Piscatorial Rumination – Salinity

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 Isotonicinternal 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.

Hormone TableAll 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.


– Chris


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.


Piscatorial Rumination – Temperature

The physiological response to abiotic and biotic cues is dependent upon the surrounding water temperature. With the exception of members from the scrombridae (mackerels, tuna and bonitos), lamnidae (mackerel sharks and white sharks) and xiphiidae (swordfish) families, the metabolic rates, bodily temperatures and consumption rates of fish are largely dictated by external thermal conditions. A plethora of temperature gradients can be found throughout the marine environment, and studies have shown that fish indeed have an optimal thermal preferendum. Throughout much of his early career, F.E.J. Fry created a temperature hypothesis regarding these gradients, and it is still valid today. His hypothesis was threefold. First, he noted thermal preferendums are species specific. Secondly, fishes will aggregate in areas where the thermal preferendum is stable. Finally, and perhaps the most important facet of his hypothesis, the final thermal preferendum of fish will coincide with temperatures in which key physiological, biochemical and life history processes can be optimally carried out. Outside the thermal preferendum fish will undergo physiological changes, including changes in gonadal growth, somatic growth and oxygen consumption, to cope with the stress of suboptimal conditions. Increasing temperatures associated with climate change will surpass the thermal preferendum of many fish and motivate such physiological changes.


codIn water, as temperatures increase the concentration of O2 will decrease. Oxygen limitation is often the biggest facet of thermal intolerance in fish. In 1975, D. M. Rowell and others found a 10°C increase more than doubled the oxidative metabolic rate in winter flounder (Pseudopleuronectes americanus). Similar results regarding the winter flounder were found by J. Cech and others in 1976. However, their data also revealed another interesting physiological change. Cech noted as temperature increased, the difference in O2 between the main artery and the main vein also vastly increased, indicating more oxygen was being delivered to tissues and organs. In winter flounder, and other species, increasing temperatures will also increase heart rate, which could be attributed to the increased temperature of pacemaker cells within the heart. The hemoglobin’s capacity for oxygen appears to be affected at higher temperatures, and in the case of the winter flounder, an elevated heart rate is the physiological response to deliver a higher flow of blood to the body to combat the onset of the decreased capacity of oxygen in hemoglobin. Phenotypic plasticity is another viable option to cope with the lack of oxygen. Portner and others sampled Atlantic cod (Gadus morhua) from various sites in the North and Baltic seas. The variation of thermal regimes at each sampling site was reflected in their data in which differences in the ratio of hemoglobin types were observed. In other words, the specific conditions found at each site would elicit a variety of hemoglobin polymorphs to cope with the local conditions.


In addition to decreased oxygen consumption, the literature is ripe with studies relating growth to temperature. In some thermally stressful instances, the energy typically allocated for growth will be reallocated to cope with increasing maintenance needs. Higher temperatures for short durations have been seen to increase food consumption rates to keep up with metabolic demands. However, if higher temperatures persist or progress towards a species’ critical thermal maximum, decreases in growth usually follow. Within Fry’s thermal preferendum, there lies an optimal temperature in which growth rate is maximized. Events or disturbances decreasing or increasing the temperature above or below this optimum will yield decreased growth rates. When relating growth rate to temperature, cod (Gadus morhua) displays classic trends. In a sub-experiment involving various sizes of cod, Bjornsson and others found an optimum temperature of approximately 8°C yielded the highest growth rate. InterestinglycodIII, they also found the optimal temperature decreased as fish size increased. This would suggest one of two ideologies. (1) The younger smaller fish have a wider optimal temperature regime, or (2) the thermal optimum shifts with age and is dependent on size. In either case, higher temperatures would allow for faster development of juveniles to adulthood, and higher temperatures stimulating maximum growth rates would not be necessary at the adult level.

Temperature is also a determining factor in the maturation of fish. Increasing temperatures will delay the various maturation schedules found in the fish community. Although it is unclear exactly how temperature affects this process, it is hypothesized to do so through alteration of maturation regulatory proteins, secretion of gonadotropins, oestrogen in the liver or mechanistically altering oocyte growth rates. This shows changes in temperature regimes carry potential recruitment and fecundity repercussions.


– Chris


References and Photo Credits

Fry, F.E.J. 1947. Effects of the environment on animal activity. Publications of Ontario Fishery Research Lab 68:1-62.Portner, H.O. 2001. Climate change and temperature dependent biogeography: oxygen limitation of thermal tolerances in animals. Naturwissenschaften 88:137-146.

Portner, H.O., Berdal, B., Blust, R., Brix, O., Colosimo, A., De Wachter, B., Giuliani, A., Johansen, T., Fischer, T., Knust, R., Lannig, G., Naevdal, G., Nedenes, A., Nyhammer, G., Sartoris, F.J., Serendero, I., Sirabella, P., Thorkildsen, S., and Zakhartsev,M. 2001. Climate induced temperature effects on growth performance, fecundity and recruitment in marine fish: developing a hypothesis for cause and effect relationships in Atlantic cod (Gadus morhua) and common eel pout (Zoarces viviparous). Continental Shelf Research 21:1975-1997.

Rowell, D.M., Cech Jr, J.J. and Bridges, D.W. 1975. Respiratory metabolic responses of the winter flounder (Pseudopleuronectes americanus) to environmental stress. In Respiration of marine organisms. Edited by J.J. Cech Jr., D.W. Bridges. and D.B. Horton. TRIGOM Publications, South Portland, Maine pp163-170.

Randall, D.J. 1968. Functional morphology of the heart in fishes. American Zoology 8:179-189.

Cech Jr, J.J., Bridges, D.W., Rowell, D.M. and Balzer, P.J. 1976. Cardiovascular responses of winter flounder, Pseudopleuronectes americanus (Walbaum), to acute temperature increase. Canadian Journal of Zoology 54:1383-1388.

Hayden, J.B., Cech, J.J., and Bridges, D.W. 1975. Blood oxygen dissociation characteristics of the winter flounder, Pseudopleuronectes americanus. Journal of the Fisheries Board of Canada 32:1539-1544.

Portner, H.O., van Dijk, P.L.M., Hardewig, I., and Sommer, A. 2000. Levels of metabolic cold adaptation: tradeoffs in eurythermal and Stenothermal ectotherms. In: Davison, W., Howard-Williams, C., and Broady, P. (Eds). Antartic Ecosystems: Models for Wider Ecological Understanding. Caxton Press, Christchurch, New Zealand, pp. 109-122.

Buckel, J.A., Steinberg, N.D. and Conover, D.O. 1995. Effects of temperature, salinity, and fish size on growth and consumption of juvenile bluefish. Journal of Fish Biology 47:696-706.

Bjornsson, B., Steinarsson, A. and Oddgeirsson, M. 2001. Optimal temperature for growth and feed conversion of immature cod (Gadus morhua L.). Journal of Marine Science 58:29-38.

Dhillon, R.S. and Fox, M.G. 2004. Growth-independent effects of temperature on age and size at maturity in Japanese medaka (Oryzias latipes). Copeia 2004:37-45.

Tobin, D. and Wright, P.J. 2011. Temperature effects on female maturation in a temperate marine fish. Journal of Experimental Marine Biology and Ecology 403:9-13.

Pankhurst, N.W. and Porter, M.J.R. 2003. Cold and dark or warm and light: variations on the theme of environmental control of reproduction. Fish Physiology and Biochemsistry 28:385-389.

Peter, R.E. 1981. Gonadotropin secretion during reproductive cycles in teleosts: influences of environmental factors. General Comparative Endocrinology 45:294-305.

Yaron, Z., Cocos, M. and Salzer, H. 1980. Effects of temperature and photoperiod on ovarian recrudescence in the cyprinid fish Mirogrex terrae-sonctae. Journal of Fish Biology 16:371-382.

Korsgaard, B., Mommsen, T.P. and Saunders, R.I. 1986. The effect of temperature on the vitellogenic response in Atlantic salmon post smolts (Salmo salar). General Comparative Endocrinology 62:191-201.