Inside Stories

In for the long haul

Bimini Biological Field Station (BBFS)

Samuel H. Gruber

Words by Samuel ‘Doc’ Gruber

Cover image

Photo by Matthew Potenski

In January 2014, a team of scientists working with lemon sharks at the Bimini Biological Field Station published a pioneering paper that represents the culmination of my research on the lemon shark extending back half a century to 1961. As a young graduate at the University of Miami, I was trained in the discipline of animal behaviour, known technically as etho- logy. My speciality was learning, psychophysics and sensory biology, with an emphasis on the vision of sharks. As such, the first 15 years of my research were spent primarily in the laboratory. Then in 1976 came an epiphany: I hadn’t signed on to spend 12 hours a day in a dark laboratory looking at the golden eye of a lemon shark. I was hankering for the reefs and sea-grass flats where these amazing creatures live.

Miraculously, I snagged a career-development grant from the US National Science Foundation and for the next decade I was privileged to combine laboratory and field research, looking at the behaviour and ecology of my favourite subject, the lemon shark. Between 1979 and 1989, we went to sea on oceanographic research vessels four times a year. The maiden cruise took us around the Bahamas in search of an appropriate site, one that teemed with lemon sharks.
My first trip to Bimini had been in 1959, when the Bahamas was still a British Crown Colony (independence was granted in 1972), and by the 1980s I knew the islands quite well. We surveyed them for lemon sharks and, much to my elation, Bimini turned out to be the best place to set up our research site. There were many reasons for its suitability, including its pristine waters, proximity to Florida and the excellent, untouched colony of lemon sharks inhabiting the lagoon.

But my elation was short-lived. In 1988 the cancer I had been fighting for a decade became critical and the doctors told me my time was over. Not being one to accept authority, I set to work with a friend and we searched the literature for experimental cancer trials. To cut a long story short, we found a promising phase-1 research project and I was able to receive the drug Fludarabine on a compassionate-treatment basis. Clearly it worked well, and here I am writing this article a full 26 years later.

I mention all this because when I recovered with a new lease on life I realised that I now had a future with options, as well as the time and experience to pursue them. Notwithstanding the university politics that had to be overcome, I was determined to establish a research station on Bimini to study my beloved lemon sharks. I was also firmly committed to creating a place where students with a passion for sharks could come and study these creatures, learning the field techniques that we had developed over the past decade. So my wife Mari and I mortgaged our house, brought a bunch of beds, pots, pans and appliances to Bimini and opened the doors of the Bimini Biological Field Station in March 1990. The saying ‘Fools rush in where angels fear to tread’ applies 150% in this case. But I digress.

Cover image

Photo by Matthew Potenski

Samuel H. Gruber Founder of the Bimini Biological Field Station

Illustration by Marc Dando

As soon as the research station was up and running, I decided to launch a twice-yearly shark census at Bimini in an effort to tag every lemon shark in the lagoon. It’s a challenge to determine population dynamics and abundance, and one that I had been wrestling with for 17 years. First there was the tricky choice of which tag-and-release model to employ: the classical Peterson method, the Jolly-Seber method, the triple-catch technique or what? Then we had to decide which tag to use and whether it might affect the sharks’ growth or mortality. For instance, we found that the standard dart tag of the National Oceanic and Atmospheric Administration (NOAA) killed at least 10% of young sharks and seriously affected the growth of others. Using it means that a lot of the data it delivers is useless – as I found out to my frustration because I chose the wrong tag when we began our research in 1979. Back then, a control study by my student Alan Henningsen showed that most kinds of tags fell out at rates approaching 90%. The exception was the NOAA dart tag, so naturally that’s the one we chose. However, if you expect almost all your marked sharks to survive but your tags injure them, you can guess what that means for your statistical analyses.

After years of searching, in 1988 we stumbled upon a tag with a very low shed rate and no deleterious effects on the little sharks. Called a passive integrated trans-
ponder (PIT) tag, this glass-encapsulated electronic tag is the size of a rice grain and needs no power source. When the body of a shark is scanned with a small hand-held reader, microwaves excite the previously implanted PIT tag and it spits out its number like a barcode scanner in a supermarket. We inject these tiny tags under the skin at the base of the shark’s dorsal fin and it usually stays there for life.

So after years of study, we thought we were ready for our new research project at Bimini. Because of the species of choice (the lemon shark) and the unique ecological conditions (more like a lake than an open, infinite marine environment), we reckoned that we were ready to carry out a unique series of comprehensive and detailed studies on population dynamics. Our goal was to yield life-history para-
meters, such as growth, survival and habitat selection. But assumptions had to be made. For example, we assumed that Bimini’s population of lemon sharks did not immigrate or emigrate out of the shark nursery for three years, and that there was no fishing mortality either – only natural mortality.

Against this background, we set up a big pen and fished large sections of the nursery at a time. We knew from John Morrissey’s comprehensive tracking study of 1988 that young lemon sharks have a restricted activity space, so at sundown we set 180-metre-long gill nets at three places in the shallow nursery and walked their length every 15 minutes during the 12-hour summer night. When we came upon an entangled shark, we removed it and rushed it by boat to the big pen, where the tagging crew weighed, measured, sexed and tagged each shark as it came in. By this extraordinary effort we were able to tag about 90 juvenile lemon sharks that first season. The plan was to return six months later and do it all over again so that we could keep tabs on our little sharks. They say the best-laid plans often go astray and this is exactly what happened. The next November we were only able to catch two sharks, which of course put a stop to our project.

It took another five years to consider starting up again. By this time, our capabilities and goals were much better focused and we organised a campaign that is still going today, two decades later. This has become the longest-ever tagging study of a single species and the centrepiece of our research at Bimini. Based on the success of this campaign, we advanced our goals to follow all the lemon sharks born at Bimini from birth to adulthood and beyond. Only this time we applied the innovative techniques of genetics research to the sharks. Our interest was to understand many of the life-history traits of lemon sharks using breeding biology as the focus of the project.

Fast forward to 2014. After tagging more than 3,500 lemon sharks over the past 20-year period and producing a genetic catalogue for each and every one, we created a pedigree of all the families of lemon sharks using Bimini’s lagoon. It is important to note that by using molecular techniques we were able to identify the parents of each baby shark by examining the maternal and paternal contributions to its genome. Thus, we never saw most of the parents nor did we need to. In addition, the method we used, called microsatellite analysis, meant that we were able to genetically fingerprint each shark so that if we ever caught that individual again, we could identify it definitively without any other tags or marks. So it was that we had Bimini’s entire lemon shark population under scrutiny for two decades. I will not go into the myriad of new findings, publications, graduate dissertations and theses that we gleaned from this study. Suffice to say that the advances in our knowledge of lemon shark biology were significant and will be the subject of numerous future articles.

Finally, to return to our recent landmark paper, many shark biologists in the past believed that many shark species, like salmon, return to their birthplace to reproduce. These biologists used statistics and calculations to suggest that this reproductive behaviour, known as philopatry or natal homing, was a fact. However, no one had actually observed natal homing. Given that it takes 12 to 15 years for a lemon shark to reach reproductive maturity, who would wait that long to find out? The answer: the shark fanatics of Bimini Biological Field Station! After waiting the dozen years, with great anticipation, we genetically examined a cohort of sharks born in 2010 to see if any of their parents had been born in 1995 and had returned to Bimini to give birth. We were thrilled to find that several sharks had indeed remembered their birthplace and come back to give birth.

But that was not enough. The editors of the journal we submitted our paper to wanted proof and required that we actually catch a mother lemon shark and directly identify her as having been born in Bimini. To cut to the chase, we did it! We’ve developed techniques to find and catch by hand (no hooks or harpoons) adult lemon sharks entering the lagoon (that’s another story for the future). These allowed us to successfully read a PIT tag that had been placed under the skin of one mother shark so many years ago.

We have learned many lessons from this long-term study. One of the most important is that it is very easy to drive a local population to extinction. This can be done either by destroying the population’s nursery habitat or by killing off the breeding stock of mothers that return to the nursery where they are born to give birth. Understanding these local populations and where their most important habitats lie is imperative to conservation and management plans, particularly spatial protection measures such as marine protected areas, for these apex predators.

Illustration by Marc Dando