Building a Better Pound Net
By Takashi Ishihara, Yoshimasa Matsuzawa, John Wang, and Hoyt Peckham
Accidental capture, or by-catch, in fisheries is among the greatest threats to sea turtle populations globally. By-catch in Japanese pound nets represents a major obstacle to the recovery of the endangered North Pacific loggerhead population. A pound net consists of a leader net that is run from the seafloor to the surface and that is set perpendicular to the coast to direct fish into a system of standing nets, which, in turn, entrain the fish into an enclosed trap. When mounted underwater, these traps prevent incidentally captured sea turtles from reaching the surface to breathe, thereby resulting in high numbers of drowning mortalities. Numbering in the thousands and varying greatly in size and design, some Japanese pound nets can be massive, with leaders up to two kilometers long, traps measuring in excess of 10,000 cubic meters (13,000 cubic yards), and initial construction costs exceeding US$2 million per net.
Although modifications to trawls and longlines—such as turtle excluder devices and circle hooks—have been shown to greatly reduce sea turtle by-catch, until recently no by-catch mitigation technology had been designed for pound nets. That began to change in 2006 when pioneering work by Osamu Abe (National Far Seas Fisheries Institute) and Daisuke Shiode (Tokyo University of Marine Science and Technology) discovered that trapped turtles consistently search pound net roofs. Taking advantage of this behavior, their studies indicated that specially designed hatches on pound net roofs could facilitate turtle escape.
Building on those findings, in 2009 we launched an international, multidisciplinary program to develop pound net escape devices (PEDs) that could enable turtles to escape while retaining target fish. From the outset, we engaged fishermen, fisheries managers, marine scientists, gear manufacturers, and journalists from three countries—Japan, Mexico, and the United States—that host the Japanese nesting loggerhead population. To design, test, and promote the adoption of PEDs, we convened a series of collaborative workshops at Suma Aqualife Park and Minamichita Beachland Aquarium in Kobe and Mihama, Japan, respectively. During each workshop, we assembled a model pound net in a massive aquarium with an adjacent underwater viewing area from which turtle and fish behavior could be observed (see image). Participants collaborated during the workshops to design, build, and test PEDs, thus running multiple trials for each experimental PED by introducing a turtle into the trap and observing if and how the animal encountered the PED and successfully escaped.
During the first two workshops, we designed several types of PEDs that proved to be highly successful for turtle escapes. Although we were all pleased with these promising results (PEDs can release loggerheads from pound nets!), the fishermen among us emphasized that turtle escape was only half the goal. PEDs still needed to be engineered to ensure retention of target fish.
In the most recent workshop (October 2011), we tested a range of PED designs for fish retention as well as turtle escape. For those trials, we introduced juvenile hamachi, an important target fish, into the trap along with the turtles. Extensive trials were run on numerous PED designs, with additional fish retention trials on the most promising of the designs. Ultimately, several PED designs demonstrated both ease of turtle escape and effective fish retention. In 2012, we plan to begin field trials in conjunction with Japanese fishermen.
Beyond developing a range of promising PED designs, the workshops were quite effective in raising awareness among the first—and perhaps most important—audience: fishermen. By seeing firsthand that schools of hamachi were retained in the nets while turtles escaped through the PEDs, fishermen realized that turtle-safe pound nets will not diminish their fisheries’ profitability. Members of the public and press also joined workshop participants to observe the trials from within the aquarium, thereby offering them firsthand views of turtles struggling and escaping the pound net traps to reach the surface to breathe.
The experience was transformative for many and resulted in extensive press coverage of the by-catch problem and the collaborative efforts to develop PEDs. Prominent stories appeared in national newspapers and on television, reaching tens of millions of readers and viewers. Public and official commentary on the workshop focused on the development of solutions, which was a considerable departure from prior debate of the problem. In the end, the awareness raised throughout this program has been as influential as the PED research itself. Collaborative refinement and eventual adoption of PEDs by pound net fishermen could eliminate one of the gravest threats to the endangered North Pacific loggerhead.
This article originally appeared in SWOT Report, vol. 7 (2012). Click here to download the entire article as a PDF.