Application of an acoustic–trawl survey design to improve estimates of rockfish biomass

Biomass estimates of several species of Alaskan rockfishes exhibit large interannual variations. Because rockfishes are long lived and relatively slow growing, large, short-term shifts in population abundance are not likely. We attribute the variations in biomass estimates to the high variability in the spatial distribution of rockfishes that is not well accounted for by the survey design currently used. We evaluated the performance of an experimental survey design, the Trawl and Acoustic Presence/Absence Survey (TAPAS), to reduce the variability in estimated biomass for Pacific ocean perch (Sebastes alutus). Analysis of archived acoustic backscatter data produced an acoustic threshold for delineating potential areas of high (“patch”) and low (“background”) catch per unit of effort (CPUE) in real time. In 2009, we conducted a 12-day TAPAS near Yakutat, Alaska. We completed 59 trawls at 19 patch stations and 40 background stations. The design performed well logistically, and Pacific ocean perch (POP) accounted for 55% of the 31 metric tons (t) of the catch from this survey. The resulting estimates of rockfish biomass were slightly less precise than estimates from simple random sampling. This difference in precision was due to the weak relationship of CPUE to mean volume backscattering and the relatively low variability of POP CPUE encountered. When the data were re-analyzed with a higher acoustic threshold than the one used in the field study, performance was slightly better with this revised design than with the original field design. The TAPAS design could be made more effective by establishing a stronger link between acoustic backscatter and CPUE and by deriving an acoustic threshold that allows better identification of backscatter as that from the target species.

Effect of sampling interval on estimates of larval supply

Estimates of larval supply can provide information on year-class strength that is useful for fisheries management. However, larval supply is difficult to monitor because long-term, high-frequency sampling is needed. The purpose of this study was to subsample an 11-year record of daily larval supply of blue crab (Callinectes sapidus) to determine the effect of sampling interval on variability in estimates of supply. The coefficient of variation in estimates of supply varied by 0.39 among years at a 2-day sampling interval and 0.84 at a 7-day sampling interval. For 8 of the 11 years, there was a significant correlation between mean daily larval supply and lagged fishery catch per trip (coefficient of correlation [r]=0.88). When these 8 years were subsampled, a 2-day sampling interval yielded a significant correlation with fishery data only 64.5% of the time and a 3-day sampling interval never yielded a significant correlation. Therefore, high-frequency sampling (daily or every other day) may be needed to characterize interannual variability in larval supply.

Evaluating the quality of bycatch data and bycatch estimates among disparate fisheries

In 2006, the National Marine Fisheries Service, NOAA, initiated development of a national bycatch report that would provide bycatch estimates for U.S. commercial fisheries at the fishery and species levels for fishes, marine mammals, sea turtles, and seabirds. As part of this project, the need to quantify the relative quality of available bycatch data and estimation methods was identified. Working collaboratively with fisheries managers and scientists across the nation, a system of evaluation was developed. Herein we describe the development of this system (the “tier system”), its components, and its application. We also discuss the value of the tier system in allowing fisheries managers to identify research needs and efficiently allocate limited resources toward those areas that will result in the greatest improvement to bycatch data and estimation quality.

Estimates of total seabird bycatch by Atlantic pelagic longline fisheries from 2003 to 2006

Results of recent seabird bycatch studies in the International Commission for the Conservation of Atlantic Tunas Convention Area were combined to estimate total seabird bycatch of pelagic longline fishing in the Atlantic Ocean, and bycatch per selected species. Available studies do not apply to the full spatial and temporal extent of the fishing effort, so assumptions were made to account for missing information. Over the 4 years from 2003 to 2006 the total seabird bycatch estimate was 48,500. Results indicate that about 57% of the pelagic longline seabird bycatch was albatrosses (Diomedea, Phoebastria, Thalassarche, Phoebetria spp.). This mortality is at a level to cause concern for the smaller and more vulnerable albatross populations in the region. Variation in annual seabird bycatch was caused by variation in total fishing effort, and movement of effort away from areas of higher seabird bycatch rates.