Marine Mammal Bycatch Research at the NEFSC
Reliable quantitative information about the bycatch of marine mammal species is one of the essential pieces of information needed in assessments of the marine mammals and in the management of human-induced mortalities of the marine mammals.
For all bycaught marine mammal species and all fisheries, the total number of marine mammal takes (B) from a fishery is defined as the product of the observed bycatch rate of that species from that fishery and the amount of total annual fishing effort: B = bycatch rate * total effort, where the bycatch rate is defined as the number of animals taken per unit of fishing effort.
Since 1990, estimates of seal and cetacean bycatch rates were derived from data collected by the Northeast Fisheries Observer Program (NEFOP). Bycatch rates have been calculated using ratio and modeling estimation methods. The unit of fishing effort used in the bycatch rate is defined as the most appropriate measure of fishing effort for a particular fishery. An appropriate measure is defined as a measure that increased as the number of takes increased, and is available in the observed sample and for the entire fishery.
Fishing effort information is obtained from multiple databases: the Vessel Trip Report (VTR) database, the Northeast Dealer Report Landings database (also sometimes referred to as the Weighout database), the Allocated Commercial Fisheries database, the North Carolina Division of Marine Fisheries trip ticket landings database, and the Virginia Marine Resource Commission landings database. For some fisheries, information from the VTR or NEFOP databases is used to prorate landings from a particular fishery to fishery-and-bycaught species-specific strata.
Standard bootstrapping techniques are used to derive the confidence intervals and CV for the bycatch estimates. The resampling unit used is usually an entire trip rather than individual hauls to ensure that any within-trip dependence was carried over into the bycatch estimate’s variance.
drawing of sink gillnet fishing gear and humpback whale by Tora Johnson