Bonefish Physiology

Living in an environment with changing conditions presents many challenges for residents such as bonefish. The tropical ocean coast can be just as harsh as northern coasts. As water temperature has been found to be more important than any other single factor, it can also be considered a source of stress to fish when temperatures change. 

Daily fluctuations in water temperature can be dramatic in tidal flats environments with up to a 15.5°C change over a 24-hour period. These changes, in turn, influence oxygen concentration as warmer waters hold less oxygen. How a fish will respond to such environmental changes will depend on whether they’ve had a chance to get used to the conditions. Unfortunately, environmental factors such as how temperature influences bonefish have barely been addressed until now.

As part of the ongoing studies conducted by the Flats Ecology and Conservation Program at the Cape Eleuthera Institute in The Bahamas, we’re investigating the temperature tolerance of bonefish. The first part of the puzzle is determining the maximum water temperature bonefish can withstand. To do this, the lab study exposes fish to a constantly increasing water temperature. When the water reaches a temperature that a fish in unable to tolerate, it loses equilibrium (i.e., rolls over). Once this happens, the fish is quickly removed, sampled for blood and placed back in its original temperature tank to recover. 

This experiment provides us with what exact water temperature a bonefish can handle with no other stressors involved. However, since it’s common for more than one stressor to act on a fish at one time, it’s useful to see how bonefish respond in the field to exercise at different temperatures. In addition, by examining the movements of transmitter-implanted bonefish in areas where temperature data loggers are placed, we can attempt to understand the strategies bonefish use to cope with changes in temperature.

Results of our lab study found that bonefish conditioned to water temperatures around 27°C can tolerate a maximum temperature of 36.4°C, whereas fish acclimated to a temperature of around 30°C can tolerate a maximum temperature of 37.9°C.  Temperature loggers placed in tidal flats recorded water temperatures of over 38°C at the mouths of tidal creeks and over 41°C in the backwaters of creeks. 

When we compared the blood from fish exposed to high water temperatures to bonefish that did not undergo the experiment, it was clear that exposure to increasing temperatures was physiologically stressful. Outside the lab where bonefish were exercised for either short or long periods of times at both cooler and warmer temperatures, similar results were found with higher temperatures being more stressful to fish in addition to the duration of exercise. Conversely the shortest duration of exercise at the lowest water temperature resulted in the least amount of stress on bonefish. 

Preliminary examination of the movements of tagged individuals seem to show that bonefish spend less time in nearshore tidal flats/creeks in the warmer months than they do when the water is cooler. This suggests that bonefish do indeed thermoregulate by avoiding potentially stressful warm water. 

The implications of our findings to bonefish apply to both their basic ecology and also to understanding the potential impacts of recreational angling. In light of predicted climate change models, if water temperature within flats environments continues to increase gradually, this may affect the amount of time bonefish can spend feeding in nearshore areas. With regards to recreational angling, the field survey demonstrated that combined stressors such as temperature and exercise duration had a greater effect on fish physiology. Shorter fish fights when water temperature is high will reduce stress. 

Future research into the influence of water temperature on bonefish physiology and behaviour will continue to strengthen our ever-growing knowledge about them.

For more information contact Karen Murchie ( This e-mail address is being protected from spambots. You need JavaScript enabled to view it )