A Statistical Analysis for Estimating Fish Number Density with the Use of a Multibeam Echosounder

Fish number density can be estimated from the normalized second moment of acoustic backscatter intensity [Denbigh et al., J. Acoust. Soc. Am. 90, 457-469 (1991)]. This method assumes that the distribution of fish scattering amplitudes is known and that the fish are randomly distributed following a Poisson volume distribution within regions of constant density. It is most useful at low fish densities, relative to the resolution of the acoustic device being used, since the estimators quickly become noisy as the number of fish per resolution cell increases. New models that include noise contributions are considered. The methods were applied to an acoustic assessment of juvenile Atlantic Bluefin Tuna, Thunnus thynnus. The data were collected using a 400 kHz multibeam echo sounder during the summer months of 2009 in Cape Cod, MA. Due to the high resolution of the multibeam system used, the large size (approx. 1.5 m) of the tuna, and the spacing of the fish in the school, we expect there to be low fish densities relative to the resolution of the multibeam system. Results of the fish number density based on the normalized second moment of acoustic intensity are compared to fish packing density estimated using aerial imagery that was collected simultaneously

Near resonance acoustic scattering from organized schools of juvenile Atlantic bluefin tuna (Thunnus thynnus)

Schools of Atlantic bluefin tuna (Thunnus thynnus) can exhibit highly organized spatial structure within the school. This structure was quantified for dome shaped schools using both aerial imagery collected from a commercial spotter plane and 400 kHz multibeam echo sounder data collected on a fishing vessel in 2009 in Cape Cod Bay, MA. Observations from one school, containing an estimated 263 fish within an approximately ellipsoidal volume of 1900 m3, were used to seed an acoustic model that estimated the school target strength at frequencies between 10 and 2000 Hz. The fish\u27s swimbladder resonance was estimated to occur at approximately 50 Hz. The acoustic model examined single and multiple scattering solutions and also a completely incoherent summation of scattering responses from the fish. Three levels of structure within the school were examined, starting with fish locations that were constrained by the school boundaries but placed according to a Poisson process, then incorporating a constraint on the distance to the nearest neighbor, and finally adding a constraint on the bearing to the nearest neighbor. Results suggest that both multiple scattering and spatial organization within the school should be considered when estimating the target strength of schools similar to the ones considered here

Assessing juvenile Atlantic bluefin tuna schools in the Northwest Atlantic using sonar data and aerial imagery

Over the past 2 years, a feasibility study has been conducted in order to establish a methodology for assessing the biomass of juvenile Atlantic bluefin tuna (Thunnus thynnus). Over several days in August 2009, a 400 kHz Reson 7125 multibeam sonar installed on a commercial fishing vessel was used to collect acoustic backscatter from tuna schools. The multibeam sonar was oriented on the starboard side of the vessel to image a vertical slice of the water column. Because the fishing vessel was led to the tuna schools by a spotter plane, we were restricted to examining only near‐surface tuna schools that were visible from the air. The same spotter plane collected aerial images of the same schools that were examined with the multibeam sonar. The multibeam sonar data allowed us to estimate attributes such as the maximum depth, cross sectional area, and morphology of the fish schools in a vertical plane, while metrics such as the nearest neighbor distance and number of fish were estimated from the aerial photographs. Taken together, the sonar data and aerial imagery provide a viable methodology for assessing juvenile Atlantic bluefin tuna

Estimating Atlantic Bluefin Tuna number density using the second moment of intensity

Fish number density can be estimated from the normalized second moment of acoustic backscatter intensity [Denbigh et al., J. Acoust. Soc. Am. 90, 457–469 (1991)]. This method assumes that the distribution of fish scattering amplitudes is known and that the fish are randomly distributed following a Poisson volume distribution within regions of constant density. It is most useful at low fish densities, relative to the resolution of the acoustic device being used, since the estimators quickly become noisy as the number of fish per resolution cell increases. The method was applied to an acoustic assessment of juvenile Atlantic Bluefin Tuna, Thunnus thynnus. The data were collected using a 400 kHz multibeam echosounder during the summer month of 2009 in Cape Cod, MA. Due to the high resolution of the multibeam system used, the large size (approx. 1 m) of the tuna, and the spacing of the fish in the school, we expect there to be low fish densities relative to the resolution of the multibeam system. Results based on the normalized second moment of acoustic intensity are compared to fish packing density estimated using aerial imagery that was collected simultaneously

Mid-frequency backscatter from spatially organized fish schools

Schools of Atlantic bluefin tuna, Thunnus thynnus, can exhibit highly organized spatial structure. A stochastic simulation has been used to investigate the impact of this spatial structure on thebackscattered acoustic field at frequencies below 10 kHz. The simulations are seeded with realizations of schools of juveniles based on field observations from 2009 in Cape Cod Bay. The field observations, which consist of both aerial imagery and 400 kHz multibeam echo sounderbackscatter, have been used to characterize the school morphology, number of fish, and spatial structure within the school. The simulation examines various degrees of structure within the school, starting with fish locations that are constrained by the school boundaries but are otherwise the result of a Poisson process, and gradually incorporating components of school structure such as nearest neighbor distance and quasi-crystalline school sub-structures containing different numbers of fish. Results of the simulation suggest that multiple scattering is negligible except at low frequencies near the swimbladder resonance. Above resonance, even a modest degree of structure within the school (e.g., spatial constraints on pairs of fish) results in appreciable changes to the scattered field. [Work supported by ONR.