Echosounders may be calibrated by suspending a solid metal sphere in the acoustic beam and adjusting the system gain so that the measured target strength (TS) equals the theoretical value for the sphere. The suspension apparatus (i.e. net bag or loop of line glued into a hole, and control lines) may appreciably scatter sound, potentially affecting the sphere-echo intensity and phase, and hence the calibration uncertainty. Here, we characterize the effects of conventional suspension materials and configurations on measurements of sphere TS and recommend approaches to mitigate the observed effects on echosounder calibrations. We show that the effects of suspension lines on both calibration accuracy and precision change with line type and increase with line diameter and acoustic frequencies above similar to 100kHz. Compared to commonly used polyamide (nylon) line, ultra-high-molecular-weight polyethylene (PE) line has an acoustic impedance closer to that of water and therefore has a lessened effect on measurements of sphere TS. The effects may be mitigated in a practical manner using a 0.38-mm diameter, multifilament, PE line for the suspension.

During the last century, the population of Pacific sardine (Sardinops sagax) in the California Current Ecosystem has exhibited large fluctuations in abundance and migration behavior. From approximately 1900 to 1940, the abundance of sardine reached 3.6 million metric tons and the “northern stock” migrated from offshore of California in the spring to the coastal areas near Oregon, Washington, and Vancouver Island in the summer. In the 1940s, the sardine stock collapsed and the few remaining sardine schools concentrated in the coastal region off southern California, year-round, for the next 50 years. The stock gradually recovered in the late 1980s and resumed its seasonal migration between regions off southern California and Canada. Recently, a model was developed which predicts the potential habitat for the northern stock of Pacific sardine and its seasonal dynamics. The habitat predictions were successfully validated using data from sardine surveys using the daily egg production method; scientific trawl surveys off the Columbia River mouth; and commercial sardine landings off Oregon, Washington, and Vancouver Island. Here, the predictions of the potential habitat and seasonal migration of the northern stock of sardine are validated using data from “acoustic–trawl” surveys of the entire west coast of the United States during the spring and summer of 2008. The estimates of sardine biomass and lengths from the two surveys are not significantly different between spring and summer, indicating that they are representative of the entire stock. The results also confirm that the model of potential sardine habitat can be used to optimally apply survey effort and thus minimize random and systematic sampling error in the biomass estimates. Furthermore, the acoustic–trawl survey data are useful to estimate concurrently the distributions and abundances of other pelagic fishes.

The abundances and distributions of coastal pelagic fish species in the California Current Ecosystem from San Diego to southern Vancouver Island, were estimated from combined acoustic and trawl surveys conducted in the spring of 2006, 2008, and 2010. Pacific sardine (Sardinops sagax), jack mackerel (Trachurus symmetricus), and Pacific mackerel (Scomber japonicus) were the dominant coastal pelagic fish species, in that order. Northern anchovy (Engraulis mordax) and Pacific herring (Clupea pallasii) were sampled only sporadically and therefore estimates for these species were unreliable. The estimates of sardine biomass compared well with those of the annual assessments and confirmed a declining trajectory of the "northern stock" since 2006. During the sampling period, the biomass of jack mackerel was stable or increasing, and that of Pacific mackerel was low and variable. The uncertainties in these estimates are mostly the result of spatial patchiness which increased from sardine to mackerels to anchovy and herring. Future surveys of coastal pelagic fish species in the California Current Ecosystem should benefit from adaptive sampling based on modeled habitat; increased echosounder and trawl sampling, particularly for the most patchy and nearshore species; and directed-trawl sampling for improved species identification and estimations of their acoustic target strength.

Pacific sardine (Sardinops sagax) and other coastal pelagic fish species (CPS) have long been surveyed off the west coast of the United States of America using combined echosounder and trawl sampling. The challenges of the acoustic-trawl method are to first estimate and survey the potential habitat; identify the contribution of target species to the total acoustic backscatter; estimate the mean acoustic backscatter per individual fish of each target species, and combine this information to estimate their biomass densities, total biomasses, and geographic distributions. Total uncertainty, including random and systematic components of measurement and sampling error, is then estimated. Using equipment and methods resulting from over fifty years of technological maturation, the total biomass of sardine in the northern sub-population was estimated from the summer 2008 acoustic-trawl survey data as 0.679 Mt (CV = 30.9 %), compared to 0.7 Mt from an assessment model. Biomass estimates of jack mackerel (Trachurus symetricus) and Pacific mackerel (Scomber japonicus) were estimated from the survey as 0.448 Mt (35.7 %), and 0.055 Mt (53.3%), respectively. The distribution of acoustically-mapped CPS matched the distribution of trawl catches with CPS. The sardine biomass was located mostly off the coasts of Oregon, and Washington as predicted by a generalized additive model (GAM) of potential sardine habitat. For future surveys, the GAM also indicates that acoustic-trawl surveys of sardine in the northern sub-population may be most efficiently conducted during the months of June and July, when the habitat is compressed along the coasts of Oregon and Washington and the fish are generally north of Point Conception and south of the Strait of Juan de Fuca. Also during this period, daytime survey effort is maximum and the survey analysis can be augmented with fishery catch data from the same general time and place.