animal Care Centre at the University of British Columbia, and by the Fond du Centenaire - Université de Montréal.
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