Fish and How we Count Them

Orangepeel Angelfish (Centropyge shepardi)

Fish are some of the most conspicuous residents of the coral reef environment and, along with corals, tend to get the majority of the attention.  These communities also tend to be the ones we get the most questions about.  Recently we received some excellent questions from Mr. Hill's period 2, 5 and 6 classes.  They ask the following:

• "Have any of the scientists on your team ever discovered a new species of fish?"
• "Do you search for particular species of fish on your dives or do you simple count and measure all the fish that you come across."
• "What type of sharks have you identified on your trip to date? What was the biggest fish that you have seen on your trip to date?"

Ornate Butterflyfish (Chaetodon ornatissimus)

We have three teams of scientists whose job it is to assess the overall condition of the reef fish community at each of the islands we visit.  While we haven't yet discovered any new species of fish on this expedition, one of our scientists may have discovered on on an expedition a few years ago.  The process of discovering a new species is more complicated than you might imagine.  You may find something you don't recognize, but that doesn't mean that NO ONE has ever seen it before.  If you find something you can't identify you have just started down a very long but exciting path.  First you have to do quite a lot of research to make sure you really are the first person to see and describe your particular fish.  You look through all your fish guides and all the scientific papers you can find, consult regional and local experts and taxonomist at universities and museums to see if ANYONE has ever seen the fish you found.  If you really are the first person to see it you then need to describe it and publish your findings in the scientific literature to let other scientists know of your find.  You describe in extreme detail what the fish looks like so that other scientists can compare fish they may find to your notes.  You describe what color it is, how big it is, what shape it is, how many scales it has along its lateral line, how many fin rays or spines it has in each of its fins, you may even take a sample for genetic analysis so that the DNA sequence of your fish can go into a taxonomic gene bank.  This process can take years but it is very exciting.  When all is said and done, you get to name your fish!

Saddled Butterflyfish (Chaetodon ephippium)

So ... Do we count all fish we encounter? That’s an excellent question. We do count every single fish we encounter in our survey area. Of course, we know very well that we are missing some, especially the ones that are hiding deep inside the recesses of the reef, but we do the best we can.  Often we are not as much trying to get an accurate count of the total number of fish in a particular area, but more how a given site compares to other sites we visit.  For this purpose, as long as we are making our counts in the same way at each site, we are likely missing the same things each time so our counts are comparable.  The more counts we make, the more accurate we are.  Although we pay special attention to the typical “food” fishes, we also want to understand what is happening to the entire fish community and how it is changing through time.  Are we seeing different species now compared to what we saw in the past?  Is the total number of species or species diversity changing from site to site or over time?

Pyramid Butterflyfish (Hemitaurichthys polylepis)

So how do we count all those hundreds of fish belonging to various species swirling around us?  We use a variety of different methods al of which involve SCUBA.  For many of the smaller fishes: we stay at the center of our survey area, take 5 minutes to establish a species presence list, and than proceed to enumerate and size all the fish in this list, one species at a time.  It's a little tricky, but you get the hang of it and get better the more times you do it.  There is a lot of training involved.

Divers counting small fish along transects

For some of the larger species we use what we call a towed-diver or towboard method.  These large fishes tend to exist in very low densities, that is to say, we don't find very many of them in a given area.  So, you have to cover a lot of area in order to get a good idea of how many there are at a given island.  The scientists who study these fish use dive planes which are towed behind a small boat along the coral reef for distances of up to a mile or more.

A towboarder flying over the reef

We fly these dive planes at a depth of about 50 feet and count all of the large fishes we come across during a 50 minute survey.  We don't count as many fish, but the ones we see tend to be bigger.  We see a whole variety of species including a lot of sharks.  On this expedition most of the sharks we have seen have been white-tip reef sharks (Triaenodon obesus) but we have also seen a few gray reef sharks (Carcharhinus amblyrhynchos) and tawny nurse sharks (Nebrius ferrugineus).  What is the biggest fish we have seen so far?  Definitely one of the nurse sharks which was about 6 feet long.  We have also seen some big dogtooth tuna (Gymnosarda unicolor).

Blackfin Barracuda (Sphyraena qenie)

Dogtooth tuna (Gymnosarda unicolor)