A sailfish is a hardy animal for sure but imagine how one must feel after a 15 minute battle with an angler! Pretty tired for sure. We see it in the animal’s color change. Imagine an athlete running a 100 meter dash and then placing his head in a tub of water. He simply can not extract oxygen from the water in the tub. Instead he puts on an oxygen mask and inhales pure O² to recover. A sailfish is like that athlete trying to extract as much O² from the water as possible.
We have thought about this analogy since the beginning and decided to take a hard look at the data to see how the sailfish behaved after release. The PSATs collect depth and temperature data so we analyzed it specifically looking for the mode (most frequent) values for depth and temperature. While they do go hand in hand in that the deeper the water the colder it is, the absolute values are influenced by a number of factors including the season, rain, and of course upwellings of cooler water.
The first graph below represents the Maximum and Mode depth in meters of 7 PSATs during the duration of their deployment.
There are a couple of interesting observations from this data:
- The maximum depth varied from a “shallow” 57 meters (187 feet) to a rather deep 142 meters (466 feet). Granted, this is not particularly deep for fish and marine mammals which have been know to search for food at much deeper depths.
- However, looking at the Mode depth we see that each tagged sailfish spent most of their time in less than 10 meters (33 feet) of water. They may have been swimming in thousands of feet of water but they most frequently were in the top 10 meters. The TBRP team should now check the data for a pattern indicating that sailfish spend their day time in the top 10 meters and their night time below searching for squid and other food.
This next graph indicates the Minimum, Mode and Maximum Sea Water Temperature in degrees Celsius.
What we have learned from this data is:
- Sailfish will spend some time in what we would consider very cold water. The lowest reading was 14.3 °C (57.7 °F). This is considerably below what is normally considered to be there comfort zone.
- There was a maximum temperature of 31.5 °C (88.7 °F) recorded. This is well within the typical water temperature range that sportfishermen record when looking for sailfish, marlin, tuna and mahi. What is most interesting about these max temp figures is that the variance is only 0.3 °C (0.5 °F). This implies that there is almost an absolute temperature barrier that they will not cross.
- The mode, or most frequently observed, value for temperature indicates that these 7 fish preferred to spend their time in water from 27.6 °C (81.7 °F) to 30.4 °C (86.7 °F). With a variance of only 1.3 °C (2.3 °F) these fish were fairly precise in choosing the water temperature in which they spent most of their time.
The chart below shows the relationship between water depth and two factors critical to sailfish, dissolved oxygen and water temperature. As you would expect, the deeper the sailfish swims the colder it is and the less dissolved oxygen there is. This data only represents one sample around 10am and not on any dates of PSAT deployment represented in the other charts. While many factors influence water temperature and dissolved oxygen there is a consistent relationship between depth, water temperature and dissolved oxygen. The deeper the fish swim the colder it gets and the less dissolved oxygen there is for the sailfish to breath.
The BillfishResearch.org team will continue to collect, analyze and present this data as it accumulates. For additional information or if you have any questions please contact Dr. John Mark Dean via e-mail.
Time of Day vs Depth - Diurnal Movement Indicated
In this graph to the left it is interesting to see that as time moves through the day from midnight to dawn and eventually back to midnight this sailfish expressed a preference for the depth at which he swam.
There are three interesting observations from this scattergram:
1) From midnight to 6 AM this sailfish stayed mostly above 40 meters and substantially above 10 meters, which is around the Mode depth.
2) But around 6 AM this sailfish started venturing into much deeper water. Most of the time from 6 AM to 6 PM was spent swimming between 40 and 80 meters with a few observations below 80 meters!
3) Then as the sun began to set we see that this sailfish started working its way back up into the shallower depths of above 20 meters.
Of course the TBRP team has some questions!
Q: Is this diurnal pattern related to foraging for food? This fish swam deeper during the day. Was it because he could see his prey better as the sunlight reached deeper into the water?
Q: Is this sailfish higher up in the water column during the night because his prey is rising up every night to a point in the water column that does not require such visits to deeper water?
All good questions that the team has to now think about and attempt to answer!