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.”