Oceanic warming has forced changes in the hydrological cycle, which are to some extent manifested within the salinity regimes of the planet’s oceans. These changes have altered the stratification of isopycnals, and as a result, oceanic circulation patterns are changing. In the subtropical latitudes, accelerated evaporation due to warmer temperatures is causing an increase of salinity; in the northern latitudes, the increased temperatures are melting ice sheets, glaciers and polar ice caps causing an influx of freshwater into marine environments and ultimately decreasing salinities. Salinity is a major abiotic parameter when considering fish growth and reproduction, and it could be detrimental to marine fisheries when considering the observed haline fluctuations.
The genre of salinity effects on growth rates are well documented within the scientific literature. Haline conditions are very similar in nature to thermal optima in that there is a specific range that optimizes biochemical processes. For example, the summer flounder (Paralichthys dentate) and the Southern flounder (Paralichthys lethostigma) show the greatest growth rates and development in intermediate salinities within the 8-14 psu (practical salinity units) and 5-30 psu ranges, respectively. In addition to growth rates, embryonic development within these two species is greatly dependent upon haline conditions. In 1994, Y. Lambert, as well as others, found similar results within the gadidae (cod) and scophthalmidae (turbot) families. The Atlantic cod (Gadus morhua) and turbot (Scophthalmus maximus) also show significant growth rates at intermediate salinities. These preferences for intermediate salinities appear to be common among many marine fishes. One hypothesis lies within the reduction of osmotic stress. When in a resting metabolic state, tuna have been shown to devote as much as 54% to 68% of available energy for ion regulation. Species of lesser metabolic rates will allocate approximately 20% to > 50% of energy to ion regulation. Salinities experienced outside these intermediate ranges could further lead to poor food and nutrient absorption, which ultimately leads to decreased somatic growth.
Reducing the surrounding water to an isotonic solution will diminish the metabolic demands to maintain optimal internal osmotic gradients. This can be attributed to the decreased energy required to maintain Cl–, K+, Na+ and ATPase gradients across transporter proteins in chloride cells found in the epithelial lining of gill lamellae. The isotonic hypothesis could be the main cause of the increased growth trends in the intermediate salinity examples mentioned above. Support of this can also be seen in the work done by W. O. Watanabe and others in 1988. By subjecting red tilapia (Tilapia rendalli) to various salinity regimes, they were able to show an optimal salinity for food intake and growth. Due to the decreased osmoregulatory energy demands, the observed growth was attributed to the excess energy available to convert food items into somatic growth. Salinity has been observed to have this same effect on developing embryos.
All species are able to acclimate within some range of haline conditions. Saline intensity has an indirect effect on the central nervous system by affecting hormones associated with the osmosensitivity in prolactin cells and chemoreceptors in the pseudobranch. In marine fishes, actively drinking saltwater is a strategy to replenish water reserves lost to diffusion, and the mechanism triggering this behavior can be linked to the activation of various dual purpose hormones such as the ones listed in the table above. For example, GH (a simple growth hormone) regulates growth while also aiding in osmotic regulation. In 2001, G. Boeuf and P. Payan hypothesized the water content in the stomach would trigger hormones such as GH, and the energy typically allocated to convert food to growth would then be allocated to osmoregulation (active drinking), resulting in decreased growth. Although no data currently show a distinct relationship between the active drinking behavior and hormonal growth control, the logic cannot simply be dismissed. The involvement of GH in both processes warrants consideration that both phenomenon are in some way linked and can be altered by fluctuating salinity regimes.
References and Photo Credits
Held, I.M. and Soden, B.J. 2006. Robust Responses of the hydrological cycle to global warming. Journal of Climate 19:5686-5699.
Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M. et al. 2007. Climate change 2007: The physical science basis: Contributions of working group 1 to the fourth assessment report of the intergovernmental panel on climate change. New York: Cambridge University Press.
Durack, P.J. and Wuffels S.E. 2010. Fifty-year trends in global ocean salinities and their relationship to broad-scale warming. Journal of Climate 23:4342-4362.
Antonov, J.I., Levitus, S., and Boyer, T.P. 2002. Steric sea level variations during 1957-1994: importance of salinity. Journal of Geophysical Resources 107:8013.
Boyer, T.P., Levitus, S., Antonov, J., Locarnini, R., and Garcia, H. 2005. Linear trends in salinity for the world ocean, 1955-1998. Geophysical Research Letter. 32.
Smith,T.I.J., Denson, M.R., Heyward, L.D., Jenkins, W.E., and Carter, L.M. 1999. Salinity effects on early life stages of southern flounder Paralichthys lethostigma. Journal of World Aquaculture Society 30:236-244.
Specker, J.L., Schhreiber, A.M., McArdle, M.E., Poholek, A., Henderson, J., and Bengtson, D.A. 1999. Metamorphosis in summer flounder: effects of acclimation to low and high salinities. Aquaculture 176:145-154.
Lambert, Y., Dutil, J.D., and Munro, J. 1994. Effect of intermediate and low salinity conditions on growth rate and food conversion of Atlantic cod (Gadus morhua). Canadian Journal of Fish Aquatic Science 51:1569-1576.
Gaumet, F., Boeuf, G., Severe, A., Le Roux, A., and Mayer-Gostan, N. 1995. Effects of salinity on the ionic balance and growth of juvenile turbot. Journal of Fish Biology 47:865-876.
Dutil, J.D., Lambert, Y., and Boucher, E. 1997. Does higher growth rate in Atlantic cod (Gadus morhua) at low salinity result from lower standard metabolic rate or increased protein digestibility?. Canadian Journal Fish Aquaculture Science 54:99-103.
Imsland, A.K., Foss, A., and Gunnarsson, S. 2001. The interaction of temperature and salinity on growth and food conversion in juvenile turbot (Scophthalmus maximus). Aquaculture 198:353-367.
Bushnell, P.G. and Brill, R.W. 1992. Oxygen transport and cardiovascular responses in skipjack tuna (Katsuwonus pelamis) and yellowfin tuna (Thunnus albacores) exposed to acute hypoxia. Journal of Comparative Physiology 162:131-143.
Toepfer, C. and Barton, M. 1992. Influence of salinity on the rates of oxygen consumption in two species of freshwater fishes, Phoxinus erythrogaster (family Cyprinidae) and Fundulus catenatus (family Fundulidae). Hydrobiologia 242:149-154.
Watanabe, W.O., Ellingson, L.J., Wicklund, R.I., and Olla, B.L. 1988. The effect of salinity on growth, food consumption, and conversion in juvenile, monosex male Florida red tilapia. In: Pulin, R.S.V., Bhukaswan, T., Tonguthai, K., Maclean, J.L. (Eds). The second International Symposium on Tilapia in Aquaculture, ICLARM Conference Proceedings A5. Department of Fisheries, Bangkok, Thailand and International Center for Living Aquatic Resources Management, Philippines, pp. 515-523.
Boeuf, G. and Payan, P. 2001. How should salinity influence fish growth?. Comparative Biochemistry and Physiology 130:411-423.