loggerhead sea turtle – Shark Research & Conservation Program (SRC)

By Abby Tinari, SRC intern

South African scientists, Diane et. al (2017), studied how organic matter is transferred through the food web and how this influences consumer populations. Specifically, they chose to look at the interaction between sea turtle eggs and meiofauna in sandy beaches where organic matter is limited.

Loggerhead turtle, species who provided the turtle eggs. (Photo from Wikipedia Commons)

Meiofauna are small invertebrates that live in both marine and fresh water environments. The main taxa present in the study were nematodes, insect larvae, collembola, and halacarid. Sandy beaches lack an abundance of organic matter. Diane et. al predicted that the seasonal influx of turtle eggs would generate an increase in meiofauna abundance. To test this, they chose to look at how the meiofauna abundance differs between naturally predated nests and the sand around it as well as how quickly the meiofauna utilized the resource pulse.

The abundance study, looking at the differences in meiofauna population between the preyed upon sea turtle nests and the surrounding sand, sampled 15 nests taking three samples at 5, 20, and 40-centimeter depth from each nest. This was also done for the surrounding sand 2 meters from each of the selected nests to act as the control. These nests were naturally predated on by honey badgers and ghost crabs among other predators. 50 turtle eggs were taken from active nests and placed into 10 pseudo-nests (baskets) for the temporal study. The eggs were cracked to simulate natural predation. Diane et al. took samples at three depths (5, 20 and 40cm) in each the control and the experimental baskets. Samples were taken every day for 20 days. In each experiment the meiofauna was sorted into major taxonomic groups and the density of individuals was counted.

Mean abundances (±SE) of major meiofauna taxonomic groups in the experiment baskets (with turtle eggs: left panel) and control baskets (without turtle eggs: right panel) over the sampling period for the in-situ experiment.(a-b nematodes; c-d halacarid mites; e-f insect larvae; g-h collembolan; i-j unknown 1) (Diane et al. 2017)

Diane et al. found that the meiofaunal and nematode abundances were significantly different between the experimental and control nests for both the abundance and temporal study. Nematodes were the dominant taxa in both studies. In the first experiment, for abundance, the meiofaunal diversity was higher in the naturally predated nests than in the control nests around them. Collembolans were the second most dominant taxa in the predated nests while in the control nests insect larvae was the second most abundant taxa. The abundance of most meiofauna decreased with depth except for the collembolans which had similar abundances at the shallowest and deepest depths. In the temporal study the species richness was higher in the experimental baskets than in the control baskets. Diane et al. also found a significant difference in abundance at the middle and bottom depths. Most meiofauna were in the deepest samples close to the eggs. Abundances and maximums varied with time. Nematodes were the first to arrive and die out, followed by insect larvae, halacarid mites and collembolans.

Studying the interactions between trophic levels is useful to know, even on a macroscopic scale. Some of the meiofauna observed were insect larvae that will become consumers and prey for other taxa. Diane mentions that nematodes feed on bacteria and other macrofauna and this could be linked to higher trophic levels. The sea turtle eggs bring a predictable annual source of nutrients that allow the meiofauna to thrive. Other spawning events may have a similar increase in meiofauna populations and abundances with the pulse of organic matter they bring. Knowing the interactions between trophic levels can help scientists better understand the effects of an abundance shift in one of the levels.

Works cited

Diane Z.M., et Le Gouvello, Ronel Nel, Linda R. Harris, Karien Bezuidenhout. 2017. The response of sandy beach meiofauna to nutrients from sea turtle eggs. Journal of Experimental Marine Biology and Ecology 487:94-105.

By Ryan Keller, SRC Intern

Bycatch of megafauna (larger organisms) is a serious negative side effect that stems from the practice of commercial fishing worldwide. Often fishing practices such as long lines or using nets are effective at catching the target species but also will entrap many other organisms. Often for organisms that breath air this means mortality as they may be stuck underwater for a longer period of time then they able to hold their breath. Baja California Sur, Mexico has some of the highest recorded megafauna bycatch rates of anywhere in the world due to heavy use of bottom-set nets. Unfortunately this area also happens to be a foraging mecca for endangered loggerhead turtles.

Pictured above: An endangered loggerhead turtle swimming over a reef. Loggerheads when caught in nets cannot get to the surface to breathe leading to death. https://commons.wikimedia.org/wiki/File:Loggerhead_turtle.jpg

Between 2007-2009 Stanford University researches worked with local fisherman to compare the megafauna’ bycatch rates between traditional nets (with buoys) and bouyless nets. Both types of nets were set near each other during the trials and the difference in bycatch recorded. The nets were checked on a regular basis to try and prevent mortality of any turtles caught in the nets. Local fisherman were compensated for setting two of the experimental nets and having a researcher with them on their trips. Later in the experiment partner fisherman were hired to fish exclusively the bouyless nets. All turtles that were caught in the nets were tagged, measured and released. In all trials the fisherman were allowed to keep their catch and bring it to market. The difference in the monetary value of catches between types of nets was also calculated.

As seen above: A green sea turtle (Chelonia mydas) stuck in a fishing net. Turtles often get stuck in nets that have broken free and are floating in the currents throughout the ocean, these nets are termed “Ghost nets”. https://commons.wikimedia.org/wiki/File:Sea_turtle_entangled_in_a_ghost_net.jpg

There was found to be a 67% mean reduction in the number of turtles caught in the bouyless nets compared to traditional nets with a minimal impact on the quantity of target species. There was a decreased market value in the catch from the bouyless nets but this is most likely due to higher than average amounts of certain species being caught and brought to market at one time driving the price down. There has been other research done in this area that shows small changes such as illuminating nets at night also reduce the amount of bycatch. It is not practical to just ban the present commercial fishing methods completely. Finding ways to make small changes that have minimal impact on the fisherman and their income while drastically decreasing bycatch is the best way to gain acceptance and support from the industry. If the fisherman are minimally impacted they are much more likely to take up the new practices and not resist or revert back to prior methods.

Peckham, S. H., Lucero‐Romero, J., Maldonado‐Díaz, D., Rodríguez‐Sánchez, A., Senko, J., Wojakowski, M., & Gaos, A. (2015). Buoyless Nets Reduce Sea Turtle Bycatch in Coastal Net Fisheries. Conservation Letters.

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