carbon dioxide emissions – Shark Research & Conservation Program (SRC)

By Shannon Moorhead, SRC Intern

In recent years, it has become apparent that increased CO₂ emissions have farther reaching consequences than simply raising the temperature of Earth’s atmosphere. A significant amount of CO₂ is absorbed by the ocean, which raises its acidity through chemical reactions with water molecules. This process, termed ocean acidification, has a large number of potentially detrimental effects on biodiversity, interactions between species, and individual species fitness. Elevated CO₂ levels encumber the ability of certain invertebrates, such as corals and snails, to build calcium carbonate skeletons and can drive habitat shifts that degrade ecosystems. Laboratory experiments have also shown that prolonged exposure to CO₂ negatively affects the ability of fish to perform predator-avoidance behaviors.

Nagelkerken et al 2015 explores the effects of ocean acidification with in situ study, meaning the experiment was done in the natural habitat of the subject species. A field study allowed for the researchers to account for the indirect effects of changes in habitat on fish behavior and abundance, necessary to more accurately predict species responses to ocean acidification, as well as their potential consequences for ecosystems. The authors observed habitat coverage, fish habitat association, fish escape response performance, and fish and predator population density at 2 separate locations: White Island, New Zealand and Vulcano Island, Italy. Both sites are characterized by vents that naturally produce CO₂, maintaining the acidity of the surrounding water at a much higher than average level, levels that the rest of the oceans may reach by the end of the century.

a,b, Habitat cover at White Island (a) and Vulcano Island (b). c,d, Fish density in each habitat at White Island (c) and Vulcano Island (d)

At both locations, habitat coverage changed significantly between the control sites (far enough away from the vents to not be affected by the elevated CO₂) and the sites near the vents. Increased acidity caused ecosystem phase shifts; complex ecosystems mottled with vegetation, algae, patches of rock or sand give way to simpler communities dominated by either algae or sand near the vents. The fish species observed, the common triplefin and Bucchich’s goby, both associated primarily with algae and sand or rock bottom areas. Increased biomass of preferred habitat, along with higher levels of prey abundance in these habitats, most likely contribute to the significant difference in fish density observed between the vent and control sites. Fish density near the vents was greater than double density measured at control sites. This may have also been a result in part of the lack of predatory fish observed near the CO₂ vents.

a, fish escape speed at White Island (top panel) and Vulcano Island (bottom panel). b, jump distance (distance fish moved while escaping) at White Island (top panel) and Vulcano Island (bottom panel). c, startle distance at White Island (top panel) and Vulcano Island (bottom panel)

Though effects of elevated CO₂ actually improved fish abundance, it still had negative effects on the behavior of the fish species. Fish at vent sites escaped threats more slowly than fish at control sites and usually waited until the threat was closer to begin moving away, indicating CO₂ exposure lessened the ability of the fish to avoid predation. One exception to this, in the algae dominated habitats at the Vulcano Island site, there was little difference between the startle distance (distance from the threat to the fish when the fish starts its escape) of fish living at the control sites and fish living near the vents. Fish may begin their escape response later in this habitat because they feel more relaxed knowing they have easy access to shelter.

a, fish density at White Island and Vulcano Island. b, predator density at White Island and Vulcano Island

This study is the first example of the negative direct effects of ocean acidification on fish behavior being counteracted by indirect effects that actually increase fish abundance and survival. Contrasting laboratory-based predictions that less productive and simpler ecosystems would harm fish populations, this paper demonstrates the need for more in situ studies on the effects of elevated CO₂ levels. Indirect effects, such as changes in predator and prey abundance and habitat phase-shifts, must be considered when attempting to accurately predict the consequences of climate change.

Nagelkerken I, Russel BD, Gillanders BM, Connell SD (2016) Ocean acidification alters fish populations through habitat modification. Nature Climate Change 6: 89-93

By Gabi Goodrich, RJD Intern

Everyone is well aware of the problem of overfishing. Fishing fleets go out and fish until they meet their quota. While over fishing is a major problem for the oceans health, another, less talked about side of the issue is the fuel consumption of those fishing fleets. As the “fight to fish” grows, fleets have become bigger and more powerful fleets. With the public concern for green products, the use of high emission energy sources has been put into the spotlight. In an article titled “Fuel Consumption of Global Fishing Fleets: Current Understanding and Knowledge Gaps,” Robert W R Parker and Peter H Tyedmers studied more than 1,600 records of fuel use by fleets worldwide using all types of fishing methods.

A shrimp trawler hauling in nets. Photo Credit – Robert Brigham, NOAA Photo Library

It is apparent that some are bigger offenders than others. Some of the most popular foods hold the top ten spots, with shrimp and lobster coming in at number one for worst offenders. It is interesting to see that the global difference between fishing practices. While globally, shrimp and lobster hold an average of 2932 liters per ton, the fuel use intensity (FUI) is 783 liters of fuel to catch one ton of Maine Lobsters from traps, while the Norway Lobster takes 17,000 liters of fuel per ton in the North Sea (Parker 2014). These variations, however, can be attributed to different fishing styles, gear, and availability and magnitude of what they are trying to catch. Those to have to travel farther and longer distances to find the desired catch use more fuel than those who have to travel shorter distances and have greater potential to land the desired catch. Catches like the Peruvian Anchovy, Atlantic Mackerel, and Australian Sardine are some of the most efficient fisheries and are some of the largest fisheries globally, by volume of landings. The use of purse seine gear or other surrounding nets average an FUI of 71 liters per ton while trawling for small pelagic fish has an FUI of 169 liters per ton (Parker 2014).

A boat and lobster pots. Photo credit – Bob Jones

So what does this all mean? According to the FUI records, the median value is 239 liters per ton. That’s roughly an average of two kilograms of carbon dioxide produced per kilogram of seafood caught and landed. To put that into perspective, beef has an average of just over 10 kilograms of CO₂ per kilogram produced, pork has just less than 6 kilograms of CO₂ per kilogram. While compared to other sources of food, the production and fishing of seafood is relatively low. The study does have some serious implications. The most efficient sources of fishing (small pelagic fisheries) are often overlooked as a viable food source in developing countries and are instead used for aquaculture and livestock. Furthermore, with fuel prices on the rise and carbon emission regulations and laws growing stricter, the profitability of the fishing industry will be compromised. Parker and Thyedmers say the most effective way to improve the energy performance of fisheries is to rebuild stocks and manage capacity effectively.

Posts

Ocean acidification alters fish populations indirectly through habitat modification

Fuel consumption of global fishing fleets