Temperature preference of juvenile Lemon sharks, Negaprion brevirostris, in the south bimini nursery.
Jurgen Batsleer & Dr. Samuel H. Gruber
The question why or how organisms select a particular habitat is an important topic in ecology. Environmental factors, for example salinity, pH and temperature, play a vital role in habitat selection. Beitinger and Fitzpatrick (1979) have argued that temperature exerts greater control over fish behaviour than any other abiotic factor in the environment.
Sharks in particular play a vital role as top predators in most pelagic and coastal ecosystems, but little study has been conducted to examine the environmental variables that may influence their habitat selection and movement patterns. Bimini is known to be an important nursery area for juvenile lemon sharks, the mangroves in the north and south of the island give the juveniles a better chance of survival by protecting them from predation and at the same time it's the habitat of their prefered prey-species.
It's known that the water temperatures in the south nursery area show seasonal and diel fluctuations. During this six month study, from August 2005 - February 2006, I'll focus on the water temperature in the south nursery area of Bimini and profile the temperature regime. Previous laboratory based research in has shown that sharks are able to detect and use temperature gradients, therefore my second focus is on the movement pattern of the juvenile sharks to see whether their movement is regulated by a temperature preference.
For this study I-buttons® are used to measure the temperature. The I-button® is a computer chip enclosed in a 16mm stainless steel housing. 60 Environmental I-buttons® are evenly spread over the entire south nursery area to profile the temperature regime. These I-buttons® record the temperature every 20 minutes for a duration of 28 days. Each month the buttons are collected, downloaded, reset and placed back at the same location.
To get an idea which water temperature the juvenile sharks prefere, an I-button® is externally attached underneath the first dorsal fin of the sample specimen. The I-button® records the temperature every 10 minutes for about 14 days. The specimen is then recaptured and the I-button® has removed. As it's difficult to study animals that have a large home range we use manual acoustic telemetry to study the movement of the sharks. Coded transmitters are surgically inserted into the peritoneal cavity so they can be followed with a hydrophone. During the two week period the shark is released we track the shark so we can get a more detailed image of the movement patterns of the juveniles.
So far the obtained data gives us an interesting view of the movement patterns of the juvenile sharks. During my stay at the lab I have temperature data of the water during two tropical storms (Katrina in August and Rita in September) and one hurricane (Wilma in October). The data of the sample specimens out during the storm and hurricane show that they seem to find warmer water than is present across the south nursery environment. Of course a more in depth analysis of the data has to be conducted to see whether movement and temperature preference of juvenile lemon sharks are corelated.
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