Turtle bycatch

When you’re standing knee-deep in a stream, branches growing together into a leafy green tunnel above your head, slippery cobbles under your feet, expecting to find a steelhead or two in the net you just jerked out of the water, it can be a little startling to see a five-inch crayfish staring back at you, waving its claws.

Sometimes at work, when I’m lucky, I get to step outside my office for a little while and help the graduate students in my lab as they collect samples and data in the field. The project I work on the most involves scooping up fish in hand-held nets, but occasionally we capture other things, too – organisms we weren’t targeting or trying to catch, like that five-inch crayfish. In the fisheries world, this accidental take is called bycatch. Usually when scientists and managers are studying or trying to regulate bycatch, they’re dealing with marine life, but it happens in freshwater fisheries, too – and not just with crayfish.

When I catch a crayfish in my net, I pull it out and set it back in the stream where it came from (carefully, to preserve my fingers), but that’s not always possible in other situations. Some fishing nets are set underwater, and left to soak and collect fish for several hours before the catch is retrieved; that can be a problem if air-breathing aquatic animals – like turtles – also get caught.

A team of Canadian scientists recently published a case-study of freshwater turtle bycatch in a small lake in the journal Aquatic Conservation: Marine and Freshwater Ecosystems. They estimated the effectiveness of two methods for reducing turtle bycatch in Lake Opinicon, in southeast Ontario, Canada, which supports a small commercial fishery: adding “exclusion devices” to fish net openings that would keep most turtles out but let fish in, and shortening the fishing season.

The researchers measured the shells of four species of female turtles, and found that the smaller exclusion device they evaluated (which had an opening about two inches wide) would keep most turtles out of the fishnets – between 92 and 100 percent of three turtle species in the lake had shells too large to fit through the device. For the fourth and smallest turtle species the scientists studied, however, the exclusion device was not as successful – only 27 percent of those turtles were bigger than the opening.

If managers close the fishery during the time when turtles are most active, that can reduce turtle bycatch; to evaluate the effect of closing the fishery a month earlier in the spring, the scientists measured how active the turtles were during May and June. Only one turtle species was most active right before the current closing date of June 20, but, as the authors note, “[w]hile seasonal activity rates vary among species, a shortened fishing season would still decrease the total number of turtles captured, provided that there is no compensatory increase in fishing effort.”

Based on demographic traits like the current number of turtles in the lake, how old female turtles are when they first reproduce, and the proportion of female turtles that reproduce each year, the researchers estimated how the population of each turtle species might increase or decrease over the next 500 years under different bycatch scenarios. They determined that for three of the turtle species, just one or two female turtle deaths due to bycatch each year could lead to a complete loss of the population from the lake within that timeframe.

When I catch a crayfish in my hand-held net I can let it go right away, but turtles caught in underwater fishnets don’t have that option. The unintended capture of turtles during fishing can have disastrous consequences – as the authors write, “it is imperative that appropriate bycatch mitigation measures . . . are put in place to ensure the long-term persistence of freshwater turtles.”

Researchers studied four turtle species in Lake Opinicon, in southeastern Ontario, Canada, including the painted turtle. 

(Image by Micheal Jewel via Flickr)

Flashing and flickering squid

“Humboldt squid can be very cannibalistic,” Stanford University graduate student and researcher Hannah Rosen told me via email, “and if they sense a weakness in another individual they will attack and eat it.”

I had asked her to speculate on the fate of one particular Humboldt squid. “Though I can’t say with any certainty what its fate was since the camera was ripped off, I’d say there is a good chance it was killed,” she replied.

Rosen and a team of researchers from Stanford University and National Geographic recently reported the results of a study in which they outfitted three Humboldt squid, large cephalopods that can grow up to four feet long (not including their arms and tentacles – scientists report squid sizes in terms of the length of the mantle, the torpedo-shaped part of the body above the head), with video cameras so they could spy on their underwater color-changing behavior without the interference of divers or submersible vehicles.

Two of the squid were lit only by natural light filtering down through the water; the recordings they gathered showed Humboldt squid exhibiting two types of dynamic color-changing behavior: ‘flashing,’ a whole-body, rhythmic and rapid change in color, and ‘flickering,’ a wave-like scattering of color across the skin that, the authors write, “mimic[s] reflections of down-welled light in the water column,” much like the play of light against the bottom of a pool. (Humboldt squid chromatophores, the small organs in their skin that they reveal to change their appearance, are a single, reddish-brown color; unlike some other species of squid which have chromatophores of many different colors, Humboldt squid are either white, when their chromatophores are hidden, or red, when they’re exposed.)

Flickering, the authors suggest, may be a form of camouflage for the squid, helping them to blend into their environment and perhaps avoid being eaten. Flashing occurred primarily when the squid were in groups, suggesting that it may be a form of communication; the video recordings captured a number of interactions between squid, including physical contact, possible mating attempts, and arm-splaying that appeared to be directed toward the squid wearing the camera.

One of the cameras also captured an aggressive episode of “numerous attacks” in which, the researchers write in their paper, “several other squid . . . tore the camera package off the camera-bearing squid shortly after it was released.”

That was the squid I asked Rosen about. The scientists had equipped that squid’s camera with a red LED light so they wouldn't have to rely on natural illumination, allowing them to observe nighttime behavior. The red light apparently had the unintended consequence of aggravating the surrounding squid, leading to the attacks. 

“We were hoping the red light would be out of their visual range,” Rosen said in an email, “but were obviously wrong.”

Rosen and her team didn’t give up there, though. Last year they tried infrared lighting, which did not lead to the same problems as the red LEDs; unfortunately, Rosen said, “it also didn’t provide enough light to really see anything that was happening around the squid.”

Moving forward, Rosen intends to continue studying Humboldt squid, their color-changing behavior, and how they control their chromatophores. In the meantime, she hopes that non-scientists embrace the importance of studying animals that experience the world in a completely different way than humans do.

“It’s easy to assume an animal is stupid just because of how it looks,” she said, “but just because an animal doesn’t act the same way we do, that doesn’t mean it isn’t smart, it might just have skills we can’t imagine because they aren’t something we would ever need.”

Researchers attached a video camera to a cloth sleeve slipped onto the mantle of each squid. The cameras were programmed to detach and float to the surface at a specified time. 

(Image by Joel Hollander)

Ocean acidification

Last week, there was a flurry of news reports on the effects of ocean acidification on shellfish stocks in the U.S. (in response to a paper on the subject published in Nature Climate Change), and the news wasn’t good – according to one report, “U.S. shellfish producers in the Northeast and the Gulf of Mexico will be most vulnerable to an acidification of the oceans.” Ocean acidification refers to the global phenomenon of decreasing pH in marine waters as a result of human-induced carbon dioxide emissions dissolving in the ocean, and it can have a big effect on marine organisms. Acidic ocean water holds less calcium carbonate – meaning it’s less available for the organisms that need it to build their exoskeletons and shells – and it can even dissolve already formed shells.

New research recently published in the journal Aquatic Toxicology suggests that the mechanisms behind the negative effects of ocean acidification are not always straightforward – ocean acidification can weaken organisms’ immune systems and make them more susceptible to other, more sporadic threats. 

A team of Swedish researchers exposed tanks of Norway lobsters (a small lobster native to northern Europe) to saltwater with a pH lowered to the level predicted to occur by 2100. They measured the lobsters’ immune response to an injection of a common marine bacteria, and the amount of bacteria that persisted in the lobsters for 24 hours after the injection – if the lobsters’ immune systems were unaffected by the experiment and working well, they expected to see fewer bacterial cells after 24 hours. 

In order to test the lobsters’ response to other stressors that might occur in the presence of acidic ocean water, the scientists also exposed some of the lobsters to low oxygen, or hypoxic, conditions and high levels of manganese, a heavy metal that can have toxic effects at high exposures. (Scientists predict increasingly frequent intermittent periods of hypoxia as well as increasing levels of bioavailable manganese in marine environments as the world’s climate continues to change.)

Based on counts of immune cells, acidic ocean water on its own did not appear to affect the immune response of Norway lobsters; however, when the lobsters were exposed to hypoxia or manganese as well as acidic conditions, they had fewer immune cells than lobsters that didn’t experience hypoxia or high manganese levels. (Lobsters that were exposed to manganese under current pH levels also had fewer immune cells.)

Bacterial counts, however, told a slightly different story – the lobsters in the acidic condition tanks that didn’t experience any additional stressors weren’t able to reduce their bacterial loads, suggesting that their immune cells, though not reduced in number, were not functioning properly. (The lobsters in the hypoxic and high manganese conditions also had high bacterial loads, as did the lobsters exposed to manganese but not acidic ocean water; only lobsters in water with current pH levels and either no additional stressors or low oxygen were able to reduce the amount of bacteria in their bodies.)

As the acidity of the world’s oceans continues to increase, lobsters and other marine organisms may find themselves in an increasingly challenging environment in which they struggle to fight off infections, build their exoskeletons and shells, and survive.

Norway lobsters are typically about seven to eight inches long, including claws and tail. 

(Image by Hans Hillewaert via Wikimedia Commons)

Bioaccumulation

“Silent Spring,” writer and ecologist Rachel Carson’s most famous work, focuses on a discussion of the detrimental effects of chemical pesticides on the environment, and on humanity. The book was published in 1962, but the issue of contaminants reverberating throughout the ecosystem is still pertinent today; though Carson’s writing inspired reforms, the problems she exposed are far from solved.

The authors of a study recently published in the journal Ecological Applications consider Carson’s seminal work relevant enough to cite it in the opening of their paper, an investigation of the pathways that mercury can take as it moves through a food web spanning an ecosystem that includes both aquatic and terrestrial organisms. (‘Boundaries’ between aquatic and terrestrial areas are often permeable to a reciprocal flow of food and energy resources.)

Mercury, a toxic heavy metal, is produced by both natural and anthropogenic sources; in aquatic environments, it’s easily converted to a bioavailable form that humans and other organisms can absorb, methyl mercury. Methyl mercury can reach high levels in predators like large fish as it biomagnifies, or bioaccumulates, instead of dissolving or metabolizing – as organisms consume contaminated prey, methyl mercury builds up in their flesh.

The scientists measured the methyl mercury concentrations of several species of terrestrial spiders, insects, and mites (all types of arthropods) around two Icelandic lakes; because of bioaccumulation, they expected the organisms that fed predominantly on aquatic food sources, which contain elevated methyl mercury levels, to display higher contamination levels than organisms that consumed terrestrial foods.

What they found instead was the opposite – arthropods that had an aquatic-based diet had much lower concentrations of methyl mercury in their bodies than those that ate terrestrial foods. The arthropods collected within a few feet of the lake with a large population of midges – an aquatic insect that was a likely food source for the spiders, insects, and mites – contained, on average, less than half the amount of methyl mercury found in arthropods collected far from the lake (and therefore, presumably, far from an aquatic-based food source like the midges).

The researchers also found that arthropods with an aquatic diet were at the top of a shorter food web than those with a terrestrial diet; with fewer steps in the food web, the scientists hypothesize, there was less bioaccumulation of methyl mercury, creating what they termed a ‘trophic bypass.’ As they write in their paper, “direct consumption of aquatic inputs result[ed] in a trophic bypass that create[d] a shorter terrestrial food web and reduced biomagnification of [methyl mercury] across the food web.”

The findings of this study were unexpected, and contrary to other research that revealed higher levels of terrestrial contamination in areas adjacent to polluted streams; as the authors of the study write, “our research highlights the fact that we still know little about the potential implications of [aquatic-terrestrial] linkages for terrestrial food webs and ecosystems, particularly with regard to societally important applied issues such as contaminant bioaccumulation.”

Contaminants and pollutants in the environment are a perennial problem; a problem that, in some ways, is just as obscure as it was more than half a century ago, when Rachel Carson first brought it to the attention of the public with “Silent Spring.”

Rachel Carson and colleague Bob Hines collecting marine samples in Florida in 1952.

(Image by U.S. Fish & Wildlife Service via USFWS National Digital Library)