A Summer Spent Catching Sharks? Count me in!

Author: Catherine Fox, Marine Biology & Environmental Science Student, Florida Southern College; Advised by Dr. Gabriel Langford

Picture this: it’s winter 2021, covid has been a pandemic for nearly a year now. The research project I was supposed to do last summer was canceled due to covid and now I’m searching for something for next summer. I’ve applied to what seems like hundreds of places (when maybe it was only ~20) and haven’t been accepted to any. I can’t spend another summer at home waiting for something to come my way, so I keep searching. Then, one day, I get an email from my professor asking if I would be interested in working with sharks over the summer. Of course, I’m interested! I tell her I have experience operating and docking boats of various sizes and I’ve been saltwater fishing since I was young. Next thing I know, I’m accepted to participate in summer research! The plan is to take out the brand-new boat a couple times a week and capture bonnetheads, bull sharks, and blacktips. By the time May rolls around I’m so excited to get out on the water. But as with most research, there were many hiccups along the way. First, the boat wasn’t ready. Then we couldn’t get someone to drive it down to Tampa. Then there were issues with the insurance. As days bled into weeks, we tried to keep ourselves busy. We learned how to mend the nets, set up camera traps, and spent many hours cleaning and organizing labs in the science building.

Finally, the day had come! The boat was ready, it was in Tampa at the marina, and it was finally insured. We met at the crack of dawn, yawning, and stretching out the kinks in our bodies as we loaded the van with all our equipment. The car ride was filled with excited chatter as we talked about what the day would hold. It was a beautiful day, and the sun was shining brightly in the sky. We caught one juvenile bull shark that day, but it died. We were saddened by this, and our excitement evaporated quickly. When we got back, we practiced the workup on our fake blow-up shark so we could be faster and more confident next time. We also mended the holes in the net (at this point I didn’t realize just how many hours that summer would be spent mending those holes).

We got faster and faster over the next few weeks and caught over a dozen sharks. Then a massive red tide bloom started killing everything. Thousands and thousands of dead fish floated on the water, leaving a putrid stench to the air. All the rays that we had been catching were dead and there was no sign of the sharks. Day after day we went out searching for them but we didn’t find them. We thought this was the end. We wouldn’t catch anything else, maybe we should change our research project? But late July we decided to go out once more just to make sure. And thank goodness we did! That day we caught a record number of sharks in one gill net!

We only caught more from that point on. Bull sharks made the return and shortly after cownose rays did. But we never saw another southern stingray. I wonder if they all died or if some were able to survive? Maybe they went upriver or left the bay. I guess we’ll never know. That summer was filled with so much fun, laughter, learning, and too many sunburns and even though things didn’t always go as planned, I wouldn’t trade it for anything.

Coastal shark community assemblages

by Clark Morgan, University of North Florida, Master’s student

As animals grow and mature, their needs change. Throughout their life cycle, many animals relocate into different habitats to enhance their own survival, a process known as ontogenetic habitat shifts. Many shark species aggregate by size and life stage.  Smaller, younger sharks are found in “nursery” areas that have a high abundance of food resources and offer protection from predators (e.g. larger sharks). When a shark reaches sexual maturity, it often moves to another location where other like-minded (and bodied) sharks of the same species aggregate. These movements often correspond with changes in resource requirements for larger individuals. Multiple species often share resources, geographic areas, and prefer many of the same environmental conditions that deem a habitat suitable for use and consequently, complex communities are formed. Researching how these communities change in time and space can be a daunting task, but through the use of multiple methodologies, dynamic ecological questions can be answered for many scientific applications. Thus, understanding the relationship between sharks and their environment is crucial for sustainable management and conservation of shark populations (Simpfendorfer and Heupel, 2012).

Many coastal shark species of the southeastern United States are Carcharhinids, a family of fish known as the “requiem sharks.” A characteristic feature of this group is placental viviparity, in which pregnant females provide nutrients to their pups in uterovia placental connections before live birth. As a result, newborn pups have openings in their ventral surface that are essentially equivalent to a belly button in humans. These open umbilical scars heal quickly, but the size of the remaining wound can provide birthday estimations for these animals. Once completely closed, the presence of a healed umbilical scar is still useful for identifying an animal as a young-of-year (YOY), an important life-stage distinction.

A partially healed umbilical opening on a neonatal sandbar shark (Carcharinus plumbeus)

A shark’s length is the most useful way to determine its life stage, a result of many lethal studies that measured the development of internal reproductive organs at different sizes. Male elasmobranchs possess external reproductive organs known as claspers that are used in the internal fertilization of a female. These structures harden via calcification as an individual matures, which allows for a quick and easy assessment of life stage by a researcher. Quantifying species-specific life stage abundances and the corresponding environmental parameters of their habitats provides the framework for understanding ecosystems. This is becoming increasingly important for coastal ecosystems as the negative results of anthropogenic disturbances such as pollution, increased human populations, and coastal development can result in habitat degradation (Pan et al. 2013).

A mature male blacknose shark (Carcharhinus acronotus), with visibly large and developed claspers

Another useful method for assessing community interactions is known as Stable Isotope Analysis (SIA), which measures the naturally occurring ratios of heavy and light chemical elements found in animal tissue. Carbon (δ¹³C) and nitrogen (δ¹⁵N)are the most common isotopes measured. δ¹³C values trace the original base source of dietary carbon of a consumer, while δ¹⁵N values are indicative of relative trophic position (Peterson and Fry 1987, Post 2002).These ratios allow researchers to infer trophic levels, niche widths, and temporal foraging patterns, which provide deeper insight into how these communities may be competing for resources.To insure successful application of SIA, one must also consider the varying turnover time of different tissue types. Tissues that are less active in metabolic processing, such as muscle tissue, take longer for a change in diet to be reflected in a consumer’s isotopic signature (~1 year) while blood plasma provides dietary insight on a much shorter time scale (~2-3 months) (MacNeil et al. 2006; Matich et al. 2011). Comparing δ¹⁵N values of different tissue types from the same animal reveals temporal dietary changes which are common with ontogeny, while δ¹³C values indicate spatial dietary changes indicative of movements into areas of different carbon sources.

A combination of ecological factors like environmental characteristics, resource abundance and distribution, and the presence of other competing species influences nearshore habitat use by sharks (Knip et al. 2010).  Considering the known ecological importance of sharks, identifying influential factors on coastal shark habitat use is imperative to understand how shark species will respond to future changes in the environment (Heithaus et al. 2008, Pan et al. 2013, Yates et al. 2015).

References

Matich, P., Heithaus, M.R. & Layman, C.A. 2011. Contrasting Patterns Of Individual Specialization And Trophic Coupling In Two Marine Apex Predators. Journal Of Animal Ecology, 80, 295–304.

Post, D. M. 2002. Using Stable Isotopes To Estimate Trophic Position: Models, Methods, And Assumptions. Ecology 83:703–718.

Peterson, B. J., And B. Fry. 1987. Stable Isotopes In Ecosystem Studies. Annual Reviews In Ecological Systems 18:293–320.

Macneil, M. A., G. B. Skomal, And A. T. Fisk. 2006. Stable Isotopes From Multiple Tissues Reveal Diet Switching In Sharks. Marine Ecology Progress Series 302:199–206.

Heithaus, M.R., Frid, A., Wirsing A.J., Worm, B. 2008. Predicting ecological consequences of marine top predator declines. Trends in Ecology and Evolution 23: 202-210.

Knip, D. M., Heupel, M. R., and Simpfendorfer, C. A. (2010). Sharks in nearshore environments: models, importance, and consequences. Marine Ecology Progress Series 402, 1–11.

Pan, J., Marcoval M.A., Bazzini, S.M, Vallina, M.V., De Marco, S.G. 2013. Coastal Marine Biodiversity Challenges and Threats. Marine Ecologist in a Changing World. 43-67.

Yates, P. M., Heupel, M. R., Tobin, A. J., and Simpfendorfer, C. A. 2015. Ecological drivers of shark distributions along a tropical coastline. PLoSOne 10(4), e0121346.

Simpfendorfer, Colin A., and Heupel, Michelle R. (2012) Assessing habitat use and movement. In: Carrier, Jeffrey C., Musick, John A., and Heithaus, Michael R., (eds.) Biology of Sharks and Their Relatives. CRC Marine Biology Series. CRC Press, London, UK, pp. 579-601.