Evaluation of low impact pressure waves as a quantitative sampling method for small fish in shallow water

Low impact pressure waves (LIMP) generated by small (< 1 g) underwater ignition capsules have been used to qualitatively and semi-quantitatively, as catch per unit effort, sample young-of-the-year (YOY) and juvenile fish in shallow (0-3 m) waters. In order to evaluate this method on small fish, four categories, 0+ and 1+ year Eurasian perch, 1+ year roach and 0+ to 1+ year pike, were subjected to the LIMP at distances of 0.5-5 m from the detonation point. At a distance < 1 m all specimens were stunned and either floated to the surface or sank to the bottom, whereas almost all were unaffected at distances > 3 m. The efficiency radius of the LIMP varied between 1.5 and 3.0 m, and the effective area was 7-28 m² depending on habitat and species. The average effective area for the two extensively studied categories (0+ perch and 1+ roach) was 17.9 m². Both depth and vegetation affected the effective area. The smallest area was obtained at < 1 m depth with vegetation present, and the largest at 1-2 m depths with no vegetation present. Within each fish category, the method sampled fish independently of length suggesting that a size/year-class is sampled equally. The proportion of affected fish that sank was large and species-dependent. In addition to the netting of surfaced fish, collecting fish from the bottom is recommended for achieving an accurate quantitative estimate. The tested parameters of the LIMP, P_max and Impulse, declined as a function of distance following a negative exponential growth model with a baseline. The similar-shaped responses among the fish categories suggest that the method works at depths < 3 m also in heterogeneous environments, and that the method can be used to quantitatively estimate abundance of small fish with swim bladders. A model for quantitative abundance estimates based on the evaluation is presented.