Mesopelagic fish are difficult to study due to their unique anatomy. Many of these fish have swim bladders to help them control their buoyancy, which makes them hard to sample because those gas-filled chambers typically burst as the fish come up in nets and the fish die. Scientists in California have made progress on mesopelagic fish sampling by developing a submersible chamber that can keep fish alive on the way up to the surface under a controlled atmosphere and pressure. A passive method to estimate mesopelagic fish abundance is by echosounding to locate the 'deep scattering layer' through the backscatter received from these acoustic sounders. A 2015 study suggested that some areas have had a decline in abundance of mesopelagic fish, including off the coast of Southern California, using a long-term study dating back to the 1970s. Cold water species were especially vulnerable to decline.